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Indentation Load Deflection Explained (Mattresses)

If you’re looking to buy a new mattress then you might be wondering what indentation load deflection (ILD) is.

Indentation load deflection (ILD) is a measurement of how ‘hard’ or ‘soft’ the foam in a mattress is. More specifically, ILD is a value that expresses the amount of force required to compress a 100 mm thick piece of foam by 25% – with the reading being taken after 60 seconds of compression.

The rest of this article explains what ILD is in more detail and why relying on these values alone might not be the best way to guage mattress comfort.

Alternatively: check out my list of the best mattresses to buy in 2021 for an easier way to find the most comfortable mattress for you.

Indentation Load Deflection Explained

Below is a full explanantion of what ILD is:

ILD vs IFD

The term ‘indentation load deflection’ (ILD) is often used interchangeably with that of ‘indentation force deflection’ (IFD).

And for practical purposes, you can take the two to mean the same thing when looking at ILD values.

The alternate terminology is simply a reflection of the differences between the imperial and metric systems.

How is ILD Measured?

ILD is calculated by taking a section of the foam measuring 100 mm in thickness – with an area of 500 mm by 500 mm – to which a flat, circular indenter with a surface area of 323 cm squared is applied [1].

The indenter is depressed into the foam until an indent of 25% of the foam is achieved – at which point a force reading is taken after 60 seconds.

Ratings are then expressed in pounds/force (US) or newtons (EU).

What Do the ILD Ratings Mean?

The bottom line is that a ‘lower’ ILD rating indicates a softer foam, whilst a ‘higher’ ILD rating maps to a harder foam.

ILD ratings are typically expressed as a range.

So in theory, you could potentially use ILD ratings to figure out how firm your mattress is likely to be.

And I’ve created the table below so that you can see how various ILD ratings might correlate to potential mattress firmnesses.

But the reality of the matter is that different ILD ratings could correspond to totally different firmnesses.

I’ll explain why in the next section.

ILD Rating (lbs)Possible Mattress Firmness
<16.5Extra plush
16.5 – 22.5Plush
22.5 – 26.5Soft
26.5 – 30.5Medium
30.5 – 34.5Firm
34.5 – 38.5Extra-firm

Why ILD Ratings Aren’t Reliable

Using ILD ratings alone to choose the right mattress firmness might backfire on you.

Because I consulted with several sources online to help me compile the ILD table above and they each said something different.

To me, it appears that whilst ILD ratings may indeed be objective by foam testing industry standards – this value could become lost in translation when trying to directly equate it to a mattress firmness and therefore gauge overall comfort.

For example, memory foam mattresses typically have different support and comfort layers that could have their own individual ILD ratings.

Put these together with any additional layers and two identical ILD ratings for two different brand mattresses could result in different real-world firmnesses.

Here’s a few more factors that could skew the relationship between ILD ratings and actual mattress firmness.

Material Type

Comparing the ILD of memory foam mattresses and latex mattresses is like comparing apples and oranges.

Because the sample size for latex is around 152 mm compared to the normal 100 mm.

Matters are complicated further when you consider that brands like Tempur-Pedic have their own proprietary foam blends that make their Tempur-Pedic mattresses different to the other memory foam mattresses on the market.

Material Thickness

The thicker the material layer being tested, the more force will be required to compress it to achieve a 25% indentation rate due to the effects of compounding.

So even if the sample size of the material used in the mattress is standardised at 100 mm for the ILD test – the actual thickness of the material that you’re going to be lying on in the mattress will vary.

Combine this with your body weight and sleeping position and ILD values start to become less meaningful in the real world in my view.

Temperature

The apparent firmness of your foam mattress may decrease in the summer months because an increase in temperature and humidity can make the material less dense and therefore skew the relationship between the original ILD value and the actual firmness.

Body Weight

Even if all ILD values mapped exactly to the same level of firmness – the final feel of the mattress would still differ because we are different weights.

For example, a 130 lb individual is likely to find that a mattress feels different to what a 230 lb person would think of the same mattress when lying on it – simply due to the difference in force being applied to the material.

Sleeping Position

In addition to your body weight, the position that you sleep in will affect how the mattress feels to you.

Sleeping on your side for example, increases the pressure on your hips and shoulders when compared to sleeping on your back or front – which tends to put more pressure on your lower back.

Simply switching between positions on the same mattress can give it a different feel regardless of its ILD rating.

How Can ILD Help You Choose the Right Mattress?

My final opinion on using ILD values to help you choose your ideal mattress goes against the grain of the other articles that I’ve read online:

I don’t think that ILD ratings are a good way to compare mattress firmnesses and potential comfort levels simply because the ratings can be skewed by so many variables.

That’s why I put together the list of the best mattresses that you can buy online in 2021 so that you can find the most comfortable mattress for you quickly – click the button below to get started:


Sources and References

[1] Wikipedia – Indentation Force Deflection. Accessed 30/12/19.

Image Attribution and Licencing

Main image: ‘Memory Foam Mattress’ by Slobodan Miljevic (Getty Images Signature) – used with permission under the terms of Canva’s One Design Use License Agreement.

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