Box sheets, fitted sheets and shaped sheets

Short answer: Measure the mattress length, width and height. The height is the figure most people forget, and it decides whether the sheet stays on. The names box sheet, envelope sheet and fitted sheet matter less than the depth of the edge.

The three measurements

Length and width are the easy ones. Height is measured from the top of the mattress to the bottom, and it decides how deep the sheet's edge must be. Write all three down before you look at sheets, so you can compare them directly with the product description. See bamboo sheets.

Height is what people forget

Most people know the mattress size from the packaging but not the height. Yet the height decides whether the sheet fits. A mattress can be fifteen centimetres high or twenty-five, and the sheet must be chosen accordingly. If you no longer have the packaging, measure yourself with a ruler or tape along the side.

How to measure the height

Lay a ruler or tape along the side of the mattress and read the height from bottom to top. Measure both sides and a few spots, since mattresses give a little. If there is a topper or pad on top, measure the total height. Count what you actually sleep on, not just the mattress itself.

If the height is too small

The sheet pops off the corners at night and ends up crumpled under you. It is not the elastic that is poor, but the edge that is too short for the mattress. A sheet with too short an edge pulls at the corners. See bamboo envelope sheets for deeper edges.

The edge and the extra centimetres

A sheet needs an edge at least as deep as the mattress, plus a couple of centimetres to grip underneath. Those extra centimetres keep the sheet in place when you turn in your sleep. Choose a slightly deeper edge rather than one that is too short, because the elastic gathers the fabric under the mattress.

Topper and pad

If you have a topper, the height must be included. Two layers of mattress often call for a sheet with a deep edge, and that also applies if you use a thicker protective pad. Always measure the total height when something lies on top of the mattress. See bamboo fitted sheets for tall mattresses and toppers.

The names are used differently

Box sheet, envelope sheet and fitted sheet are used slightly differently by different retailers. So the name is not the safest guide. Look instead at the number that gives the edge depth in centimetres and compare it with your mattress. A box sheet has a deeper edge meant for tall mattresses.

Shrinkage in the first wash

Bamboo fabric can shrink a little in the first wash, so keep that in mind when you choose the size. Do not pick a sheet that fits exactly if your mattress is tall or large. Follow the care label and use a gentle programme so the sheet keeps its shape and stays on.

How to choose

Start with the three measurements and add two or three centimetres to the height if you are unsure. Then choose the edge depth that fits and the size that matches the mattress. If you hesitate between two sizes, take the one with the deeper edge. That way you avoid a sheet that pops off at night.