Put the watermelon on a flat surface where it is not able to roll off and break.
Choose one section of the watermelon to measure. Each end of the watermelon tapers from its center. Therefore, its circumference gradually decreases from its middle outward.
Place the measuring tape flat on the table. Roll the watermelon onto the tape at the point you want to measure.
Press the beginning of the tape at one point on the watermelon, then bring the measuring tape around the width of the watermelon until it overlaps.
Take the circumference reading. Look at the number on the measuring tape at the point where the beginning crosses over with the other end. This is the circumference of the watermelon.