Foam Bed Firmness

Foam bed firmness is measured in a lab by pushing a rigid, flat surface into the surface of foam.

Indentation Force Deflection (IFD) - A measure of the load bearing capacity of flexible polyurethane foam. IFD is generally measured as the force (in pounds) required to compress a 50 square inch circular indentor foot into a 4 inch thick sample, typically 15 inches square or larger, to a stated percentage of the sample's initial height. Initial Load Deflection (ILD) is another term meaning the same thing.

 

Lower IFD = softer foam

Higher IFD = firmer foam

 

Common values for the top layer of a memory foam mattress range from 12 – 15 IFD. Foam bed base layers typically range from 20 – 40 IFD

Firmness is not the same thing as support. Good foam beds should be both soft and supportive, but some bad mattresses are both too firm and offer little support.

 

The most common IFD value measures indentation to 25% of the total height of a piece of foam.  This is called 25 IFD and tells us how soft or firm the surface of a particular foam feels.

 

While you want a foam bed to softly contour to your body, you don’t want it to completely compress or “bottom out.”  Good foam does two things; it contours and supports.  Support is determined by taking measurements deeper into the foam and is called support factor.

Flex fatigue is the normal softening of foam beds with use and over time. You can expect your mattress to get 5-15% softer by the end of its useful life. Higher density foams (3 lbs and up) hold their original firmness best. In general, the higher the density, the better it will retain its firmness.

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