To make sure a twin comforter will fit your bed, you should get the exact measurements of your mattress. A standard twin size mattress is 39 inches wide and 75 inches long. A twin extra-long mattress is 39 inches by 80 inches long. Twin comforters range in size from 66 to 68 inches wide and 86 to 88 inches long. Determine the exact size you need in a comforter to get the amount of coverage you want.

Since mattress depths vary by manufacturer and design, you can't just assume that any twin comforter will fit your twin mattress. For instance, many mattresses have an additional pillow top added for comfort, and that can add two or more inches to the mattress depth. Just adding a couple of inches of pillow top will reduce the amount of coverage you'll get from a comforter.

Another factor that will reduce the coverage is the type of comforter you buy. For instance, a down comforter or other very fluffy comforter material will 'puff' up rather than lay flat on the bed, thus, reducing the amount of coverage. You should allow a few extra inches in the size of the comforter to cover the sides of the bed.

Many twin comforters are part of a bed set that includes the sheets, shams, pillow cases, and skirt. This is a convenient way to color coordinate all bedding; however, it is particularly important that you pay attention to the measurements of the twin comforter in combined sets to make certain it is the size you need.