The most obvious way of measuring the circumference of your wheel is by rolling it across the floor. Stick a strip of masking tape across the tyre so you'll know exactly when one revolution has been made. The slightest wobble off a straight line will result in a wrong measurement so best use a tiled floor (pencil marks come off more easily, too).
There is a far simpler way: measure the diameter of the wheel in the same way that you measure a person's height. Click photo for enlargement.
Multiply the wheel's diameter by pi to find its circumference – just use your calculator.
681mm (tyre diameter) x pi = 2140mm (circumference)