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Understanding Foam Grades

Density Firmness Thickness User Weight

Density, Firmness, Thickness & User Weight

📌 Quick Answer

Foam is graded by density (kg/m³), firmness (Newtons/ILD), and thickness (volume). Heavier users need higher density foams, thinner cushions need firmer grades, and sitting feels softer while lying feels firmer due to weight distribution. Understanding foam grades Density, firmness , thickness & user weight is vital for making the correct foam choice


Most people don’t think about foam until the cushions sag, the sofa feels tired, or the campervan mattress leaves you waking up stiff. That’s when confusing terms like density, firmness and ILD start cropping up.

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This guide cuts through the jargon and shows you the three things you must consider when choosing foam, how the UK grading system works, and why the same foam can feel completely different depending on thickness, use, and even your body weight.


🧩 The 3 Things to Consider When Choosing Foam

1️⃣ Density (Durability)

How much material is packed into the foam (kg/m³). Think Yogurt vs Moose. The pots are the same size, the moose is whipped up and full of air so you get less of it in reality (still nice though!) Hope this analogy helps understanding foam grades

💬 Customer Insight: Density ≠ Firmness
“I picked a high-density foam from another online suppler thinking it would be really firm. But when it arrived, it was soft as anything and sank right down when I sat on it. I had no idea density and firmness weren’t the same thing.” Mrs J Bristol

  • Higher density = longer lifespan and better resilience.
  • Lower density = cheaper, breaks down quicker.
  • Heavier users usually need higher density foam to avoid early sagging or bottoming out.
Same Volume Different Weight
Same Volume Different Weight

2️⃣ Firmness (Comfort)

How resistant the foam feels when you sit or lie on it.

  • Measured in Newtons (UK) or ILD (international).
  • Density and firmness are not the same thing.
    • High-density can still be soft (memory foam).
    • High-density can also be very firm (gym mats).
Firm Foam vs Softer Foam
Firm Foam vs Softer Foam

3️⃣ Volume (Thickness & Size)

Customer Insight: When Firm Foam Still Feels Too Soft

One customer told us:

“I ordered a firm foam because I’m a bit on the heavier side, but the cushion still felt too soft. Foam4U explained that firmness wasn’t the only factor — volume was the key. The cushion simply needed to be thicker. Once I swapped to a deeper cut, the difference was night and day.” J McClusky Inverkip

The same grade feels very different depending on thickness.

  • Thin pads (2″) → need firmer foam, or you’ll bottom out on the frame.
  • Medium cushions (3–4″) → medium to medium-firm usually works.
  • Thick cushions (5–6″+) → too firm here feels like sitting on a brick. Softer grades are often more comfortable at this depth.

👉 Part of understanding foam grades is the importance of the effect of User weight

  • A lighter person compresses the foam less, so it feels firmer.
  • A heavier person compresses further, so it feels softer — which is why heavier users benefit from firmer, higher-density foams.

Thicker foam, Same Grade, More Support.

🔢 How Foam is Graded in the UK

Here’s a typical grade you’ll see:

RX39/200

  • RX → Nothing more than a brand prefix. Could say RX, CM, or even Brian. Ignore it.
  • 39 → The density, measured in kilograms per cubic metre (kg/m³). A cube of foam 1m × 1m × 1m weighs 39kg.
  • 200 → The firmness, measured in Newtons. Higher numbers = firmer feel. A 200 Newton foam is firm compared to, say, a 120 Newton soft grade.

👉 Focus on the numbers, not the letters. Density = quality/lifespan. Firmness = comfort/feel.


⚖️ What About ILD?

In the UK we measure firmness in Newtons, but internationally you’ll see ILD (Indentation Load Deflection).

ILD measures how much pressure it takes to squash foam by 25% of its thickness.

  • Lower ILD = softer feel.
  • Higher ILD = firmer feel.

It’s just another way of expressing firmness — same principle, different scale.


🪑 Sitting vs 🛏️ Lying – Why Foam Feels Different

How foam feels under you depends on two things: weight distribution and thickness.

  • When you sit → Weight is concentrated on hips and thighs. Foam feels softer, because you’re pressing it harder in one area.
  • When you lie down → Weight is spread across shoulders, back and legs. Foam feels firmer, because no single point is under heavy pressure.

👉 Snow test: stand in shoes and you sink; stand in skis and you stay on top.

Why thickness + user weight matters even more

  • Thin seats (≤2″) → need firmer foam, especially for heavier users.
  • Medium cushions (3–4″) → balance firmness to user weight (lighter = softer grade feels fine; heavier = firmer grade needed).
  • Thick cushions (5–6″+) → a firm grade here can feel like a brick for lighter users. Softer grades usually work better, while heavier users may still need firmness for support.

✅ Key Takeaways for Understanding Foam Grades

  • Foam grades show density (kg/m³) and firmness (Newtons).
  • Density = durability and quality.
  • Firmness = comfort and feel.
  • Thickness (volume) completely changes perception of firmness.
  • User weight matters: heavier = firmer/high-density; lighter = softer options work.
  • Sitting vs lying shifts the feel: sitting = softer, lying = firmer.

❓ FAQs – Real Questions Customers Ask

Need more info Contact us
Need more info Contact us

1. What does RX39/200 actually mean?
It’s the grade: RX (brand), 39 (density in kg/m³), 200 (firmness in Newtons).

2. Why do my new sofa cushions feel like bricks?
If they’re 5–6″ thick and made in a firm grade, they’ll feel overly stiff — especially for lighter users.

3. My dining chair pad is only 2″ thick. What foam should I choose?
Go firmer, or you’ll bottom out. Heavier users especially need a denser, firmer foam at this thickness.

4. What firmness is best for sofa cushions?
For most users: medium to medium-firm at 35–40 kg/m³ density. For heavier users, step up the firmness to prevent sagging.

5. Why does the same foam feel soft when I sit but firm when I lie down?
Sitting = pressure concentrated → foam feels softer.
Lying = weight spread → foam feels firmer.

6. What foam works best for campervan or boat mattresses?
A higher density (for durability), medium firmness (for comfort), and reticulated (quick-dry) foam for damp conditions.

7. How long will foam cushions last?
Standard foams 2–3 years, higher density 5–10, and premium HR/memory foams up to 10 years — depending on use and body weight.

8. Do I need fibre (Dacron) wrap on my seat foam?
Yes. It fills the cover, softens the edges, and gives a plumper look without reducing support.

9. I’m a heavier build — what grade should I choose?
Always favour higher density and firmer foams. Lower density cushions will sag and bottom out faster under extra load.

10. Is UK foam fire safe?
Yes — all upholstery foam sold in the UK must comply with BS 5852 for domestic use (and Crib 5 for contract).


🎯 Final Word

Foam isn’t complicated once you know what the grades mean. Remember:

  • Density tells you how long it lasts.
  • Firmness tells you how it feels.
  • Thickness (volume) changes everything.
  • User weight makes a big difference.
  • Sitting vs lying changes how you perceive firmness.

Get those right and you’ll have foam that feels good, supports properly, and lasts.

Foam4U has been cutting foam for over 40 years — from sofas and armchairs to campervans, boats, and bay windows.

We recognise that sometimes you need independent advice about foam & Upholstery. Visit Furniture Industry Research Association (FIRA)


  • Use our foam cut-to-size service for instant quotes.
  • Or send us your covers and we’ll handle the lot with our cushion refill service.
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🪑 The Complete Guide to Disability Cushions, Boosters & Bed Extenders

Disability Products

When mobility gets tougher, the right cushion or support can make all the difference. From wheelchair cushions to booster seats and even bed extenders, people ask a lot of questions about disability cushions before buying. Let’s tackle the most common ones.


🦽 What’s the best cushion for a wheelchair?

High-density or memory foam wheelchair cushions distribute weight evenly and reduce pressure sores. They’re the go-to for long-term comfort.
👉 See our wheelchair cushions cut to size.


📏 How thick should a booster cushion be?

Booster cushions are usually 7–12cm thick, depending on the chair and user’s height. The aim is to make sitting and standing easier without throwing posture off.
👉 Browse our booster cushions cut to size.


🌟 Do pressure relief cushions really work?

Yes. They ease strain on hips and legs, improve circulation, and make long periods of sitting far more comfortable — especially for elderly users or post-surgery recovery.
👉 Explore our pressure relief cushions.


🧘 What’s the best cushion for posture and back support?

Foam wedges, lumbar rolls, and cut-to-size back cushions improve alignment and reduce strain. Good posture = less pain.
👉 Shop our posture cushions.


🛏 Can I make my bed longer or wider without replacing it?

Yes — a bed extender lets you widen or lengthen your mattress in seconds. Perfect for:

  • Adding length for taller sleepers
  • Joining two singles into a double
  • Creating more room without buying a whole new mattress

👉 Order your foam bed extender cut to size.


❓ Other Common Questions

Can you get disability cushions made to measure?
Yes, we cut every cushion to your exact size and shape.

How often should cushions be replaced?
Every 2–3 years, or sooner if the foam goes flat.

Are memory foam cushions worth it?
Absolutely — they offer superior support, especially for long-term use.


🏠 Final Word

Comfort, posture, and independence all start with the right support. Whether it’s a wheelchair cushion, booster seat, pressure relief pad, or a mattress extender, the right foam product makes daily life easier.

👉 Explore our full range of Disability Products →

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The Ultimate Dog Bed Guide – 20 Questions Every Owner Asks

A dog’s bed isn’t just a cushion on the floor – it’s their home inside your home. Owners ask a lot of questions before buying one, and rightly so. Here are the 20 most common questions answered in plain English.

📏 1. What size dog bed does my dog need?

Measure nose-to-tail when they’re stretched out, then add 15–20cm for wiggle room. Better too big than too small.


🛋 2. Do dogs really need a bed or will the sofa do?

Yes, they need a bed. It protects joints, keeps them comfy, and gives them a personal safe space. Your sofa is not a long-term solution.


🦴 3. What’s the best bed for an older dog with arthritis?

Orthopaedic or memory foam beds. They support hips, spine, and joints, helping reduce stiffness and pain.


⏳ 4. How often should I replace a dog bed?

Every 1–2 years, or sooner if the foam’s gone flat, the cover stinks, or your dog has destroyed it with chewing and digging.


💤 5. Are memory foam dog beds worth it?

Absolutely. Memory foam distributes weight evenly and is especially good for large or older dogs.


🧼 6. How do I wash a dog bed properly?

Removable, washable covers are a must. Foam cores should only be spot-cleaned. Never throw the whole bed in the washer.


🐕 7. What’s the difference between an orthopedic dog bed and a standard one?

Orthopedic = firm, high-density foam that supports joints. Standard = cheap filling that flattens quickly.


🛗 8. Are raised dog beds better than flat ones?

Raised beds allow airflow, keep dogs cooler, and get them off cold floors. Great for summer or outdoors.


🦷 9. Which dog beds are most chew-resistant?

Heavy-duty canvas, ballistic nylon, or reinforced stitching. No bed is 100% chew-proof, but some are close.


🩺 10. Can a dog bed help with joint pain or hip dysplasia?

Yes. Proper support eases strain on bones and joints. Memory foam or orthopedic foam is best.


🐾 11. What’s the best bed for a puppy?

Go waterproof and washable. Expect accidents. Skip memory foam until the chewing stage is over.


📐 12. How thick should the foam be in a dog bed?

Small dogs: 5–7cm.
Medium to large: 10–15cm.
Bigger dog = deeper foam.


💧 13. Are waterproof dog beds really waterproof?

Only if they have an inner waterproof liner. A removable waterproof cover protects foam best.


👃 14. How do I stop a dog bed from smelling?

Wash the cover weekly, vacuum hair, sprinkle bicarbonate of soda, and air the foam in sunlight.


🐩 15. Should I get one big bed or separate beds for multiple dogs?

Most dogs prefer their own. One bed often leads to squabbles.


🛌 16. Do dogs prefer soft beds or firm ones?

Medium-firm is ideal. Too soft = no support. Too firm = uncomfortable.


🧵 17. What’s the safest fabric for a dog bed?

Durable, tightly woven, non-toxic fabrics like canvas or polyester. Avoid loose threads or cheap fillings.


🔥 18. Are heated dog beds safe?

Yes – but only pet-specific ones. Never use a human electric blanket.


🚐 19. What’s the best dog bed for travel, camping, or campervans?

Foldable foam mats or raised travel beds – portable, washable, and durable.


😴 20. Why does my dog refuse to sleep in their bed?

Could be smell, placement, or comfort. Move it somewhere quiet, wash it, or add a blanket with your scent.


🐕 Find the perfect bed for your dog today.

Pick the right bed and you’re not just buying a cushion – you’re investing in your dog’s health and happiness. Supportive foam, washable covers, and the right size make all the difference.

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How to Measure for Perfectly Fitted Foam Cushions.

Perfectly fitted foam cushions

📏 Why Accurate Measuring Matters (what happens if your measurements are wrong?)

A perfect fit starts with accurate measurements. Even a few millimetres off can make the difference between a cushion that sits neatly in its cover and one that bulges, wrinkles, or looks baggy.

Good measuring means:

  • Snug fit with no gaps or bunching.
  • Even wear and longer foam life.
  • A better-looking, more comfortable seat.

Not a fan of measuring? Our Cushion Refill Service lets you send us your covers — we’ll measure, cut, and fit the foam for you.


🛠 What You’ll Need (what’s the best way to get accurate measurements?)

  • A rigid tape measure (fabric ones stretch).
  • A flat work surface.
  • Pen and paper (or our downloadable Measuring Sheet PDF).
  • Your cushion cover or old foam insert.

1️⃣ Measuring from the Cushion Cover (should I measure the cover or the old foam?)

  1. Remove the old insert completely.
  2. Lay the cover flat on a table — no stretching.
  3. Measure seam-to-seam for width and depth.
  4. Measure thickness at the highest point of the side panel (border).

💡 Important: The border is not always the finished foam thickness. See “The Border Rule” below.


🔍 The Border Rule – Foam vs Dacron (how much thickness does Dacron add?)

If you’re replacing old foam, the border measurement is a guide — but you’ve got choices.

When the border is 10 cm (4″) or more:

  • Foam matches border + 12 oz Dacron – Looks rounded and plump, but with less foam volume so support is softer.
  • Slightly deeper foam, no Dacron – 12.5 cm (5″) foam gives more volume and firmer support, but with a squarer finish.

💡 Borders under 10 cm usually work better with the Dacron method for a smooth fit.


⚠ Swapping Fibre or Feather for Foam (why doesn’t foam fit the same as feather cushions?)

Feather and fibre cushions squash into spaces — foam won’t.
If your cover width is 30″ on a two-seater, you’d expect a 60″ arm gap. In reality, it might be less. Fine for soft fills, but foam needs the full space.
If in doubt, contact us for advice before ordering.


Cushions to big for the available space

2️⃣ Measuring from the Old Foam Insert (can I just copy the old insert size?)

  1. Place foam on a flat surface — no compression.
  2. Measure the widest width and depth.
  3. Add a couple of mm if edges have pulled in over time.

3️⃣ Measuring Irregular Shapes (how do you measure domed or curved cushions?)

  • Start with the widest width and deepest depth.
  • Break the shape into smaller sections and measure each.
  • For curves/angles, make a paper template for precision.
  • Compare to our Foam Shapes Guide before ordering.

4️⃣ Allowances for a Snug Fit (should foam be cut bigger or smaller than the cover?)

  • Foam: Subtract 5 mm from width and depth for easier fitting.
  • Fibre or Feather: Use exact measurements.

🚫 Common Measuring Mistakes (what causes most wrong cushion orders?)

  • Guessing without a tape measure.
  • Measuring with the cushion still in the cover.
  • Ignoring warped shapes.
  • Forgetting to measure thickness accurately.

✅ Ready to Order? (how to order your new cushions quickly and correctly)

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Foam Care & Maintenance: How to Make Cushions Last Longer


🛋 Why Cushion Care Matters

Foam cushions aren’t just there to make sitting softer — they directly influence how you sit. Poor posture, especially over months or years, can cause or worsen back pain. In many cases, the culprit isn’t your back at all, but the furniture you’re sitting on. Good foam care & maintenance is essential.

New, supportive foam can transform the way your body is positioned. Correct posture takes strain off your spine and muscles, and for some people, that can mean the difference between daily aches and being pain-free.

Poor Posture = Back Ache

For more information on how sitting affects back health, the NHS guide to sitting and back pain has clear, practical advice.

Quick Posture Check for Sofas & Chairs (how to check your seating posture)

Most modern sofas are low, with deep, soft-filled cushions that feel great in the showroom — but can encourage poor posture at home. Here’s a quick way to check if yours needs improvement:

  1. Sit upright on your sofa or chair.
  2. Slide your heels back until they touch the frame or valance of the sofa.
  3. From there, slide your feet forward by roughly one foot-length.
  4. Look at your knees:
    • If your knees are above your hips – the seat is too low or too soft. This tilts your pelvis backwards, putting extra pressure on your lower back.
    • If your knees are level with your hips – better, but still not ideal.
    • If your knees are slightly below your hips – this is the best position for spinal alignment and long-term comfort.

Even small improvements matter. You may not get your knees perfectly below your hips depending on the sofa’s design, but firmer, higher-density foam can lift you enough to make a real difference — sometimes turning constant back pain into no pain at all. The Chartered Society of Physiotherapy’s sitting posture advice is another useful reference.


1️⃣ Cleaning Your Foam Cushions (how to clean foam cushions without damage)

Regular cleaning keeps foam fresh, odour-free, and performing well. Dirt and oils break down fabrics and can even affect the foam inside if left too long.


How to Clean your Sofa

2️⃣ Rotate & Flip for Even Wear (why rotating sofa cushions matters)

Without proper Foam care & maintenance cushions compress where they’re used most. The most support comes from the centre of the foam block, where weight is distributed evenly. If you regularly sit on the edge, you’re compressing a thinner section of foam — support drops, the shape changes, and you may start “rolling” toward the centre when you sit properly.

Rotating and flipping cushions keeps the foam wearing evenly, making them feel firmer for longer. Proper rotation also protects the sofa’s springs and frame. Once a cushion sags, the structure underneath takes more strain, and that can mean costly repairs.

Top Tip Fibre or feather cushions should be plumped & turned on a daily bases. Foam cushions no less the once per week,


Turn & Plump your Cushions

3️⃣ Re-Wrap to Revive Comfort (how to make an old sofa cushion look new again)

If your cushions feel flat but the foam is still springy, the wrap is probably worn. Wrapping the foam in fresh polyester wadding restores volume and comfort instantly. Once you added the Polyester wrap ensure you practice good Foam care & maintenance

For professional upholstery standards, you can reference the Furniture Industry Research Association.


4️⃣ Store Cushions Properly (best way to store outdoor cushions in winter)

For outdoor furniture cushions, storage is where most people go wrong:

  • Never store foam and covers together in a damp shed or garage — moisture will create mould and odours.
  • Best practice: remove the covers at the end of the season, wash them, and store them indoors until spring.
  • Never store outdoor cushions outside under a tarp or cover. This traps moisture in, and you’ll find mould spots long before summer returns.

For more outdoor furniture care tips, see Better Homes & Gardens’ patio furniture maintenance guide.


Never store outdoor cushions outdoors in the winter
Never store outdoor cushions outdoors in the winter

✅ Keep Comfort for Years

Caring for your foam cushions isn’t just about looks — it’s about posture, comfort, and protecting your furniture. With a little maintenance, you can add years to their life. And when the time comes, Foam4U’s Foam Cut to Size service makes replacing them quick, easy, and perfectly fitted.

[Order Your Replacement Foam]

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Fibre back cushions need to be oversized to fill out the cover properly. This guide explains how to take accurate measurements and get the right fit when ordering replacements.


🪑 Replacing Fibre Back Cushions?

They need more fill than you think

Fibre compresses very easily. If you order a cushion to the exact size of your cover, you’ll end up with something flat, baggy, and uncomfortable. To get a full, supportive cushion that sits right, it needs to be made larger than the cover’s actual size.


📏 How to Measure

Add 5cm to both directions

Lay the empty cushion cover flat and measure:

  • Side to side (width)
  • Top to bottom (height)

Measure seam to seam internally. Then add 5cm to both measurements when ordering. This gives us enough material to fill out the shape properly and avoids that underfilled, saggy look.

Example:
If your cover measures 60cm wide x 50cm high, order a 65cm x 55cm fibre cushion.


📐 What About the Depth and Shape?

We calculate the crown height for you

Back cushions don’t usually have a thick border like seat cushions. Most are single-chambered with a flat stitched edge. However, fibre still needs a raised centre – what we call the crown – to give it shape and support.

You don’t need to calculate this. Once we know your cover measurements, we work out the correct crown height based on the type of cushion, its purpose, and the fill required. This means your cushion will sit full and upright, not flat or baggy.


🧼 Daily Plumping Required

It’s part of the deal

Fibre back cushions will go flat without daily attention. They need to be plumped up regularly to hold their shape and keep their crown. If you want something lower maintenance, we’d recommend looking at foam or feather alternatives.


📦 Send Us Your Covers

We’ll fill them for a perfect fit

For the most accurate result, send us your empty covers after placing the order. We’ll build your new back cushions to suit the exact shape, making sure the crown height and fill level are spot on.

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Caravan, Campervan & Boat

Foam4U Unveils Tailored Comfort: Your Ultimate Guide to Customized Mattresses for Adventures”

Welcome to the Foam4U blog, where we embark on a journey into the world of bespoke comfort for caravans, campervans, boats, and motorhomes. With over a century of expertise, Foam4U takes pride in crafting premium mattresses, cushions, and upholstery designed to elevate your travel experience.

Tailored Comfort on the Road and Water: Foam4U’s Product Range
Bespoke Caravan Mattresses:

Immerse yourself in unparalleled comfort with mattresses tailored to fit your caravan seamlessly.
Choose from a variety of materials and sizes, ensuring a restful night’s sleep on every adventure.
Campervan Comfort:

Our custom campervan mattresses

guarantee a snug fit, providing the perfect retreat after a day of exploration.
Explore standard and made-to-measure options for personalized comfort.
Sailing in Luxury: Boat Mattresses and V Berths:

Experience the luxury of specially crafted boat mattresses designed for unique sleeping spaces.
V berth mattresses offer added comfort for your maritime journeys.
Motorhome Bliss:

Navigate the limited space of motorhomes with our specially designed mattresses.
Comfortable and durable options to transform your motorhome into a true home on wheels.
Pop-Top Roof Tent Adventures:

Optimize your pop-top roof tent experience with standard or made-to-measure mattresses.
Ensure a rejuvenating night’s sleep wherever your travels take you.
Enhance Your Bed: Memory Foam Mattress Toppers:

Elevate your existing mattress with our premium memory foam mattress toppers.
Choose from standard or custom-made options for an added layer of comfort.
Personalize Your Space: Custom Cushions and Covers:

Infuse your space with personality using our custom-made cushions and covers.
Select from a variety of fabrics and designs to suit your style.

While Foam4U won’t be at the upcoming events, we encourage you to draw inspiration from gatherings like the Caravan, Camping & Motorhome Show at the NEC (February 13th to 18th) and Boat Life at the NEC (February 13th to 18th). These events are a great opportunity to gather ideas for your ideal travel setup.

Consider the possibilities showcased at these events and envision the ultimate comfort for your journeys. Then, turn to Foam4U to bring your ideas to life at a far more affordable cost. Our commitment to crafting customized mattresses, cushions, and upholstery ensures that your travel adventures remain extraordinary, with your comfort as our top priority.

Stay connected for valuable insights, tips, and exciting updates on achieving bespoke comfort for your on-road and on-water escapades. Your adventure truly begins with Foam4U – where tailored comfort meets affordability.

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Rg50 Firm

Without a double the best foam for a Pop-Top. There a 3 elements to support Firmness, Density & Volume. Given most Pop-Top are less than 5cm thick one of the three elements (volume) is greatly reduced hence why a firm, high density foam is a good choice. Don’t be put off with the notion of firmness. It support that matters. Our customers have told us that without a doubt this is the best foam they have ever used in a Pop Top.

Highly Recommended