Foam specifications are widely misread, and the most common error is treating density as a measure of how firm something feels. It is not, and the confusion leads people to buy the wrong product.
Density
How much the foam weighs per unit of volume, usually given in pounds per cubic foot or kilograms per cubic metre.
It describes how much material is in the foam. That correlates with durability, support over years, and how well the foam recovers its shape, which is why it is the specification worth reading.
It does not describe how firm the foam feels when you sit or lie on it.
Firmness
How much force is needed to compress the foam by a set amount, expressed by measures such as ILD or IFD.
This is what you actually feel. High-density foam can be soft, and low-density foam can be firm. They are independent properties.
Why It Matters
- Density predicts how long the foam lasts before it flattens
- Firmness predicts how it feels on the first day
- Low-density foam that feels firm will soften quickly
- High-density foam that feels soft will still feel that way in three years
- Cheap products advertise firmness and omit density
Manufacturers who publish both, or at least state which specification a claim refers to, of the kind at xtreme-comforts.com, are making a comparison possible; a listing giving neither is asking you to trust a photograph.
The Cheap Cushion Pattern
Recognisable once you know it. A low-density foam cushion feels supportive in the shop and flattens within months, because there was never much material in it.
That is why an inexpensive seat cushion frequently feels excellent for six weeks and then like sitting on a chair again. Nothing broke; the foam compressed permanently.
Memory Foam Specifically
Viscoelastic foam behaves differently again: it softens as it warms, so its effective firmness changes over the course of use.
A memory foam cushion feels firmer when you first sit down and conforms over the following minutes. That is the material working, not it giving way.
Temperature Sensitivity
The same property explains why memory foam feels hard in a cold room and soft in a warm one.
For a cushion kept in a car overnight in winter, the first ten minutes will feel unlike the rest of the journey.
What to Look For
For anything you will use daily for years, density is the more important figure. For anything bought on how it feels, firmness is what you are judging.
Where neither is stated, reviews mentioning performance after six or twelve months are the practical substitute.
Weight Is a Clue
In the absence of published figures, a heavier product of the same size generally contains denser foam. It is crude and it is better than nothing.