Get the whole bed to fit.

The standard US sizes are Twin at 38 by 75 inches, Twin XL at 38 by 80, Full at 54 by 75, Queen at 60 by 80, King at 76 by 80, and California King at 72 by 84. A king is 4 inches wider and 4 inches shorter than a California king. Sizes such as Wyoming and Alaskan King are specialty rather than standard, so their dimensions vary by manufacturer.

It is not measured by any standard test, it is declared. Each manufacturer places its own models on a 1 to 10 scale relative to the rest of its own range, so a 6 from one brand can feel like a 7 from another. ILD is a genuine measurement but it describes a single foam layer rather than a finished mattress, and most brands do not publish it.

It depends on construction rather than size. Basic metal platforms are commonly published at 250 to 500 lb, wood platforms at 400 to 800, and heavy duty steel platforms at 700 to 1,500. Those are category ranges, not a promise about any one frame. The mattress counts toward the load, and a king hybrid can exceed 100 lb on its own.

Match it to the mattress plus anything on top of it. Standard pockets fit up to 12 inches, deep pockets 13 to 17, and extra deep pockets 18 to 22. Above 22 inches there is no standard class, so look for a sheet that states its exact depth. A topper adds its full thickness, which is the most common reason sheets stop fitting.