Measure ILD at home?

Is there a not-too-complex way to measure the ILD of a mattress at home? Does ILD even apply to innerspring mattresses?

The purpose would be to know roughly what firmness of mattress I have if I go shopping for a topper. If I know my current mattress is a 32, for example, I can ask to try toppers on an in-store mattress of similar firmness. Obviously it wouldn’t be an exact comparison, but it might help me make a better decision.



I’ve wondered the same thing, and the answer appears to be no. Ild is a specific measurement, taken with a certain size core (6" thick I believe), and hardly even perfectly standardized at that.

I’ve wished you could tho… And it’s not an impossible measurement that I’d guess you’re looking for. But, nobody has done it. What I suspect you want is basically something more along the lines of:

For a given surface (i.e. a mattress), how many lbs force is required to reduce a square foot of its height by x inches, and then plot the graph for all values of x from 0 inches to some reasonable upper bound (maybe 3/4 of the mattress thickness) - sort of like a torque curve for car engines. Such a graph would combine the ild and compression modulus or whatever it’s called into a single graph, and could be applied to an entire mattress with all the layers, and not just a specific material. With multi zone mattresses, you’d have different areas of the mattress to test.

… But that’d be far too scientific, and arguably make it much easier to duplicate the feel of a mattress. You’d have a nice graph of exactly how much force required to sink in a little deeper.

This is something you could probably make some sort of smallish apparatus to measure to some degree, and test beds with. You’d look a bit strange mind you :wink: a string across the mattress to provide a reference level, a ‘foot’ measuring say 1x1, that you push down, and with a ruler on its side to measure the depression vs the string. A lbs scale on top you’re pushing on. Start at 0, then press in scale until foot is 0.5", 1", 1.5", 2", etc (or 0.25" increments) and record pounds of pressure off the scale. Not perfect but… Also, at some point it’ll get really hard to press further down so you might need to exert a lot of pressure to get some of the later readings.

…bonus points if you can time the readings too - pressure to compress, and pressure 5, 10, 15, 20, etc seconds after being compressed. Some materials, once compressed, will tend to stay compressed (I.e. It might take a lot of effort to compress, but once compressed it generally doesn’t push back as hard) (e.g. Memory foam?), whereas others tend to consistently push back (e.g. Latex)

…huge bonus points for doing it in reverse too (I.e. once compressed, if you let up x inches, how much pressure is required to hold it at the new level). This would help you capture how elastic / responsive the material is to return to its original form. E.g. Slow memory foam vs talalay latex.

I had a longer answer, which I subsequently deleted. No, there’s not really a way to measure ild at home. It’s a not a uniformly standardized measure in the first place, because it has several variables. (One would imagine a vendor would define their own standards for each variable, but, across vendors there’s no standards as to what the variables need to be).

Ild is how much force is required for a fixed size ‘foot’ to depress a variably sized material (e.g. 20 inches by 20 inches), of variable thickness (example 6 inches), by a variably defined amount (example 25% - or compress in by 1.5" in this example). I thought it was 6" of the material, but maybe it’s 4". (Edit: it’s variable). The most immediate trouble being, you likely don’t have an appropriately sized piece of what you’re measuring. And even if you did, it’s by no means the full picture trying to compare different materials. More ‘formal’ info is here:

For example, a fully stated ild measure might be:
Ild of 24, fully stated could mean: 24 lbs force to compress a 4" thick, 20" x 20" sample by 25%
Ild of 24, could also mean: 24 lbs force to compress a 6" thick, 16" x 16" sample by 25%
Ild of 24 could also mean: 24 lbs force to compress a 6" thick, 24" x 24" sample by 40%

  • Even though all have a perfectly legitimate ild of 24, because of the variation in measurement if you were to re-measure all 3 materials but use a consistent thickness, size, and compression amount, they would actually end up having very different ild’s.

The above article does a good job explaining the intricacies of what Phoenix has said:

  • it’s difficult or impossible to compare ild across vendors
  • ild is by no means scientifically precise, and doesn’t speak to other measures of comfort such as the Support Factor
  • variability within a batch of foam is somewhat significant
  • variability of the environment when the foam is created affects ild (they speak to humidity / the season when foam created)
  • it’s difficult to get a consistent ild of 2 ‘identical’ foams (I.e. 2 batches of otherwise ‘identical’ foam from the same vendor) due to the variability of the foam making process


Thanks for that info and link.

Sounds like they are trying to standardize on

  • 20" x 20" sample
  • 50 square inch round foot (7.98" diameter)
  • All their examples are for 25% deflection

That leaves thickness as the variable and pounds as the result.

So, yeah, not easy to do at home, but if I had two or three 8" dumbbell plates I’d still stack them on the mattress to see how far down they sink :wink: .

I didn’t have more info than that link, I.e. I was just trying to summarize it. So if you’ve read that document, you have the same info as me. Also, if I say something which is contrary to that document, the I’ve simply summarized wrong and believe the document and not me :wink:

  • I think in the document it says that one standards body allows 15" x 15" sizes, and that many vendors have that info. They recommend 20" x 20" since you don’t get the ‘side effect’ interfering with the measurement.

  • I think near the bottom it says that in Europe, ild’s are sometimes measured as 40% compression vs 25%.

I only skimmed it. I don’t doubt that there are variables. It seems more like a story than a standards document.

The Scope section of ATSM standard D5672 would seem to indicate that tests are to be done to 25% on a 4" thickness:

That standard refers to a “referree method”:

which should be in D3574 Standard Test Methods for Flexible Cellular Materials—Slab, Bonded, and Molded Urethane Foams.

I’m not up for buying $100 worth of ATSM documents to see all the nitty gritty but it’s interesting nonetheless.

The store where I’ll probably be trying this topper is using Latex International foam. After searching LI’s site, I see even they don’t discuss their ILD testing methods. Oh well!

I’d pay to see video of you walk into a variety of mattress stores with a 15lbs bowling ball, and drop it from like 5 feet in the air and make a bunch of gestures / comments to indicate you are trying to measure the ‘softness’. Make like a crazy physics dude and babble a bunch about ILD/ IFD/ Gravity/ acceleration/ bowling balls, etc., etc. ATSM standards, international standards, etc… extra if you yell eureka after some test, and then run out of the store.

LOL love it :P. That could go viral!

Hi sdmark,

As dn has mentioned … the short answer is no. There are different ways to measure ILD (usually used for latex) or IFD (usually used for polyfoam) but ILD/IFD only measures firmness to a specific percentage of compression and the thickness of the layer (or mattress) that is being tested makes a significant difference. You could use a bowling ball or other very dense object to measure how far it sinks in to various mattresses as a relative softness comparison (although a bowling ball would probably be too light for a complete mattress) but this wouldn’t relate to ILD/IFD because the variable with this type of test would be the depth of compression on each mattress while with ILD or IFD the variable is the force used to compress it to the percentage that was being used by the test (usually 25% or 40% depending on the area of the world).

ILD alone is also only one of the specs that determines the softness of a material in “real life” because very few people sink into a mattress or a foam layer by exactly 25% and each material has a compression curve that gets firmer at different rates (compression modulus). The variance in compression modulus means that a mattress could be softer than another with a lighter testing weight but firmer than the same mattress with a heavier testing weight.

There is more about ILD/ILD in post #2 here and in post #6 here.

Springs don’t have an ILD and are measured by their spring rate. A cylindrical coil will have a linear response curve (vs foam which is more banana shaped and absorbs some energy as it compresses) but different shapes of springs or variable rate springs will also have a non linear compression. There’s more about innerspring rates and some comparison to foam compression curves in post #2 here.

The “bottom line” with all of this is that any home based method may be able to check a mattress for relative softness at certain compression levels (such as the bowling ball method) but this wouldn’t be comparable or “translatable” to ILD/IFD.