If your mattress protector keeps popping off the corners, the problem usually isn't the bed size — it's the depth. Modern mattresses, especially pillow-top and pocket-sprung ones, are far thicker than they used to be, and a standard protector simply can't stretch over them. Here's how to get it right.
UK bed sizes vs mattress depth — they're different things
"King size" tells you the width and length (150 x 200cm). It tells you nothing about how thick the mattress is. Two king mattresses can be 18cm and 32cm deep — and a protector made for the shallow one will never stay on the deep one. Always check both.
How to measure your mattress depth in 60 seconds
- Strip the bed so the bare mattress is exposed.
- Take a tape measure and stand it vertically against the side of the mattress.
- Measure from the very bottom seam to the very top seam — just the mattress, not the divan base underneath.
- Note the number in centimetres. That's your depth.
Most UK mattresses fall between 20cm and 32cm. Anything 28cm or over counts as "deep" and needs a protector with a deep, fully elasticated skirt.
What to look for in a deep-fit protector
- A stated depth rating (e.g. "fits up to 30cm") — not just a bed size.
- A fully elasticated skirt all the way round, not just elastic on the corners.
- A little room to spare — if your mattress is 28cm, a protector rated to 30cm gives a secure grip.
Still slipping off?
If a correctly-sized protector still won't stay put, it's almost certainly not deep enough. Our Extra Deep Fit collection is made specifically for thick mattresses (28cm+), with deep elasticated skirts that grip properly and stay put all night.
Measure once, order the right depth, and you'll never fight with a slipping protector again.