How to Measure a Mattress
Before measuring your mattress, take off any blankets, sheets, or pillows on it and then measure the height, width, and length of the mattress. If you're measuring for new bedding, and you have a mattress topper on your mattress, include the topper in your measurements. When you're buying a new mattress, measure any doorways or halls you'll need to carry it through when you bring it home to make sure it will fit.
Take off any sheets, blankets, or pillows on your mattress. You'll get the most accurate measurements when your mattress doesn't have anything on it. If you're measuring your mattress for fitted sheets or a comforter, and you have a padded mattress topper on your mattress, leave it on when you do the measuring. Measure from the left side to the right side of your mattress to get the width. Use a tape measure or fabric ruler to do your measuring. Mattresses can curve outward on the sides, so make sure you're measuring from the widest point on each side of the mattress. Write your measurements down on a piece of paper as you go so you don't forget them! Measure from the back to the front edge of your mattress to get the length. If your mattress curves outward at the back and front edges, measure from the furthest point out on each edge. Measure from the bottom to the top of your mattress to get the height. If you're measuring for fitted sheets or a comforter, and you have a padded mattress topper, include the mattress topper in your measurement. Otherwise, only measure the height of the mattress itself. Use the width and length of your mattress to determine what size it is. There are 6 standard mattress sizes: twin, twin XL, full, queen, king, and California king. Knowing the size of your mattress will help you choose bedding that fits. The different mattress sizes are based on how wide and long a mattress is. Twin mattresses are 38 inches (97 cm) wide by 75 inches (190 cm) long. Twin XL mattresses are 38 inches (97 cm) wide by 80 inches (200 cm) long. Full mattresses are 53 inches (130 cm) wide by 75 inches (190 cm) long. Queen mattresses are 60 inches (150 cm) wide by 80 inches (200 cm) long. King mattresses are 76 inches (190 cm) wide by 80 inches (200 cm) long. California king mattresses are 72 inches (180 cm) wide by 84 inches (210 cm) long. Measure the doorways and halls in your home before you buy a new mattress. This measurement is important because you need to know whether the new mattress will fit through them when you bring it home. Choose a mattress with a width that's smaller than the smallest door or hallway you'll need to carry it through. Check for any low-hanging light fixtures that might get in the way when you're moving in your new mattress. Measure the width and height of any staircases you'll use when you're moving the mattress. Measure a bed frame before you buy it to see if your mattress will fit. Bed frames are sized just like mattresses, but the sizing can vary between manufacturers. When you find a bed frame you like, measure the width, height, and length of the part of the bed frame your mattress will sit in. If the width or length is less than the width and length of your mattress, your mattress won't sit in the bed frame properly. If the part of the bed frame your mattress will sit in has a greater height than your mattress, your mattress will be too low in the frame. 
Take off any sheets, blankets, or pillows on your mattress. Measure from the left side to the right side of your mattress to get the width. Measure from the back to the front edge of your mattress to get the length. Measure from the bottom to the top of your mattress to get the height. Use the width and length of your mattress to determine what size it is. Measure the doorways and halls in your home before you buy a new mattress. Measure a bed frame before you buy it to see if your mattress will fit. 