A bedsheet starts with the mattress measurements
The size name on the bed is less useful than three measurements and a look at the layers underneath.
Three numbers before a colour
Write down the mattress length, width and depth before opening a bedsheet catalogue. Names such as double, queen and king are convenient shortcuts, but they are not a substitute for the dimensions used by a particular manufacturer. Two sheets with the same size name can be intended for different mattresses.
Measure the mattress itself rather than the outer bed frame. Include a topper if it stays beneath the sheet, and keep the protector in mind when checking the finished depth. A sheet that fits the bare mattress tightly may struggle once the usual layers are back in place.
Fitted and flat solve different problems
A fitted sheet needs the right length and width plus enough pocket depth to reach around the mattress. Check the stated maximum mattress thickness and any guidance about the fabric's stretch. More depth is not always better: a very deep pocket on a thin mattress can leave surplus fabric that needs constant adjustment.
For a flat sheet, think about the overhang you want and the amount available to tuck underneath. A wide bed placed against a wall may be difficult to dress even when the sheet is generously sized. Measure the access as well as the mattress, especially if lifting a heavy corner is uncomfortable.
If you are replacing a sheet that works well, measure it after washing and compare those dimensions with the new product's specification. Keep in mind that a manufacturer may quote an unwashed size or give a shrinkage allowance. Read how the size is described rather than assuming every listing uses the same convention.
Check the whole set
A photograph of a made bed can show pillows, a duvet and decorative cushions that are not included. Read the contents list. Count the pillowcases and compare their measurements with your actual pillows. An attractive set loses some of its appeal if you immediately need to buy another pair of cases.
Think about how the sheet will be washed and dried. A fabric that asks for a routine you cannot manage is not a bargain merely because it fits today. Check the care label before buying, and follow it on the first wash. Do not increase the heat in an attempt to make an oversized sheet shrink to an exact size.
Keep the useful details
Save the mattress measurements in a note on your phone and add the model and size of a sheet that fits successfully. If you have several beds, identify which measurements belong to which room. This prevents a later order being based on a vague memory of the word 'king'.
When the new sheet arrives, check the label and dimensions before removing packaging you may need for a return. Follow the retailer's inspection and return conditions. A quick fit check is easier than discovering after washing that the sheet was intended for a different depth of mattress.
