Firmness in a mattress is quite a subjective description. What I think is firm may seem medium firm to you. This is not such a big deal if you are purchasing a mattress from a brick and mortar mattress store, you can lay on the floor model and see for yourself and not rely on anyone’s description or opinion.

The problem is that more and more people are buying their mattresses through the internet or off of TV infomercials. This often results in great frustration when the advertised firmness is not what you would expect.

These is a simple solution to this problem, and the funny thing is it has been around for years in the mattress and foam cushion industry. Its a standard measurement called Indentation Load Deflection (ILD) or Indentation Force Deflection (IFD). The two terms are interchangeable.They are nothing more than a fancy way of saying how much pressure does it take to make an ‘indentation’ in the mattress or foam.

Here’s how it works at the factory they take a 50 square inch disk  and place it on a mattress and apply weight to it. The amount of weight that is required to cause a 25% indentation in a 4 inch thick piece of foam is the 25% ILD rating. They then see how much weight is required to compress the foam down to 1.4 inches or 65% indentation. This amount of weight becomes the 65% ILD rating. The ratio of the 25% ILD to the 65% ILD is the comfort factor.

You will rarely see the comfort factor used, because the measurement is difficult to explain, but you will see the 25% ILD number advertised by some and all mattress retailers especially those who specialize in latex and memory foam mattresses should be able to provide you with this number so you can compare apples to apples.

Not all natural latex foams have the same ILD despite being made out of the same raw materials. The firmness is changed in the latex manufacturing proses by using more or fewer ‘pin’ in the mold which create empty spaces in the foam. So for example not all Talalay latex mattresses will have the same firmness, just like not all memory foam mattresses will have the same firmness.

One major problem with visco-elastic memory foam is that the ILD changes with temperature. This may not seem like a problem not but you will be told your mattress has an ILD of say 15 yet when you are actually using it the ILD is different. In my opinion they should start testing the ILD if memory foam at body temperature so they could get a rating closer to what you would actually experience with your mattress.

The good thing about latex foam mattresses is that they do not respond to heat like visco-elastic memory foam (which was specifically designed to respond to heat. Of course latex foam is marginally effected because most materials become softer with heat but the change is negligible.

  One Response to “How is Mattress Firmness Rated?: ILD, IFD and What They Mean”

  1. [...] Memory foam is also a very dense foam. That being said the density usually varies by manufacturer. The denser the foam the firmer the mattress. To learn more about density rating click here. [...]

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