Foam Hardness Explained: ILD, IFD & Newtons Compared
Foam hardness is the force needed to press into a foam by a set amount: US suppliers quote it as ILD or IFD in pounds, while UK suppliers quote it in newtons (N) tested to BS EN ISO 2439. The two tests use different sample thicknesses, indentation depths and hold times, so there is no exact conversion between an ILD rating and a newton figure. GB Foam Direct lists every upholstery grade in newtons, and this guide shows how to compare them with the ILD numbers in US guides.
Key takeaways
- ILD (Indentation Load Deflection), also called IFD (Indentation Force Deflection), is the force in pounds needed to indent a 4 inch (about 10cm) foam sample by 25%.
- UK hardness is tested to BS EN ISO 2439 with a 200mm indenter on a 380mm square sample 50mm thick, and is reported in newtons.
- One pound-force equals about 4.45 newtons, but that only converts the unit, not the test, so an ILD of 30 is not the same foam as 133N.
- Density (kg/m3, BS EN ISO 845) and hardness are separate: GB 39S and GB 39H share 37 to 41 kg/m3 but measure 100 to 140N and 180 to 220N.
- GB 42H Superior Hard is our firmest upholstery grade at 220 to 300N, and GB 39S Luxury Soft our soft back cushion grade at 100 to 140N.
- GB Foam Direct has cut foam in High Wycombe since 1976 and works to ISO 9001.
What Is Foam ILD?
Foam ILD is a US measure of firmness: the force in pounds-force (lbf) needed to compress a foam sample by 25% of its thickness.
Our foam terminology guide defines ILD as the force needed to make a 25 percent indentation on a 4 inch thick foam sample, and notes it is also called IFD. The US test method is ASTM D3574. According to the Polyurethane Foam Association, the force is measured with a 50 square inch platen (about 323 square centimetres) after one minute, and IFD is often reported at both 25% and 65% indentation. A higher ILD means a firmer foam.
How Is Foam Hardness Measured in the UK?
In the UK, foam hardness is measured to BS EN ISO 2439 and given in newtons (N), and a higher figure means a firmer foam.
According to testing equipment maker ZwickRoell, ISO 2439 uses a flat circular indenter 200 to 203mm across, pressed into a square sample 380mm wide and 50mm thick (plus or minus 2mm). The standard has several methods. Method A reads the force at 40% indentation after a 30 second hold, and method B reads it at 25%, 40% and 65%. GB Foam Direct quotes every upholstery grade in newtons tested to BS EN ISO 2439, alongside density tested to BS EN ISO 845.
ILD vs Newtons: How Do the Tests Compare?
The ILD test and the ISO 2439 test measure the same idea in different ways, so their numbers cannot be swapped directly.
| Feature | ILD or IFD (US) | ISO 2439 (UK and Europe) |
|---|---|---|
| Unit | Pounds-force (lbf) | Newtons (N) |
| Indenter | 50 sq in (about 323 sq cm) | 200 to 203mm diameter (about 314 sq cm) |
| Sample thickness | 4 inches (about 10cm) | 50mm, plus or minus 2mm |
| Main reading | 25% indentation | 40% indentation (method A) |
| Hold before reading | One minute | 30 seconds (method A) |
| Firmness ratio | 65% reading divided by 25% reading | 65% force divided by 25% force (method E) |
Because the US test presses only 25% into a thicker sample, and the main ISO reading presses 40% into a thinner one, the same foam gives different numbers on each test even after converting pounds to newtons.
Can You Convert ILD to Newtons?
You can convert the unit, but not the rating: 1 pound-force equals about 4.45 newtons, so an ILD of 30 lbf is about 133N, yet that is not the ISO 2439 hardness of the same foam.
The conversion ignores the different sample thickness, indentation depth, indenter size and hold time. Foam is also not linear, so the force at 40% is not a fixed multiple of the force at 25%. Use any converted figure only as a rough starting point, and compare foams within one system wherever possible. The table below shows GB grades in newtons with the same numbers expressed in pounds-force, purely as a unit conversion.
| GB Grade | Hardness (N) | Same Figure in lbf | Feel |
|---|---|---|---|
| GB 39S Luxury Soft | 100 to 140N | 22 to 31 lbf | Soft, back cushions |
| GB 33M Average Medium | 115 to 155N | 26 to 35 lbf | Medium, deep seats |
| GB 41M Luxury Medium | 130 to 175N | 29 to 39 lbf | Medium, seats over 4 inches |
| GB 39H Luxury Hard | 180 to 220N | 40 to 49 lbf | Firm, seats up to 4 inches |
| GB 42H Superior Hard | 220 to 300N | 49 to 67 lbf | Hardest, heavy use seating |
The lbf column is the newton range divided by 4.448. It is not an ILD rating, because the foam was not tested to ASTM D3574.
Which Hardness Figure Should You Use?
Use newtons when buying foam in the UK, and use ILD only to compare US products with each other.
| Situation | Use | Why |
|---|---|---|
| Buying foam from a UK supplier | Newtons (ISO 2439) | UK grades are tested and listed this way |
| Following a US guide or video | Feel description, not the ILD number | Soft, medium or firm transfers better than the figure |
| Comparing two UK foams | Newtons and density together | Hardness sets feel, density sets lifespan |
| Replacing foam in a US made product | Ask the supplier for a sample or a matching feel | No exact conversion exists |
| Choosing seat cushion firmness | Cushion depth and user weight | Thin seats need harder grades |
Our foam density chart maps GB grades to body weight and cushion depth, and our guide to sofa cushion foam covers seats and backs.
How to Match a US ILD Rating to UK Foam
Matching a US ILD rating to UK foam is a five minute job if you work from the feel and the use rather than the number alone.
Note the ILD and the use. Write down the ILD figure, the cushion thickness and what the foam is for, such as a seat, back or mattress layer.
Place it in a feel band. Use the US guide’s own description of soft, medium or firm for that ILD rather than converting the number.
Pick the GB grade by use. Choose GB 39S for backs, GB 39H for seats up to 4 inches, GB 41M for deeper seats or GB 42H for heavy use.
Check the density. Choose a Luxury grade (37 to 43 kg/m3, fatigue class V) for daily use, as density decides how long the foam lasts.
Ask for advice if unsure. Call or email our team with the ILD, thickness and use, and we will suggest the closest grade.
Mistakes to Avoid
- Multiplying ILD by 4.45 and stopping there. That converts the unit, not the test, so the result is not an ISO 2439 hardness.
- Confusing density with hardness. A high density foam can feel soft, so always check the newton figure as well as kg/m3.
- Ignoring thickness. The same foam feels softer in a thick cushion than a thin one, so thin seats need a harder grade.
- Treating ILD and Shore hardness as the same. Shore hardness is a different scale used for firmer materials and does not convert to ILD or newtons.
- Choosing by number alone. Two foams with similar hardness can still feel different because of cell structure and recovery.
Why Choose GB Foam Direct?
GB Foam Direct has converted foam in High Wycombe since 1976, originally for the furniture trade, and lists every upholstery grade with its density and hardness so you can compare like with like. Our upholstery foam sheets run from 1/2 inch (12mm) to 4 inches (100mm), and our high density foam guide explains each grade. We work to ISO 9001 and give a 5 year guarantee against faults in manufacture.
Speak to Our Team
If you need help choosing a foam grade from a US ILD rating, call us on 01494 441177, email sales@gbfoam.com or fill out our contact form. We are in the office from 9am to 5pm Monday to Thursday and 9am to 3pm on Friday.
Foam Hardness FAQs
What does ILD mean for foam?
ILD stands for Indentation Load Deflection, the force in pounds needed to compress a foam sample by 25% of its thickness. It is also called IFD, Indentation Force Deflection, and a higher ILD means firmer foam.
Is ILD the same as IFD?
Yes, ILD and IFD describe the same firmness measurement. IFD, Indentation Force Deflection, is the term used in the US test method ASTM D3574, while ILD is the older name still used by many mattress and foam sellers.
How do you convert ILD to newtons?
You can convert the unit by multiplying pounds-force by about 4.45, so 30 lbf is about 133N. That figure is not an ISO 2439 hardness, because the US and UK tests use different sample thicknesses, indentation depths and hold times.
What is foam hardness in newtons?
Foam hardness in newtons is the force needed to press a 200mm indenter into the foam by a set percentage, tested to BS EN ISO 2439. A higher newton figure means firmer foam, and UK foam suppliers list their grades this way.
What hardness of foam is best for seat cushions?
Firm foam suits seat cushions up to 4 inches (10cm) thick, and GB recommends GB 39H Luxury Hard at 180 to 220N for these. Deeper seats suit GB 41M Luxury Medium at 130 to 175N, and heavy use suits GB 42H at 220 to 300N.
Does higher density mean firmer foam?
No, density and firmness are separate measurements. GB 39S and GB 39H both have a density of 37 to 41 kg/m3, but GB 39S measures 100 to 140N and GB 39H measures 180 to 220N.
What is a support factor in foam?
A support factor is the force at 65% indentation divided by the force at 25% indentation. ISO 2439 calls this the compressive deflection coefficient, and a higher ratio means the foam firms up more as you sink into it.
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