Place the end of the measuring tape at the head of the mattress. Have someone hold it in place, or find something heavy to hold it down. Unroll the tape until you reach the foot of the mattress and make your measurement. For United States queen sized beds, this should equal approximately 80 inches long.
Perform the same action for the width, measuring from one side of the mattress to the other. For United States queens sized beds, this should equal approximately 60 inches long.
Pick one of the four corners of the bed and place the end of your measuring tape at the bottom of the corner. Unroll the tape and make your measurement at the top of the corner to get the depth of the bed.