What is the ILD measurement that is used by mattress manufacturers?

ILD (Indentation Load Deflection) is a value that represents the force needed to compress natural latex, polyurethane and several other types of foam. It is used across many industries from mattresses, car seats and furniture.

The ILD measurement value is given to an individual layer of foam and not the entire finished product. In the case of a mattress, it will carry a general firmness rating like soft, medium or firm, while the individual natural latex foam layers within the mattress will have their own ILD rating.

Latex manufacturers like Spindle, Avocado and Brentwood Home will sometimes use Ild for mattress descriptions. In this case, the ILD value only refers to the individual layer within a particular mattress model.

ILD (Indentation Load Deflection)

ILD is a unit of measurement and refers to the number of pounds (lbs) of pressure or force that is needed to indent foam to a depth of 4” by 25%. This test is performed on an area of 50 square inches. The rating is referred to as the ILD rating or the 25% ILD rating.

Specialized Equipment Is Used To Measure ILD and IFD

An ILD testing machine ready to test a sample of polyurethane foam. https://www.admet.com

In an ILD test, the measuring instrument operates much like a press. The surface area of the indenter is 50 square inches and is circular with 8″ in diameter.

The sample is usually 100mm thick (4-inches) and has a surface area of 500mm X 500mm.

The foam is placed on a flat surface that is perforated. The perforations in the surface allow air to escape the foam as it is being compressed.

The foam is “warmed up” or “strained” before the ILD or IFD test. It is compressed to 75% of its thickness and released again. There is a 6-minute interval between the 75% “strain” compression and the 25% compression that measures the ILD.

A lower ILD score means that the foam is softer or plusher while a higher ILD score means that the mattress is firmer.

In the United States, the ILD measurement is given in pounds-force, while in Europe it is measured in Newtons.

An ILD (IFD) measuring Device in action

In the video below you will be able to see the machine that performs the test in action. The test procedure is as follows:

  1. The machine pre-flexes the foam before making the final ILD test. The machine will compress the foam twice to 25% of its original thickness at a speed of 4mm/sec.
  2. There is a 6 minute wait time before the final test begins.
  3.  The foam is then compressed by 25% of its thickness at a rate of 0.83 mm/sec. The force required to compress the foam at a 25% depth is then recorded in Newtons. This value is recorded after 60 seconds of compression.
  4. The machine then continues to compress the foam to 65% of its thickness at a rate of 0.83mm/sec. The force required to compress the foam at a 65% depth is then recorded in Newtons. This value is recorded after 60 seconds of compression.

*Most manufacturers only use the 25% test when sharing ILD results for natural latex foam. 

The Final Results

In the video below, the polyurethane foam is 4-inches in thickness.

The 25% test revealed that the foam had an ILD of 19.1.

The 65% test revealed that the foam had an ILD of 50.3.

The verdict: If we had to attribute a firmness feeling to the polyurethane foam in the video, it would fall into the “plush” category. With an ILD of 19.1 on the 25% test, the foam falls into the “plush” category. You will be able to reference the firmness ranges on the scale below.

Speak English to me, mattress man!

Some descriptive words have been assigned to ranges of values on the ILD scale. This makes it easier for people that don’t feel the need to get too scientific about natural latex foam. Note that an ILD value is only given to an individual layer of foam and not to an entire mattress.

Natural Latex foam is manufactured in the following ranges of ILD. In order to achieve a greater ILD, the latex foam manufacturer makes a denser and heavier foam layer. The higher the ILD, the smaller and fewer the air bubbles the foam will have.

ILD Firmness Guide

  • Plush has a range of 16.5 ILD to 22.5 ILD
  • Soft has a range of 22.5 ILD to 26.5 ILD
  • Medium has a range of 26.5 ILD to 30.5 ILD
  • Firm has a range of 30.5 ILD to 34.5 ILD
  • Extra-Firm has a range of 34.5 to 38.5 ILD

Most people sleep comfortably on mattresses that have a 25-35 ILD.

An ILD  value is objective

The ILD unit of measurement is a fantastic invention because it has enabled the manufacturer to objectively describe the firmness of individual mattress foam layers. With so many factors going into mattress choice like your body weight, height and preferred sleeping position, it is difficult to attribute a firmness description to a mattress (soft, medium, firm).

The ILD scale removes personal opinion from the description of a foam’s firmness and attributes a standardized value to it.

Two Different Terms That Mean The Same Thing

Ild which is an acronym for indentation load deflection is also sometimes called Indentation Force Deflection or IFD. ILD and IFD mean the same thing and are interchangeable. However, in the mattress industry, ILD is more commonly used.

A Simplified Scale 1 – 10

While ILD represents an amount of force needed to compress foam, it does not give a very good indication of the mattress firmness as a whole to the average consumer. People relate much better to a scale of 1 to 10, which is why some companies rate the firmness of their mattresses on a scale of 1 to 10 while forgoing the ILD measurement entirely.

Mattress manufacturers are aware of the particular ILD of the foam within the mattresses they make, but some whish to not make it public, as the unit of measurement can seem confusing to the average person. The ILD or IFD units of measurement are only used in the polyurethane and natural latex foam manufacturing industries, so it is unlikely that someone that is not working in this industry would quickly relate to the unit of measurement.

Avocado Vegan Mattress available with optional pillowtop. On a scale of 1 to 10, the pillowtop has a rating of 6, while the base model has a rating of 7.

A 1-10 firmness rating example

Avocado Green does not mention any ILD measurements on the Vegan Mattress page. They simply rate the entire mattress on a scale of 1 to 10 with 10 being the firmest.

  • The Vegan base model by Avocado Green is rated at 7 on a scale of 1-10. They call it a “gentle-firm” and is best suited for back sleepers, stomach sleepers, children and adolescents.
  • Vegan Pillowtop model by Avocado Green is rated as a 6 on a scale of 1-10. Avocado says it “offers just the right amount of added luxury and buoyant cushioning”.

ILD is not the only determining factor for firmness

Note that an ILD or IFD value only pertains to the foam layer inside a mattress like the Avocado Green Vegan.

There are a few other components that influence the overall firmness of the mattress: the 5-zone coil layer, wool batting and cover will influence the overall firmness of the mattress.

With this being said, the ILD rating of the foam layers inside a natural latex mattress is a good indicator of the overall mattress firmness.