How to measure a mattress for custom bedding
Measure the mattress itself — width, length and depth — not the old sheet or the bed frame.
Start with the mattress, not the bed frame and not the sheet you already own. Those two numbers are almost always different.
Measure width across the sleeping surface, length from head to foot, and depth from the top fabric down the side — including any topper that stays on overnight.
Write the three numbers in centimetres or inches. For a duvet cover, measure the insert. For pillowcases, measure the pillow.
Use the Simple ol' Me calculator, or a regional preset if your mattress is a published size. Leave the fields blank until you have the tape in your hand.