How to Measure Mattress Depth (and Which Fitted Sheet Depth You Need)
Quick answer: measure your mattress depth from the bottom seam to the top seam at the side of the mattress, including any topper or mattress protector you use. Then choose a fitted sheet whose pocket depth is the same or a few centimetres deeper. Most standard UK mattresses (up to about 25 cm deep) suit a 25 cm pocket; thicker mattresses, pillow-tops and beds with a topper need an extra deep 30–40 cm pocket.
Why mattress depth matters
Most "my fitted sheet keeps popping off" problems are not about width or length – they are about depth. If the pocket is shallower than the mattress, the elastic cannot reach underneath, so the corners ping off as soon as you move. If you have added a topper, your bed may now be 10 cm deeper than the sheets you already own.
How to measure mattress depth (step by step)
- Strip the bed down to the mattress, but leave on anything that always stays on it – a topper or a mattress protector.
- Stand at the side of the bed, not the corner (corners are often rounded and give a smaller reading).
- Hold a tape measure upright against the side, starting at the bottom seam where the mattress meets the base.
- Read the number at the top seam – or at the top of the topper, if you use one.
- Measure in two places (middle of the side and near the foot) and use the bigger number. Older mattresses are often thicker at the edges than in the middle.
Which fitted sheet depth do you need?
| Your total mattress depth | Typical bed | Fitted sheet pocket to choose |
|---|---|---|
| Up to 20 cm | Budget or children's mattress, bunk beds | Standard or 25 cm pocket |
| 20–25 cm | Most open-coil and standard pocket-sprung mattresses | 25 cm extra deep |
| 25–30 cm | Thicker pocket-sprung, hybrid and memory foam mattresses | 30 cm or deeper |
| 30–40 cm | Pillow-top mattresses, or any mattress plus a topper | 40 cm extra deep |
Rule of thumb: pick a pocket depth equal to your mattress depth or up to 5 cm more. Slightly too deep is fine – the elastic gathers the extra fabric underneath. Too shallow never fits.
Don't forget toppers and protectors
A topper adds its full thickness to the bed. Our 4-inch (10 cm) mattress topper on a 25 cm mattress makes a 35 cm bed – so a 25 cm fitted sheet will no longer fit, but a 40 cm extra deep fitted sheet will. Mattress protectors add only a few millimetres, but they need a deep enough skirt too – SAIMAZ quilted and bamboo protectors fit mattresses up to 30 cm deep.
Which SAIMAZ sheet fits your mattress?
- 25 cm Extra Deep Fitted Sheet – for mattresses up to about 25 cm. Polycotton, all-round elastic, Single to Super King.
- 40 cm Extra Deep Fitted Sheet – for deep mattresses, pillow-tops and beds with a topper. Single to Super King.
- Extra Deep Frilled Valance Sheet – a fitted sheet with a 25 cm frill to hide the bed base.
- Our bedding sets with a fitted sheet (for example the satin 6-piece set) have a 25 cm pocket – check your depth first if you use a topper.
How to put a deep fitted sheet on so it stays put
- Find the label or the seam – it usually marks the foot end or a long side.
- Fit the two top corners first, pulling the elastic right under the mattress.
- Fit the two bottom corners, lifting the mattress slightly if it is heavy.
- Run your hand along each side so the elastic sits evenly underneath.
Frequently asked questions
How deep is a standard UK mattress?
Most UK mattresses are between 20 and 30 cm deep. Pillow-top and luxury hybrid mattresses are often 30–35 cm, and adding a topper can take the total to 40 cm or more.
Can I use a 40 cm fitted sheet on a 25 cm mattress?
You can, but the extra fabric will gather underneath and the top may look slightly loose. A 25 cm or 30 cm pocket gives a neater finish on a 25 cm mattress.
What does "extra deep" mean?
There is no official standard, but in the UK "deep" usually means a pocket of around 25 cm and "extra deep" means 30 cm or more. Always check the pocket depth in centimetres rather than relying on the name.
Do I measure with the topper on?
Yes. The fitted sheet has to go over everything on top of the mattress, so measure the total height including the topper.
Find the right fitted sheet
Browse all SAIMAZ fitted sheets in 25 cm and 40 cm pocket depths, in Single, Double, King and Super King. Not sure which size your bed is? Read our UK bed sizes guide. Free UK delivery in 2–4 working days and free 30-day returns.
