A cot, also known as a baby crib in the United States, requires close measurements to ensure that you purchase the proper sized mattress to fit inside. A mattress too loose inside the crib can trap the baby against the bars or smother the child. Carefully measuring and checking the measurements for a cot mattress will prevent potential tragedy.
Measure the inside of the cot along the length in inches. Typical cot mattresses measure 51 5/8 inches in length, according to Babies R Us.
Stretch the measuring tape across the width of the cot to measure the width. Most cot mattresses have a standard measure of 27 ¼ inches, according to Babies R Us.
Make a selection of cot mattress based on that closest to your measurements, but do not get one more than 1 ½ inches -- or about two adult finger widths -- shorter than either the length or width.
Place the mattress into the crib, and check for a snug fit by inserting two fingers between the mattress and the bars on all sides. If you cannot fit two finger widths in the space, you purchased the correct cot mattress.