A king size mattress measures 76 inches wide by 80 inches long. It is the widest standard mattress size available, making it a good choice for couples who want more space to spread out or for those who sleep with their children or pets. Single sleepers may also enjoy the spaciousness of a king mattress. It is important to note that a king size bed has a surface area of 6,080 square inches, which means it requires a significant amount of floor space to ensure there is enough room for the mattress.