The most effective way to measure a mattress is to lay it flat on the ground or frame and use a tape measure to find the distance for the width and length, up to the edge of the fabric. To measure the depth of the mattress, place it on the ground, and use a ruler or tape measure.
Placing the mattress on a flat surface low to the ground is helpful when taking measurements because it removes the possibility of outside factors affecting the measurements. Tape measures are also useful because they are typically long enough to track the full distance of each dimension with a single action. Measure from the outer edge of the seam, rather than the inside, in a straight line. When measuring the depth, start at the ground or as close to the bottom of the mattress as possible.
Most mattresses use a uniform sizing system to express size, which ranges from Twin, the smallest, to King and California King, the largest. Though these terms represent a standardized sizing system, finding the exact measurements of a mattress is often helpful in determining compatibility with accessories, such as sheets, blankets and pillows. Length and width are the most important measurements, as they represent the total surface area of the unit that supports sleeping. Some mattresses may feature varying depths, which also depend on the mattress material and structuring.