How mattresses are rated for firmness is part science and part personal preference. A "regular" innerspring mattress (one with a box spring and a mattress) is rated by a number of factors, but the coil count and the coil gauge are the most important ones to look for when shopping. Latex and memory foam mattress manufacturers take another approach to ratings, given the fact they are made of a solid materials, without coils. The most common referred to ratings for these mattresses are the density and Indentation Load Deflection rating.
Innerspring Coil Count
In the past, it was generally believed that the more coils a mattress had -- the better support and firmness it would provide. That thinking is outdated. What matters most in determining the firmness is how the coils are constructed, based on the size of the mattress. Most mattresses have a range of 300 to 800 coils. In general -- more is more. The more coils -- the firmer the mattress. Use a baseline of 400 coils for a queen size mattress to gauge the firmness, less for a full or twin, and more for a king. Look for an innerspring mattress to have 400 coils if it is rated "firm," and to have 500 coils if it is rated "extra firm," and to have more than 600 coils if it is rated "super firm."
Innerspring Coil Gauge
The gauge is the size of the wire used to make the coil and relates to the firmness rating. The gauge is measured in 1/13th of an inch. If the mattress has a lot of give way and "bounce" it is probably a sign that it is constructed with a lighter gauge coil wire. However, it is not necessarily true that a high gauge coil equates to a firmer mattress. Manufacturers may compensate for a thinner or lower gauge, but give the mattress more coils. Most retailers will display a cutaway view to show what the mattress looks like inside. Be sure to review this information when shopping. Look for a gauge of 13 to 15 as an indication of firmness, durability and quality.
Latex and Memory Foam - ILD
In general, the higher the Indentation Load Deflection, the firmer the mattress. However, because memory and latex foam mattresses respond to the weight of the body, the ILD number may or not be relevant. The mattress will respond differently to the body of a 40-pound child than it will to a 200-pound adult. Regardless of the ILD rating that you see on a mattress label in a store, it is important to test the mattress and let your body be the judge of the firmness and comfort.
Latex and Memory Foam Density
The density rating is determined by the quality of the foam and materials (including chemicals) that are used to manufacture the mattress. While density is actually a technical rating and corresponds more to durability, consumers get a "feeling" of firmness when they test the mattress. From that perspective, you should look for a density rating of 1.81 Kilogram or higher when selecting a latex or memory foam mattress for greater assurance that the mattress will maintain its firmness and last anywhere from three to five years.