mattresses

In the United States, the standard mattress sizes are (the people in the drawings are 6 feet tall):

Type of mattress	Usual dimensions
Type of mattress	width (inches)	length (inches)
twin	39	75 or 76
long twin	39	80
full or double	54	75 or 76
queen	60	80
king	78 or 79	80
California king	72	84

In Europe, mattresses are currently 200 centimeters (78.7 inches) long and are offered in various widths: 100, 140, 160, 170, 180 and 200 centimeters.

Verbal descriptions

Phrases such as “extra firm,” “firm” and so on may help in comparing different mattresses within a line, but they have no meaning in comparing mattresses from different manufacturers.

In general, mattresses and box springs should be more than 6" thick.

Coils

A double bed innerspring mattress should have more than 300 coils,
a queen size more than 375, and
king size more than 450.

The gauge of the wire from which the coils are made is also significant; 13-gauge springs are sturdier than 16-gauge.

Foam

A foam mattress should have a density of more than two pounds per cubic foot.

Trying one out

The edges and the corners in particular should not feel flimsy, no matter what type of mattress it is.
If the mattress is for two people, the two of you should try it out simultaneously.
Note whether your hips and shoulders are supported comfortably when you lie on your side.
Roll around and listen for strange noises; there shouldn't be any.

The top of a mattress should be about 18 inches above the floor. In a bunk bed, the top of the lower mattress should be at least 30 inches below the bottom of the upper mattress above.