A bed can have one, both or neither, and the terminology is not helpful: a flat sheet is flat, but a fitted sheet is not "fitted" in any obvious sense until you see the elastic.
The short answer
A fitted sheet has elastic and goes directly on the mattress. A flat sheet is a plain rectangle that goes on top of you and tucks in. You need a fitted sheet. A flat sheet is optional, and most UK households skip it.
- What is a fitted sheet?
- What is a flat sheet?
- The difference
- Do you need both?
- What "deep" means
- Ironing
- Why it comes off
- FAQs
What is a fitted sheet?
A fitted sheet is the sheet that goes directly onto the mattress. It is sewn with elasticated corners — or, on better ones, elastic running the whole way round — so it grips the mattress and stays put. It has a shaped skirt on all four sides, which is why it looks like a shallow box rather than a rectangle when you hold it up.
Its job is to protect the mattress and give you a smooth surface to sleep on. You never tuck a fitted sheet in; the elastic does that work.
What is a flat sheet?
A flat sheet is exactly what it sounds like: a plain rectangle of fabric with no elastic and no shaping. It goes on top of you, between your body and the duvet, and is tucked in at the sides and foot of the bed.
On the mattress
Elasticated, shaped, never tucked. Must match your mattress depth as well as its size.
On top of you
A plain rectangle, tucked in by hand. Depth is irrelevant — one size covers any bed of that width.
In the UK the flat sheet is the less common of the two. Most British beds use a fitted sheet and a duvet with a cover, and nothing in between. In the US, and in hotels almost everywhere, the flat sheet is standard.
Flat sheet vs fitted sheet: the difference
| Fitted sheet | Flat sheet | |
|---|---|---|
| Shape | Box-shaped with a skirt | Plain rectangle |
| Elastic | Yes — corners or all round | None |
| Where it goes | Directly on the mattress | Between you and the duvet |
| Stays put by | Elastic gripping the mattress | Being tucked in |
| Depth matters? | Yes — must match your mattress | No |
| Common in the UK? | Yes, nearly universal | Less common |
| Easy to fold? | No — the elastic fights you | Yes |
The practical difference is depth. A flat sheet in double will cover any double bed, because it is just a rectangle. A fitted sheet has to match your mattress depth as well as its width and length, or it will not stay on. That single fact causes most of the trouble people have with them.
Do you need both?
You need a fitted sheet. It is what stops you sleeping directly on the mattress, and it is what gets washed. A flat sheet is optional and largely a matter of habit.
Reasons to add one
- Keeps the duvet cover cleaner, so you wash a large heavy item less often
- A lighter layer to sleep under in summer
- What you are used to if you grew up outside the UK
Reasons to skip it
- One more thing to wash, iron and tuck in every morning
- A duvet cover already does the protective job
- Most UK households manage perfectly well without
What is a deep fitted sheet?
A deep fitted sheet is the same thing with a taller skirt, made for thicker mattresses. The words are used loosely and there is no legal standard, but in practice:
A plain mattress with nothing on top of it.
A thicker mattress, or a standard one with a thin topper.
A deep mattress, or any mattress with a proper topper.
Check the number, not the word. Because the labels are inconsistent between brands, "deep" on one site is "standard" on another. And measure with your topper on — a topper adds 5cm to 8cm, which is enough to take a sheet out of range on its own. Our fitted sheet size guide covers how to measure properly.
How to iron a fitted sheet
Most people do not need to. Take it out of the dryer while it is still faintly warm, shake it out and smooth it flat with your hands, and the creases drop out on their own. Putting it straight onto the bed does the rest — the tension of the elastic pulls the surface taut.
If you do want to iron one, work in sections. Lay a portion flat on the board with the elastic edge hanging off, press it, then rotate. Iron around the elastic rather than over it: direct heat degrades it, and a stretched-out fitted sheet is finished. Microfibre and polycotton need a cooler setting than pure cotton and generally do not need ironing at all, which is much of the reason they are popular.
Why does my fitted sheet keep coming off?
Nine times out of ten this is a depth problem rather than a quality problem. The sheet is too shallow for the mattress, so the corners sit under constant tension and the first sharp movement pulls one free.
Measure your mattress depth with the topper on and size up. The other factor is the elastic: sheets with elastic only at the corners have four points of grip and nothing along the sides, so they work loose far more readily than fully elasticated ones, where the tension runs the whole way round.
Frequently asked questions
What does fitted sheet mean?
It means a sheet shaped to fit a mattress, with elasticated edges that hold it in place, rather than a flat rectangle that has to be tucked in. The "fitted" refers to the shaping, not to how tightly it fits.
Which goes on first, the flat sheet or the fitted sheet?
The fitted sheet goes on first, directly onto the mattress. The flat sheet goes on top of it, and you lie between the two.
What is the difference between a fitted sheet and a deep fitted sheet?
Only the depth of the skirt. A standard fitted sheet is cut for roughly 25cm, a deep one for 30cm, an extra deep one for 40cm. Width and length are the same for a given bed size.
Can I use a flat sheet instead of a fitted sheet?
You can tuck a flat sheet around a mattress, and people did before fitted sheets existed. It will not stay smooth — without elastic it works loose and rucks up overnight, which is precisely the problem fitted sheets were invented to solve.
Is a fitted sheet the same as a valance sheet?
No. A valance sheet has a decorative skirt that hangs down over the bed base to hide it, and often has a flat platform rather than an elasticated one. A fitted sheet is purely functional and sits on the mattress itself.
How often should you wash a fitted sheet?
About once a week. It is in direct contact with skin all night, which is a good argument for owning two — you can strip and remake the bed in the same five minutes.
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