Mattress sizing in the United States is standardised in name only. Dimensions vary by up to an inch between manufacturers, and profile height — the thickness — is not standardised at all, which is why sheets that fit one 14-inch mattress will not stay on another.
Here are the numbers, and the one sizing decision most couples get wrong.
The standard sizes
| Size | Dimensions (in) | Dimensions (cm) | Sleeps | Minimum room |
|---|---|---|---|---|
| Twin | 38 × 75 | 96 × 191 | 1 child or adult | 7 × 10 ft |
| Twin XL | 38 × 80 | 96 × 203 | 1 tall adult | 8 × 10 ft |
| Full (Double) | 54 × 75 | 137 × 191 | 1 adult comfortably | 10 × 10 ft |
| Queen | 60 × 80 | 152 × 203 | 2 adults | 10 × 10 ft |
| King | 76 × 80 | 193 × 203 | 2 adults comfortably | 12 × 12 ft |
| California King | 72 × 84 | 183 × 213 | 2 tall adults | 12 × 12 ft |
Split King (two Twin XL mattresses side by side, 76 × 80 in total) and Split California King are the same footprints assembled from two independent halves.
The mistake: buying quality over space
Two adults on a Queen have thirty inches each. That is less width than most people have on an aeroplane in premium economy, and it is the reason couples report waking each other even on a mattress with excellent motion isolation — they are simply close enough to make contact.
If you are choosing between a King at $1,400 and a Queen at $2,400, and you currently sleep on a Queen with a partner, take the King. In our shared-bed testing, the eight inches of extra personal width made a larger difference to reported sleep quality than the difference between our lowest- and highest-scoring mattresses.
Split sizes, and when they are worth it
A Split King is two Twin XL mattresses on one base. Its two real advantages:
Different firmness per side. If one of you needs a 5 and the other a 7, this is the only construction that gives you both. It resolves an argument that no compromise firmness ever really settles.
Independent adjustable bases. If either of you wants a head-raise for reflux, snoring or reading, you need a split.
The cost is a seam down the middle. Most couples stop noticing it within a fortnight; some never do. There is also no standard Split Queen — if you want split firmness, King is the entry point.
Profile height, the unstandardised dimension
Mattress thickness ranges from about 10 to 15 inches with no standard at all, and it affects three things:
- Sheets. Deep-pocket sheets are rated to a maximum depth. A 15-inch mattress needs sheets rated to 16 or they will pop off the corners nightly.
- Bed height. Mattress plus base should put the top surface at roughly knee height when you stand beside it, around 24 to 26 inches for most people. A 15-inch mattress on a 14-inch frame is too tall for a lot of people to sit down on comfortably.
- Nothing about quality. Thickness is not a quality signal. Our highest-scoring mattress is 11.5 inches; one of our lowest is 14.
Foundation requirements
Whatever size you buy, the base matters more than most people expect. Slat gaps over three inches will let any mattress sag between them under load, and it voids the warranty on every brand we track.
If your existing frame has wide slats, a bunkie board is a $60 fix that protects a $2,000 purchase. For heavier sleepers this is not optional — see the the best mattresses for heavier sleepers guide for why.
Choosing, in one paragraph
Sleeping alone: Twin XL if you are tall and short of space, Full if you have the room. Sharing: Queen is the sensible default, King is the upgrade that is genuinely worth the money, California King only if one of you is over 6 ft 2 in. Different firmness needs: Split King. Then, and only then, choose the mattress — the the best mattresses under $1,200 guide has our picks if size has eaten the budget.