The elasticated edge should pass underneath the mattress on all four sides. It grips the mattress, rather than attaching to the cot rails, so the sheet needs enough depth to wrap around the sides and hold below the sleeping surface. A sheet that is too small may pull at the corner seams or spring loose as the child moves. One that is too large can leave folds beneath the mattress and may not remain evenly anchored. The right fit looks fairly ordinary: the fabric lies smooth on top, each corner is seated properly, and the elastic is stretched enough to hold without visibly straining.
Mattress depth deserves the same attention as length and width. Pocket depth describes the distance from the sheet’s upper edge down to its elasticated hem. If that pocket is shallow, it may fail to cover the mattress sides. If it is much deeper than needed, surplus material can gather underneath and make the sheet harder to position neatly. Measure the mattress with the protector or pad that will stay on it. A waterproof layer adds thickness, and a quilted cover can create a raised edge that changes how the sheet sits. Keep the usual layers together when checking a replacement.
For a reliable fitting routine, remove the mattress from the cot and place it on a clean, flat surface. Put the sheet over it with the seams aligned to the four corners, then work around the mattress rather than stretching one corner as far as possible before tackling the others. Check that no seam has twisted onto the top surface. If a corner feels unusually tight, stop and recheck the measurements instead of forcing the elastic. Once the mattress is returned to the cot, run a hand across the sleeping surface and inspect each edge for exposed mattress, raised ridges or loose fabric.
Washing can alter the fit, particularly if the fabric is exposed to more heat than the care instructions allow. Cotton may contract, although the amount depends on the fibre, weave and laundering method. Wash a new sheet before using it so you can assess its post-wash dimensions rather than relying on how it looked in the packet. Check the care label before selecting a wash or drying cycle, and avoid treating a difficult corner as a sign that the mattress should be compressed. If the sheet no longer fits comfortably after laundering, replace it rather than stretching it into position.
Suppose the sheet fitted well in the afternoon but a corner is loose by morning. First remove the mattress and examine the protector, pad or cover for a fold that has formed a thicker ridge. Then refit the sheet on the flat mattress, making sure the corner seams meet the corresponding corners. Inspect the elastic for lost stretch, broken stitching or sections that have pulled away from the hem. A sheet can also appear incorrectly sized when it has been placed over a shifted layer. A quick check of the mattress and its layers before remaking the cot often prevents repeated adjustments later.
The sleeping surface should be smooth, with no loose fabric available to gather beneath or around the child. Do not add pins, clips, ties or improvised fasteners to keep a sheet in place, and do not tuck spare material around the sleeping area. If the sheet repeatedly comes away despite matching measurements, retire it and use a replacement that fits the mattress without excessive tension. Keeping one clean spare sheet in the cot room is a practical habit after illness, spills or overnight accidents. For a starting point, compare fitted cot sheets with the dimensions and bedding layers you actually use.
Storage affects condition as well as cleanliness. Let the sheet dry completely before folding it, since storing damp fabric can create an unpleasant smell and may weaken fibres over time. Before putting it away, check the elastic and stitching while the sheet is flat, when small faults are easier to spot. Read the care instructions again if the fit has changed after washing, and keep the original product information if it lists a particular mattress size or depth. For related measurements and bedding decisions, the cot sheet fitting advice can be a useful reference. The practical test remains simple: the sheet must cover the mattress, stay anchored underneath and remain flat after laundering.
