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How Is Foam Density Measured?

Written by Keith Cushner

Quick Overview

Density, or weight per unit of volume, is an important variable for mattresses with foam components. Foam density also affects other factors associated with the mattress, such as durability, motion isolation, odor potential, and cost.

This guide will discuss how density is measured for polyfoam and memory foam mattress layers, and provide buying tips for selecting beds based on density measurements.

What Is Foam Density?

Density refers to how much one cubic foot of foam weighs; it is expressed in pounds per cubic foot (PCF). To calculate density, divide the mass or weight of an object by the total number of volume units. For example, a foam layer that weighs 100 pounds and measures 25 cubic feet has a density of 4 PCF; assuming the foam has a uniform consistency, every square foot of the layer will measure 4 pounds.

Two types of foam are widely used in mattresses made today. Flexible polyurethane foam, also known as polyfoam, is a synthetic material produced from polyol and isocyanate petrochemicals. The other foam type is viscoelastic polyurethane foam, or memory foam, a specialized polyfoam engineered to become softer when it comes into contact with body heat and then return to its original shape when cooled. Polyfoam is relatively inexpensive, and widely used in the support core, or base layers, of mattresses; select models also feature polyfoam in the topmost cushioning layers, known as the comfort layers, or transitional layers between the comfort and support layers. Memory foam, on the other hand, is too soft and not dense enough to serve as a support core material; it is almost exclusively used as comfort or transitional layers.

Additionally, foam density falls into three general categories: low, medium, and high. High-density foam is almost always used in mattress support cores, and may also be used as a comfort or transitional layer. Low- and medium-density foams are only found in the bed’s comfort system; if used as a support core material, these foams would lead to excessive sagging and uneven sleeper support.

Density is used to evaluate both polyfoam and memory foam, but the PCF ranges differ considerably. The table below breaks down PCF ranges for low-, medium-, and high-density polyfoams and memory foams.

Density vs. Firmness and ILD

Some mattress brands use the terms ‘density’ and ‘firmness’ interchangeably, but this is technically incorrect. While density is a weight/volume measurement for individual mattress layers, firmness refers to how soft or firm the whole mattress feels, and how closely the comfort layer material conforms to the sleeper’s body. Firmness is measured on a 1-10 scale, with 1 being softest and 10 being firmest; most mattresses sold today fall between 3 and 8.

The firmness of individual foam layers may also be measured in indentation load deflection, or ILD. ILD is calculated by placing a circular disk measuring 1 foot in diameter on a section of foam that measures about 4 inches thick. The ILD is the amount of weight needed to compress that section of foam by 25%; ILD is expressed in numerals corresponding to the weight. The foams used in mattress comfort layers usually fall between 10 and 20; transitional layer and support core foams usually have much higher ILD measurements.

Although density and firmness are technically different measurements, there is some overlap. High-density foams tend to be firmer and do not conform very closely, while low- and medium-density foams are not as firm and conform more.

How Does Foam Density Affect the Mattress?

The density of foam layers used in a mattress can impact many different aspects of the overall bed. The table below breaks down how low-, medium-, and high-density foams affect some these variables. Our ratings are based on a combination of verified owner experiences and product research and analysis.

Choosing the Right Foam Density

When selecting a mattress based on the density of its foam layers, shoppers should take their weight into account. Lighter people tend to prefer lower-density foams because they do not experience as much conforming/pressure relief on higher-density materials. The opposite is true of heavier people; sleeping on lower-density foams often leads to excessive sagging and less-than-optimal support.

Preferred sleep position is another important factor. Side sleepers usually need a softer, lower-density mattress that will cushion their shoulders and hips; this improves spinal alignment and alleviates aches and pains. Back and stomach sleepers utilize positions that naturally align the spine, but excessively soft mattresses can sink beneath the heavier areas of their body; this causes the sleep surface to become uneven, and can cause more pressure points to develop.

The table below lists average ratings for different foam densities based on these two criteria.

Foam Mattress Density Comparison

Now that we’ve discussed what foam density means and how it is used in mattress construction, let’s look at the density measurements for mattresses sold today. The table below includes comfort layer and support core density figures for some of the today’s most popular all-foam beds, along firmness levels and current price-points.

To read our reviews of these brands, please click the links found in the left-hand column; to view product pages, click the links in the second column from the left.

Additional Resources

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