You will notice on natural latex mattress packaging and advertising an ILD or IFD rating.  Usually, a product will have one or the other, though they both measure the firmness of the latex.  ILD is a measure of force (in pounds) required to cause an indention of 25% into a four-inch layer of latex.

Basically, the higher the rating, the firmer the latex.  Firmness ratings are indicated in a range because latex composition is not consistent throughout due to the production process.  It can also be impacted by the temperature of the room and other factors.  Basically, when comparing two products with different but similar ratings, there may be no difference in the way they feel.

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