---
title: "Foam Density vs Firmness: What ILD Really Measures"
url: "https://cushionguide.com/guides/foam-density-vs-firmness-ild/"
section: "The Pattern Room"
author: "Nate Kowalski, The Pattern Engineer of Detroit"
author_url: "https://cushionguide.com/masters/nate/"
published: "2026-03-23"
updated: "2026-09-24"
keywords: "foam density vs firmness, what is ild in foam, ifd foam firmness, firm foam for bench cushion, high resilience foam"
---

# Foam density vs firmness: why a dense foam can still feel soft

*Density tells you how long foam will last; ILD tells you how it feels under you, and mixing them up is how people order a brick or a marshmallow. Here are both numbers, the bands that matter, and the workshop's own foams as test pieces.*

> **The quick answer:** Foam density and firmness are two different numbers: density is weight per cubic foot and signals durability, while ILD (also called IFD) is the pounds of force needed to press a 4 in sample down 25% and signals feel. A dense foam can be medium or firm. Daily sofas and dining chairs sit around ILD 35–45; back cushions around 15–25.

![Soft lavender velvet bench cushion with eggplant piping on a pale wood bench beside a vanity with a round mirror](https://cushionguide.com/media/bench-cushions/soft-lavender-velvet-vanity-bench-eggplant-piping-1600.webp "Soft lavender velvet with eggplant piping on a pale wood bench gives a dressing corner a calm, tonal purple scheme.")

## Two numbers, two jobs

32 kg/m³ is 2.0 lb/ft³, and neither figure tells you how a cushion feels when you sit on it. That's foam density vs firmness in one sentence.

**Density** is weight per volume: pounds per cubic foot in the US, kilograms per cubic meter elsewhere, sometimes written T28 or 28 DNS. Divide kg/m³ by 16.02 to get lb/ft³. It's a durability signal: published ranges put 1.5 lb foam at 1–3 years before permanent impressions, 2.0 lb at 5–7, and 2.5 lb high-resilience foam at 10 or more.

**ILD**, also written IFD, is a feel signal: the pounds of force needed to press a 4 in (10 cm) thick sample down 25% with a round foot of 50 sq in (323 cm²).

The two travel together loosely, never exactly; the same density can be made soft or hard. The conversion table lives in [foam density in lb/ft³ and kg/m³](https://cushionguide.com/guides/foam-density-lb-ft3-kg-m3/), and the glossary defines [ILD](https://cushionguide.com/glossary/#ild), [foam density](https://cushionguide.com/glossary/#foam-density) and [cold foam](https://cushionguide.com/glossary/#cold-foam).

## ILD bands, from pillow-soft to heavy-duty

| ILD (lb) | Feel | Where it usually goes |
|---|---|---|
| 8–15 | Very soft | Pillow tops, soft backs |
| 15–25 | Soft | Back cushions |
| 25–35 | Medium | Most sofa seats |
| 35–45 | Medium-firm | Daily sofas, dining chairs |
| 45–55 | Firm | Heavy use, sitters over 200 lb (91 kg) |
| 55+ | Very firm | Beyond most home seating |

## The workshop's own foams, measured honestly

| Foam | Density | ILD on file | How it's described |
|---|---|---|---|
| Floor-sofa seat mattress | about 1.75 lb/ft³ (28 kg/m³), quoted as T28 | 42 | Medium-hard HR cold foam; softens slightly with use |
| Bench, window-seat and seat cushions | about 2.0 lb/ft³ (32 kg/m³) | None on file | Firm high-density foam, firmer than the 28 grade |
| Designer original floor sofa, as cited by buyers | about 1.87 lb/ft³ (30 kg/m³), T30 | 45 | Firmer on paper; almost no difference in feel, the workshop says |
| Back pillows and soft backrests | Shredded foam | Not rated | Softer than solid foam, but not very soft |

## Spec a seat foam in eight steps

1. **Name the job: seat, back or bed**: Seats want support, backs want give, and a floor-sofa mattress must handle sitting and sleeping. By default the workshop makes backrests softer than seats, though a back can be made equally firm.
2. **Choose density for lifespan**: For daily seating, 2.0 lb/ft³ (32 kg/m³) is the workshop's standard and a sound floor. Published ranges give 1.5 lb foam only 1–3 years before it holds impressions.
3. **Choose ILD for feel**: Daily sofas and dining chairs sit around ILD 35–45; back cushions around 15–25. Pick the band from the table, not from the density.
4. **Adjust for thickness**: Thin seats need firmer foam because there's less depth before you meet the board. One foam supplier's rule of thumb: under 4 in (10 cm), about ILD 50; 4–6 in (10–15 cm), about 44; backs, about 24.
5. **Adjust for the sitter**: Sitters over 200 lb (91 kg) are usually steered to ILD 40–55. If two very different people share a bench, spec for the heavier one.
6. **Compare like with like**: Some sellers quote hardness in newtons, often at 40% compression instead of 25%. 1 lbf is 4.45 N, so ILD 42 is about 187 N only at the same indentation. For deep support, the support factor (IFD at 65% divided by IFD at 25%) typically runs 1.5–2.6; higher resists bottoming out.
7. **Ask what's on file, then press a sample**: If a maker has no ILD for a foam, the honest answer is density plus a description, not an invented number. Ask for a small piece of foam to travel with your fabric samples.
8. **Give new foam a week**: Vacuum-packed foam needs 48–72 hours to recover and 3–4 days to reach full size. Judge firmness after that, not on unboxing day.

## A 78-inch seat and a request for ILD 44

A buyer in the US wanted a long indoor seat, 25 x 78 x 6 in (64 x 198 x 15 cm), that adults would sit on every day. The spec: foam of at least 1.8 lb density and ILD 44, so nobody would sink through.

The workshop answered straight. Its 32 kg/m³ foam is about 2.0 lb/ft³, so the density ask was met with room to spare, and the foam is firm. But there's no official ILD rating on file, so the reply gave a description, not a number. The buyer went ahead with the 6 in seat.

I'll take that answer over a guessed 44 any day, partner. A guessed number is a tolerance nobody can hold.

## Common foam mix-ups

- ✗ Treating density as firmness. A 2.0 lb/ft³ foam can be medium or firm, so ask for both numbers.
- ✗ Quoting one ILD for every thickness. A 2 in pad and a 6 in seat need different numbers.
- ✗ Comparing newtons with ILD pounds without checking the test point: 25% and 40% compression aren't the same test.
- ✗ Judging on unboxing day. One 78 x 39 x 6.5 in (198 x 99 x 17 cm) floor-sofa seat still sat flat in a corner after a week; that was a foam fault, and the workshop sent a replacement insert.
- ✗ Letting a compressed set wait for weeks. If a move delays delivery, ask the maker to hold the order and make it fresh.

> **Tailor's margin:** If a maker quotes foam density but no ILD, ask how firm it feels at your thickness; 2.0 lb/ft³ (32 kg/m³) foam can be medium or firm.

> Foam density tells you how long a cushion lasts; ILD tells you how it feels. ILD 42 means 42 lb of force to press a 4 in foam sample down 25%.
>
> — Nate

## Run your seat through the numbers

Before you pick a foam, run your seat through the [foam firmness finder](https://cushionguide.com/studio/foam-firmness-finder/); if height matters as much as feel, the [seat height calculator](https://cushionguide.com/studio/seat-height-calculator/) shows what 3 in against 4 in does to the sit.

4 in (10 cm) is the standard for everyday seating and 3 in (7.5 cm) sits trimmer. Firm foam is cut slightly larger than its cover so the cover looks full, whether it's [velvet](https://cushionguide.com/fabrics/velvet/) or linen-look. Measure twice; the fabric remembers.

What's your seat thickness, and who sits there most? The rest is arithmetic. Further reading: [the 1-to-10 firmness scale](https://cushionguide.com/guides/cushion-firmness-scale-1-to-10/), [high-density foam vs memory foam and latex](https://cushionguide.com/guides/high-density-foam-vs-memory-foam-latex/), and the [foam inserts](https://cushionguide.com/cushions/foam-inserts/) and [modular floor sofas](https://cushionguide.com/cushions/modular-floor-sofas/) hubs. [Hana](https://cushionguide.com/masters/hana/) covers how a floor sofa sleeps.

## Buyer questions: density, ILD and feel

**A common question:** Is T28 foam at ILD 42 softer than the T30, ILD 45 foam in the original designer floor sofa?

**The workshop's answer:** Slightly, on paper. The floor-sofa seats are high-resilience cold foam at T28 (28 kg/m³, about 1.75 lb/ft³) with ILD 42, which the workshop calls firm but comfortable. The original is cited at T30 / ILD 45, but the workshop says the difference in feel and wear is almost nothing.

**A common question:** Is foam density the same thing as firmness? Can I choose how firm my cushion is?

**The workshop's answer:** No, they're separate. Density is the foam's weight and quality, which drives durability; firmness is how it feels. A high-density foam can be medium or firm, and a small piece of foam can be sent with your fabric samples so you can feel it before ordering.

**A common question:** What density foam goes into your bench cushions?

**The workshop's answer:** 32 kg/m³, about 2.0 lb/ft³: high-density polyurethane with a medium-firm to firm feel. It's firmer and longer-lasting than the 28 kg/m³ foam often specified, and every custom cushion gets it as standard unless you ask for a softer one.

**A common question:** Will the floor sofa feel firm or soft?

**The workshop's answer:** In between. The seat and ottoman mattresses are medium-hard 28-density foam, comfortable for sitting and sleeping, built as an everyday living-room sofa. It keeps its shape for years without sagging and softens slightly with use.

**A common question:** Is the floor sofa cold foam or regular high-density foam?

**The workshop's answer:** Both labels fit. The base mattresses are HR cold foam at T28 with ILD 42, which also counts as high-density foam. When a buyer spotted 'cold foam' on one page and 'high density' on another, the workshop confirmed cold foam and corrected the listing.

## Foam numbers, briefly

### What ILD should a dining bench cushion be?

Published bands put daily sofas and dining chairs at about ILD 35–45. Thin pads under 4 in (10 cm) need a firmer number, around 50 by one foam supplier's rule, because there's less depth before you bottom out.

### How do I convert kg/m³ to lb/ft³?

Divide by 16.02. That makes 28 kg/m³ about 1.75 lb/ft³, 32 kg/m³ about 2.0 lb/ft³, and 40 kg/m³, a common 'high density' threshold in US product copy, about 2.5 lb/ft³.

### What makes foam high resilience?

Bounce and support rather than a single density. Retailers point to a high support factor, the ratio of force at 65% to 25% compression, usually quoted above 2.0 or 2.4. Sources disagree on HR density, so ask for the support factor when a maker uses the term.

### Does firm foam get softer over time?

A little. The workshop's floor-sofa foam is described as softening slightly with use. Published fatigue data shows low-density foam losing far more: one 1.0 lb, 30 IFD foam fell to about 16 IFD after roughly ten years.

## Related at the atelier

- [New foam cushion smell: how long it lasts and how to air it out](https://cushionguide.com/guides/new-foam-cushion-smell-airing/): New foam cushion smell is strongest in the first one to three days after unpacking and usually fades over one to four weeks with airflow.
- [Your cushion arrived vacuum-packed: what happens in the first 72 hours](https://cushionguide.com/guides/vacuum-packed-cushion-first-72-hours/): A vacuum-packed foam cushion rises in 48 to 72 hours and reaches its ordered size in 3 to 4 days, occasionally up to a week.
- [CertiPUR-US foam explained: what the seal certifies and how to check it](https://cushionguide.com/guides/certipur-us-foam-explained/): CertiPUR-US certifies flexible polyurethane foam only, not the cover fabric or the finished cushion.
- [Is the floor sofa cold foam or regular high-density foam?](https://cushionguide.com/answers/cold-foam-vs-high-density-foam-floor-sofa/): The floor sofa uses cold foam: its base mattresses are HR (high-resilience) cold foam at T28 density with an ILD of 42.
- [How do I get the foam back into my cushion cover?](https://cushionguide.com/answers/how-to-put-foam-back-in-cushion-cover/): To put foam back in a cushion cover, fold the foam, not the cover: open the zipper past both corners, slip the foam into a thin plastic bag, fold it lengthwise and fill the far corners first.
- [Can I get a softer or firmer floor sofa?](https://cushionguide.com/answers/softer-or-firmer-floor-sofa-cushions/): Partly, on request: firmer sofa cushions have been made for users over 220 lb (100 kg), and a puffier, softer filling has been offered for the seat mattresses and backrests.

*± 1/4 in — Nate*

---

Written by Nate Kowalski (https://cushionguide.com/masters/nate/) for CushionGuide (https://cushionguide.com/about/). The site explains made-to-measure cushions and the partner workshop makes them; measurements are in inches first. This Markdown mirrors the page at https://cushionguide.com/guides/foam-density-vs-firmness-ild/