Foam Grade Comparison
Foam grade comparison
Every foam we make, compared on the two numbers that decide your choice. All firmness figures are measured the same way — ILD at 40% compression, in Newtons — so they can be compared directly against one another.
The two numbers, in one sentence each
- Density (kg/m³) — how much material is in the foam. It predicts how long it lasts.
- Firmness (Newtons) — how much force it takes to compress. It predicts how it feels.
They are independent. A high density foam is not necessarily a firm one. Look down the table below: the 39S is denser than the 33H, and far softer. This catches people out constantly.
Upholstery foams — all Crib 5 fire rated
| Grade | Density | Firmness | Feel | Colour | Best for |
|---|---|---|---|---|---|
| Memory Foam 55M | 50 kg/m³ | 55N | Very soft | White | Mattress toppers, comfort layers, pressure relief |
| 39S | 39 kg/m³ | 120N | Soft | Orange | Sofa back cushions, headboards. Not seats. |
| 35M | 35 kg/m³ | 125N | Medium | Beige white | Standard sofa and chair seat cushions |
| 41M | 41 kg/m³ | 150N | Medium | Grey | Sofa and chair seats — longer-lasting than the 35M |
| 33H | 33 kg/m³ | 190N | Hard | Blue | Firm seating, caravans, boats, mattress support |
| 39H | 39 kg/m³ | 200N | Hard | Blue | Firm seating — longer-lasting than the 33H |
| 42H | 42 kg/m³ | 240N | Hard | Blue | Our firmest and longest-lasting. Dining chairs, heavy use |
Specialist foams
| Product | Density | Fire rating | What makes it different |
|---|---|---|---|
| Acoustic Foam 25A | 25 kg/m³ | Crib 5 + UL94-HF-1 | Absorbs sound. 140N. For studios and noisy rooms |
| Fireseal Class 0 | >90 kg/m³ | Class 0 · B-s1,d0 · UL94 V-0 | Building regs fire rating. Ductwork, plant rooms, marine |
| Packaging Foam HLB | 21 kg/m³ | Not fire rated | Protection and transit only. Never for upholstery |
| Plastazote LD33 | See product page | See product page | Closed cell. Waterproof, buoyant, heat mouldable |
| Reticulated 40100 | See product page | See product page | Open mesh. Water drains straight through. For outdoors |
Start from what you are making
| What you are doing | Grade |
|---|---|
| Replacing sagging sofa seat cushions | 35M, or 41M to last longer |
| Sofa back cushions | 39S |
| Dining chair pads | 42H (available down to 1/4") |
| A headboard | 39S, 1"–2" thick |
| Caravan or campervan seating | 33H, 39H or 42H depending on how much use it gets |
| A boat cushion for a dry cabin | 33H, 39H or 42H |
| A boat or garden cushion that will get wet | Reticulated 40100 — not standard polyurethane |
| A mattress | Firm support foam, with memory foam on top |
| Treating a room for echo | Acoustic Foam 25A |
| Anything under building fire regulations | Fireseal Class 0 |
| Protecting kit in transit | Packaging Foam HLB, or Plastazote for valuable items |
Two mistakes worth avoiding
Putting soft foam in a seat
The most common one. A soft foam behind your back is comfortable; a soft foam under your weight lets you sink through to the frame — which is the exact problem you were trying to fix. For seat cushions, choose 125N or above.
Putting standard foam outdoors
Standard polyurethane foam holds water like a sponge. On a garden or cockpit cushion it soaks up rain, stays damp and eventually degrades. No amount of density fixes this — it is the wrong material. Use reticulated foam, which lets water drain straight through.
Still unsure? Call us on 01494 441177. We have been supplying foam for over 40 years and we would rather talk it through than have you order the wrong thing. Or read our foam FAQs.