Belledorm's extra deep fitted sheets have a box depth of 38cm (15 inches). This refers to the depth of the sheet's pocket, the measurement from the top sleeping surface down to where the elastic tucks underneath the mattress, not the size of your mattress itself. Standard fitted sheets are typically 30cm deep and suit mattresses up to around 22cm. Belledorm's 38cm extra deep fitted sheets are designed for mattresses up to approximately 30cm deep, including any topper, protector, or pillow top on top. The additional fabric allows the sheet to wrap fully over the mattress and tuck firmly underneath, so it stays secure through the night.

The simplest way is to measure your mattress from its base to the top sleeping surface, including any topper, mattress protector, or pillow top you use. If that total measurement exceeds 22cm, a standard 30cm fitted sheet is unlikely to hold. It will pull away from the corners overnight, leaving the mattress exposed and the sheet bunched up around the edges. As a guide, Belledorm recommends choosing a sheet depth at least 8cm deeper than your mattress measurement, this allows enough fabric to tuck under securely and keeps the elastic anchored rather than stretched. For most modern UK mattresses with a topper added, a 38cm extra deep fitted sheet is the right choice.

Yes, this is precisely what extra deep fitted sheets are designed for. The average UK mattress now sits between 25–30cm deep before a topper is added. A standard memory foam or fibre topper typically adds a further 5–10cm, taking the total stack to 30–40cm. A standard 30cm fitted sheet cannot contain that depth without pulling free at the corners. Belledorm's 38cm extra deep fitted sheets are built to wrap over the combined mattress-and-topper height and tuck securely underneath. If your mattress and topper together measure more than 30cm, an extra deep sheet is the right fit. For very deep combinations, such as a thick mattress with a substantial topper, Belledorm also offers an ultra deep fitted sheet range for mattress depths beyond 38cm.

The difference lies in the sheet's box depth, the measurement of the fabric pocket that fits over the mattress. Belledorm's standard fitted sheets are 30cm deep, designed for mattresses up to around 22cm. Extra deep fitted sheets are 38cm deep, suitable for mattresses up to 30cm including any topper or protector. Ultra deep fitted sheets go further still, accommodating mattress depths beyond 38cm, typically where a deep mattress is combined with a thick topper. Most modern UK beds, particularly those with a memory foam or fibre topper, sit comfortably within the 38cm extra deep range. If you are unsure which depth applies to your bed, measure the full stack, mattress plus all layers, before choosing.

Remove all bedding, but keep any topper, protector, or pillow top in place if you use them, as these add to the overall depth. Use a tape measure and measure from the very base of the mattress, where it rests on the bed frame or divan, straight up to the top of the sleeping surface, including all layers. Note that measurement down. Belledorm recommends choosing a sheet with a box depth at least 8cm deeper than your total mattress measurement, to allow for the tuck-under that keeps the sheet anchored overnight. For a mattress measuring 28cm deep with a 5cm topper (33cm total), Belledorm's 38cm extra deep fitted sheet provides the right fit. If in any doubt, go slightly deeper rather than shallower, a little extra fabric can always tuck away underneath; a sheet that is too shallow will pull free every night.

In most cases, the sheet depth is too shallow for the mattress. This is one of the most common bedding frustrations, and it almost always comes down to sizing rather than sheet quality. As UK mattresses have grown deeper, particularly memory foam and hybrid designs, standard 30cm fitted sheets simply cannot contain the additional height, especially once a topper or protector is added. The elastic stretches over the corners but cannot tuck sufficiently underneath, so the sheet pops free during the night. Belledorm's extra deep fitted sheets are elasticated fully around all four corners, not just at the corners themselves, which provides a consistent, secure hold across the entire mattress perimeter. Choosing the correct depth is the single most effective solution to a sheet that won't stay put. 

Belledorm's extra deep fitted sheet range is available across the full range of UK bed sizes — single, small double (4ft), double, king, super king, and emperor, as well as some non-standard sizes including long single and IKEA European dimensions. The 38cm box depth applies across all sizes in the range. When choosing, match both the width and length to your mattress size as usual, then confirm that the 38cm depth is appropriate for your mattress height. The depth measurement is the same whether you are fitting a single or a super king, it is the height of your mattress stack, not the width or length of the bed, that determines whether extra deep is right for you.

Belledorm's extra deep fitted sheets are available across a broad range of fabric qualities, from practical everyday options through to finer cotton weaves. The range includes polycotton basics at 200 thread count, easy to care for and designed to hold their shape wash after wash, through to Egyptian cotton options at 400, 600, and 1000 thread count. Egyptian cotton is woven from longer-staple fibres, which produces a smoother, more consistent surface than standard cotton, and a sheet that tends to soften further over time rather than wearing thin. The higher the thread count, the denser and more refined the weave, a 1000TC sateen, for example, has a silky, close-woven finish that settles smoothly against the skin. All fabric options in the extra deep range are available at 38cm depth, so fabric choice comes down to how you sleep and what feel suits you best, rather than depth constraints.

Wash at the temperature stated on the care label, most of Belledorm's cotton fitted sheets wash at 40°C or 60°C depending on the fabric. Avoid over-drying at high heat, as this is the most common cause of elastic losing its stretch over time. If you tumble dry, use a low or medium heat setting and remove the sheet while it is still slightly damp to help it smooth out naturally. Cotton can shrink slightly after the first few washes, which is normal and usually settles after the first wash or two. To keep the elastic performing well, avoid wringing the sheet or twisting it tightly when removing it from the wash. With straightforward care, a quality extra deep fitted sheet should hold its fit and its feel for several years of regular use.