The most common reason a new mattress doesn't fit is that someone measured the old mattress instead of the bed. Mattresses spread and sag with age, so an old one can be a centimetre or two out in any direction. The bed frame is what the new mattress has to fit, so that's what to measure.
You'll need a tape measure, ideally a metal one, and a couple of minutes.
1. Measure the inside of the frame
Take the old mattress off and measure the space it sits in, from one inside edge of the frame to the inside edge opposite. Measure the width across the middle, then near the head and the foot, and do the same for the length. Frames are rarely perfectly square, so go by the smallest figure.
Write the numbers down in centimetres. Sizes in feet are rounded, but the centimetre figures are exact, so they're the ones to match.
2. Match it to a size
Most UK beds take one of these:
| Inside of the frame | Mattress size |
|---|---|
| 75 x 175 cm | Shorty |
| 75 x 190 cm | Small single |
| 90 x 190 cm | Single |
| 120 x 190 cm | Small double |
| 135 x 190 cm | Double |
| 150 x 200 cm | King |
| 180 x 200 cm | Super king |
A centimetre or so of spare room is normal, because the frame needs a little space for bedding to tuck in. If your figures fall well between two sizes, check the bed's instructions or the original listing, which usually give the mattress size it was designed for. Our size guide has every size in centimetres and feet.
3. Check whether it's a European size
If the inside of the frame is 200 cm long and 90, 120, 140 or 160 cm wide, you have a European bed. They're common among flat-pack beds and children's beds from European brands. A UK mattress on a European frame leaves a gap of about 10 cm at the foot, and sometimes across the width too. If your current mattress leaves bare slats at one end, that's usually why. Our European mattresses page covers every size.
4. On a raised bed, measure the depth limit
Bunk beds, high sleepers, mid sleepers and cabin beds have a maximum mattress line marked on the frame. Measure from the base the mattress sits on up to that line. That's the deepest mattress the bed can take while keeping the guard rail high enough, so treat it as a limit rather than a guide.
On many raised beds the line allows 15 to 16 cm, but some allow less, so the line on your own bed is the number that counts. Everything under the fitted sheet counts towards it, including a topper. If you need a slimmer mattress, everything in our thin mattresses range is 16 cm deep or less.
5. For a trundle, measure the drawer
A trundle mattress has to fit inside the pull-out drawer and still slide back under the main bed. Measure the inside of the drawer for the width and length, then measure the gap between the drawer's base and the underside of the bed above it. That gap is your maximum depth. Our trundle mattresses are made for this.
6. On a child's bed, never size down
It's tempting to fit a slightly smaller mattress into an odd-sized frame, but on a child's bed a gap between the mattress and the frame is somewhere small arms and legs can get stuck. If the frame is an awkward size, find a mattress that fills it properly. That's why we stock awkward sizes.
Not sure? Send us your numbers
If your measurements don't match a size, send us the width and length and, for a raised bed, the depth to the line. We'll tell you which mattresses fit. You'll find our details on the contact page.