What Are Fitted Bed Sheets? | The Bottom Sheet Explained

A fitted bed sheet is the elastic-cornered bottom sheet that hugs your mattress to stay put while you sleep.

If you’ve ever woken up on bare mattress because your top sheet slid off, you already understand what a fitted bed sheet does. It’s the bottom layer of bedding, cut to match your mattress shape and secured with elastic so it doesn’t shift, bunch, or slip loose during the night. Unlike a flat sheet that lies loosely over you, the fitted sheet wraps the mattress itself and gives you a snug, wrinkle-free surface to sleep on.

Fitted Sheets vs. Flat Sheets: What’s The Difference?

A fitted sheet and a flat sheet serve completely different jobs, and confusing them is one of the most common bedding mistakes. A flat sheet is a simple rectangle of fabric that lies on top of you, typically tucked between you and your blanket or duvet. A fitted sheet is shaped to the mattress, with sewn edges and elasticized corners, so it stays anchored to the bed.

The key difference comes down to construction: fitted sheets have elastic. Some use elastic on all four corners only; others run elastic around the entire perimeter, sometimes called “shower cap” style. This is what keeps the bottom sheet taut and in place all night.

• Fitted sheet (bottom sheet): Shaped to the mattress, elastic edges, stays in place.
• Flat sheet (top sheet): Rectangular, lies loosely over you, needs tucking.

You can also layer both — the fitted sheet on the mattress, the flat sheet on top for extra warmth or as a lightweight cover.

Standard Fitted Sheet Sizes and Pocket Depth

The size that fits your bed depends on your mattress dimensions, not just the label on the package. Standard US mattress sizes map to these fitted-sheet dimensions across major bedding brands, though exact measurements can vary slightly by manufacturer.

Pocket depth — the height of the fitted sheet’s sides — matters just as much as length and width. Standard pockets run 8–12 inches, deep pockets 13–16 inches, and extra-deep pockets 17–22 inches. A queen mattress with a 14-inch profile needs a deep-pocket sheet; a pillow-top or memory foam setup can easily require extra deep.

How To Choose The Right Fitted Sheet

Getting the right fit comes down to three quick measurements and one check. First, measure your mattress width, length, and height with a tape measure. Second, match the width and length to the nearest standard size — a 60″ x 80″ mattress is a Queen, for example. Third, confirm the sheet’s pocket depth is greater than or equal to your mattress thickness.

The most common mistake is buying by mattress name alone and ignoring depth. A sheet that’s too shallow will pop off the corners by morning; one that’s too deep will feel loose and wrinkle-prone. Also remember that a mattress protector adds thickness, so account for it when you measure.

Brand sizing can differ slightly, so check whether the manufacturer lists exact dimensions or a range that includes your mattress size. When you’re ready to buy, our tested picks for the best fitted bed sheets show which options hold up best across different mattress depths.

One honest caveat: fitted sheets are built for mattresses, not loose toppers or irregular surfaces. A mattress topper changes the effective height and depth your sheet needs to cover, so remeasure after adding one.