The Pattern Room · a pattern with a calculator
Foam density conversion: lb/ft³ to kg/m³ for cushion foam
One foam, two unit systems, and a lot of confusion in between. Here's the constant, a chart from budget grades to memory foam, and a checklist for reading any maker's spec without taking a nice word for a number.
SaveThe quick answer
To convert foam density, divide kg/m³ by 16.02 to get lb/ft³, or multiply lb/ft³ by 16.02 for kg/m³. The workshop's standard seat foam at 32 kg/m³ is about 2.0 lb/ft³, and its softer 28 kg/m³ grade is about 1.75 lb/ft³. Density predicts how long foam holds its height; firmness is a separate number, the ILD.
Key facts
- 1 lb/ft³ = 16.02 kg/m³; divide kg/m³ by 16.02 to get lb/ft³.
- 32 kg/m³ is about 2.0 lb/ft³; 28 kg/m³ is about 1.75 lb/ft³.
- Density predicts how long foam holds its height; ILD measures how firm it feels.
- US ILD is measured at 25% indentation on a 4 in sample with a 50 sq in round foot.
- 1 lbf = 4.45 N, so ILD and newton figures only compare at the same indentation.
One constant does all the work
A pound per cubic foot equals 16.02 kilograms per cubic meter, and that single constant is the whole foam density conversion. Divide kg/m³ by 16.02 to get lb/ft³; multiply lb/ft³ by 16.02 to go back.
Worked through, calculator out: 32 ÷ 16.02 = 2.00, so the workshop's standard seat foam at 32 kg/m³ is about 2.0 lb/ft³. Its softer grade works out to 28 ÷ 16.02 = 1.75 lb/ft³. Going the other way, a US spec of 1.8 lb/ft³ × 16.02 = 28.8 kg/m³, and 2.5 lb/ft³ comes to 40 kg/m³.
Spec sheets also write density as shorthand. T28 or T32 means 28 or 32 kg/m³, nothing more; the letter says nothing about firmness. The glossary covers foam density and high-density foam if the terms are new.
Foam density chart, budget grade to memory foam
| kg/m³ | lb/ft³ | Grade | Where it shows up |
|---|---|---|---|
| 22 | 1.37 | Budget | A cost-cutting substitute seen on one workshop quote |
| 24 | 1.5 | Budget | Low-cost seat pads; published guides give it 1 to 3 years before dents |
| 28 | 1.75 | Softer seat | The workshop's softer option and its floor-sofa mattress foam (T28) |
| 29 | 1.8 | Conventional | US conventional seat foam and dry-fast outdoor foam; a common minimum for seating |
| 30 | 1.87 | Seat | Written as T30 on some spec sheets |
| 32 | 2.0 | Seat-grade high density | The workshop's standard seat, bench and window-seat foam (T32) |
| 35 | 2.18 | Firmer seat | A figure some buyers ask for as commercial grade |
| 40 | 2.5 | High density | Where a lot of US copy starts using the words high density |
| 42 to 45 | 2.6 to 2.8 | High resilience | Premium sofa-seat grades |
| 48 to 80 | 3 to 5 | Memory foam | Toppers and memory seats; 3.5 lb/ft³ is a published minimum for a daily seat |
| about 95 | about 5.9 | Latex | Organic latex cores |
| 96 to 144 | 6 to 9 | Rebonded chip foam | Pews and heavy commercial seating |
lb/ft³ = kg/m³ ÷ 16.02, rounded. Grades and uses are as published by foam sellers and guides, and they vary by maker.
Density is durability; firmness is a different dial
Density tells you how much foam is packed into each cubic foot. Published guides tie it to how many years a seat holds its height, which is why a 2.0 lb/ft³ seat outlasts a 1.5 lb/ft³ one. It does not tell you how the cushion feels. The same 32 kg/m³ can be poured soft or hard.
Feel is measured as ILD, also written IFD: the pounds of force it takes to push a round 50 sq in (323 cm²) foot 25% of the way into a 4 in (10 cm) sample. Rough US bands run 25 to 35 medium, 35 to 45 medium-firm and 45 to 55 firm. The two numbers interact, too: one guide puts medium-firm near ILD 36 on 1.8 lb/ft³ foam but near ILD 27 on 2.5 lb/ft³ foam.
UK sellers quote hardness in newtons instead. One pound-force is 4.45 N, so ILD 42 is roughly 187 N, but only if both were tested at the same indentation, and many UK figures use 40%, not 25%. High-resilience foam is about bounce and support factor rather than a density line, and sources disagree on its density. The feel side gets its own pattern in foam density vs firmness.
Reading any maker's foam spec in eight moves
Find the density and its unit
Look for kg/m³, lb/ft³ (also written pcf) or shorthand like T32. A bare 'high density' with no number is an adjective, not a spec.
Convert everything to one unit
Divide kg/m³ by 16.02, or multiply lb/ft³ by 16.02, and write both figures side by side. Comparing 32 with 2.5 unconverted is how people talk themselves into the wrong foam.
Find the firmness number
US makers quote ILD or IFD in pounds; UK sellers quote newtons. If neither is listed, ask. The workshop's own seat foam has no official ILD certificate on file, and it says so plainly; it places the foam in the firm category.
Check the test point
An ILD at 25% and a hardness at 40% aren't the same measurement. Compare numbers only at the same indentation, and allow for a little batch-to-batch drift. Tolerance is kindness to your future self.
Look for the support factor
Some specs give IFD at 65% as well as at 25%. Divide the first by the second: typical foams land around 1.5 to 2.6, and higher values resist bottoming out. Sellers who call a foam high resilience usually quote 2.0 or 2.4 and up.
Ask what wraps the core
A polyester fiber wrap of 1/2 to 1 in is standard on seat foam. It softens the surface and rounds the edges, and it settles before the core does.
Match firmness to thickness
Thin cushions need firmer foam. One published guide suggests about ILD 50 under 4 in, about 44 at 4 to 6 in, about 30 over 6 in, and about 24 for back cushions.
Get the numbers, and the fill, in writing
Ask for density and firmness in the quote itself, and say 'with foam' or 'covers only' in words. The order is what gets built.
The buyer who asked for 1.8 lb and ILD 44
A buyer in the US wrote in with a spec of their own: foam of at least 1.8 lb density and ILD 44, because adults would sit deep on a long indoor seat every day and nobody wanted to feel the boards.
Fair ask, partner, and a good one. The workshop answered with a conversion and an honest gap. Its 32 kg/m³ foam works out to about 2.0 lb/ft³, comfortably past 1.8, and it's firm. But there's no official ILD rating on file, so nobody promised 44.
The cushion was quoted at 25 x 78 x 6 in, a deep 6 in (15 cm) seat, firm enough for adults sitting every day at that thickness, and the buyer picked a fabric with matching trim. One converted number answered the question. An invented one would only have postponed it.
Let the Studio narrow it down
If you'd rather answer questions than read tables, the foam firmness finder asks five: where the cushion lives, who sits on it, how you sit, how thick it can be and how often it's used. It returns a foam type, density, firmness and thickness, with the reasons written out.
Then hold its answer up against the spec sheet you've been sent. What does yours actually say, a number or an adjective? For inserts and everyday seats, see foam inserts and bench cushions; for how long each grade lasts, how long cushion foam lasts; and for other cores, high-density foam vs memory foam and latex.
Common mistakes when comparing foam numbers
- Setting 32 beside 2.5 and assuming the 32 is denser. Convert first: 2.5 lb/ft³ is 40 kg/m³, the denser of the two.
- Reading density as firmness. The same 32 kg/m³ can be soft or hard; firmness needs its own number.
- Comparing a US ILD taken at 25% with a UK newton figure taken at 40%.
- Treating high density as a fixed line. Some US copy draws it at 1.8 lb/ft³, some at 2.5.
- Judging firmness the day a vacuum-packed insert arrives. Foam needs 48 to 72 hours to recover, sometimes up to a week, and a cushion that looks flat or short on day one often just needs that time.
- Leaving 'with foam' or 'covers only' unsaid. One banquette reorder became a covers-only quote and needed a foam top-up to put right.
Foam density says how long a cushion holds its height; ILD says how firm it feels. A foam spec with only one of the two numbers is half a spec.
Nate Kowalski
Weigh your own foam
Measure the piece
Length, width and thickness in inches, on bare foam without a cover or wrap.
Find the volume
Multiply the three and divide by 1,728 to get cubic feet.
Weigh it
A kitchen scale for small pieces, a bathroom scale for large ones.
Divide
Pounds divided by cubic feet is the density in lb/ft³; multiply by 16 for kg/m³.
Read the result
Foam sellers usually treat 1.8 lb/ft³ and up as high density; the workshop's seat foam is about 2.0.
Have it made
Measured it? Have Nate’s workshop make it
Send the numbers in inches: length × depth × thickness, plus any angle. The workshop checks them with you, because foam has no undo button. — Nate
Leave your details and the workshop will call you to confirm measurements, fabric, foam and piping. No prices, dates or payment on this page: it is a request, not an order.
Buyer questions: the numbers people asked for
- A common question
What's 32 kg/m³ foam in pounds per cubic foot, and how firm is it?
The workshop's answerAbout 2.0 lb/ft³, which clears the usual 1.8 lb high-density requirement and holds adults without sinking through. It's high-resilience cold foam in the firm category. The workshop has no official lab ILD certificate to quote, and ILD can vary slightly between batches. Floor and modular sofa mattresses use a separate 28 kg/m³ foam, and firm seat cushions can be made 6 in thick.
- A common question
Which density goes into the floor sofa seats?
The workshop's answerFloor and modular sofa seat mattresses use 28 kg/m³ medium-hard high-density foam, sometimes quoted as 28 to 32, and one reply for a corduroy sofa said 32. The backrests are foam-filled too, where cheaper look-alikes often use low-density foam and solid-foam backs. Replacement foam is sold on its own.
- A common question
Is there foam inside the tufted floor cushions?
The workshop's answerNo foam core. Tufted and French-style floor cushions are filled with silicone balls: polyester hollow fiber rolled into small clusters that feel soft like cotton rather than springy like foam. The fill is packed densely, often with some cotton mixed in, and the cushions are sewn by hand. The workshop can send a photo of the filling.
- A common question
Which thickness do most people order, and what would you suggest?
The workshop's answer3 or 4 in covers most orders. 4 in (10 cm) is the standard for everyday seating and a tailored built-in window seat, while 3 in (7.5 cm) looks trimmer and fits tight spots below a sill. Because the foam is firm 32 kg/m³, a 2 to 2.5 in dining-nook cushion still supports well. Outdoor benches usually take 3 to 4 in.
- A common question
Can the foam be cut thinner or thicker than 4 inches?
The workshop's answerYes, thickness is fully custom. Past orders have run as thin as 1 in and 1.5 in and as thick as 8 and 9 in, with floor cushions at 6, 8 and 10 in (15, 20 and 25 cm). Pick the nearest option and write the exact figure in the personalization box. Thinner usually costs the same as the 4 in (10 cm) standard; thicker foam raises the quote.
Density questions, converted
How do I convert foam density in my head?
Use 16. Multiply lb/ft³ by 16 for kg/m³, or divide kg/m³ by 16 for lb/ft³; the exact constant, 16.02, barely moves the answer at cushion densities. So 2 lb/ft³ is about 32 kg/m³, and 40 kg/m³ is about 2.5 lb/ft³.
What density counts as high-density foam?
There's no single line. Much US seating copy treats 1.8 lb/ft³ (29 kg/m³) as the floor for seat foam and saves the words high density for 2.5 lb/ft³ (40 kg/m³) and up, while other makers call 2.0 lb/ft³ high density. Ask for the number instead of the label.
Is memory foam denser than regular seat foam?
Usually. Memory foam runs about 3 to 5 lb/ft³ (48 to 80 kg/m³), and one published guide sets 3.5 lb/ft³ as the minimum for a daily seat, at least 3 in thick. It's also soft, around ILD 12 to 16, and it stiffens noticeably in a cool room.
What do T28 and T32 mean on a foam label?
Trade shorthand for density in kg/m³. T28 is 28 kg/m³, about 1.75 lb/ft³; T32 is 32 kg/m³, about 2.0 lb/ft³. The code says nothing about firmness, so look for an ILD or newton figure beside it.
How heavy is a foam cushion?
Weight is volume times density. Multiply length by width by thickness in inches, divide by 1,728 to get cubic feet, then multiply by the density in lb/ft³. A 48 x 18 x 4 in cushion is 2 cubic feet, so at about 2.0 lb/ft³ the foam weighs about 4 lb, before the cover and wrap are added.
