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.

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).

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

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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.