Foam & cushion spec

How to choose the right foam density for a cushion

The single mistake I see more than any other is treating density and firmness as the same thing. They aren't — and getting them the wrong way round is the difference between a cushion that lasts ten years and one that's flat by next winter.

Density is the weight of the foam — how many kilograms a cubic metre of it weighs — and it's the best single clue to how long a cushion will last. A dense foam has more material packed into it, so it takes far longer to break down and go soft. Firmness is simply how hard it feels to sit on, measured in Newtons, and it's a separate property you choose for comfort. A cheap foam can be made to feel firm and still collapse within a year; a good dense foam can feel soft and still be holding its shape a decade later. UK foam carries both numbers together — a code like RX38/150 means a density of 38 kg/m³ and a hardness of about 150 Newtons — so once you can read the code you buy on quality, not on the showroom squeeze.

So the rule is simple: pick the density for how long you need it to last, then pick the firmness for how you want it to feel — and never let a soft, cheap, low-density foam into a seat just because it felt nice in the shop.

Matching foam to the job — a few rules from the bench

A seat works harder than a back, so it needs more density. A sofa or armchair seat is the hardest-working cushion in the house; I won't drop below about 40 kg/m³ on one, in an HR (high-resilience) or Reflex foam, at a medium-firm feel. A back is different — you lean on it, you don't sit on it — so it can go softer and lighter, around 24 kg/m³, or be filled with hollowfibre for a loose, squashy look.

Thin pads must be dense, not thick. A dining or drop-in seat (US: slip seat) is shallow and sits close to a hard frame, so a soft foam just bottoms out and you feel the wood underneath. Here a firm, dense pad of around 33 kg/m³ at 30–40 mm beats a soft thick one every time — when there's a board under the cushion, density matters more than depth.

Anything commercial goes up a grade. A pub, café or waiting-room seat is sat on hundreds of times a day, and contract-grade foam of 45 kg/m³ and up is the difference between a year and ten. Footstools and window seats sit in between — treat a footstool like a small seat, firm and dense, because it doubles as somewhere to perch.

Wrap the foam; don't leave it bare. A layer of polyester (Dacron) wadding (US: batting) over the foam softens the hard cut edge, stops the cover creasing straight onto bare foam, and gives the cushion a slight crown so it reads as full rather than boxy. And cut the foam a touch oversize for its cover, so it fills the corners and sits plump instead of hollow.

What about fire safety?

This one isn't optional in the UK. For domestic upholstery the foam must be CMHR — Combustion Modified High Resilience — to meet the Furniture and Furnishings (Fire) (Safety) Regulations. Most reputable foam sold for upholstery already is, but read the label rather than assume. Public and contract seating usually has to meet the stricter Crib 5 standard (BS 5852), and mattresses fall under their own rules (BS 7177). On any commercial job, confirm the venue's requirement before you spec the foam — it's the one corner you can never cut. You can check what applies with the fire regulations checker.

A worked example

Say you're rebuilding the seat cushions on a family sofa. The calculator points you at an HR/CMHR foam around 40 kg/m³ for the density — so it survives a decade of daily use — a medium-firm feel of roughly 150 Newtons, and about 120 mm finished thickness. You'd cut that foam a shade oversize for the covers, wrap each block in Dacron to soften the edges and crown the top, and you've a cushion that feels right on day one and still feels right years later. Swap to a firm 45 kg/m³ contract foam and it would last even longer but feel harder; drop to a soft 28 kg/m³ and it would feel lovely for a season, then sag. Same cushion, same size — the density decides the lifespan, the firmness decides the feel.