High school geometry class gives the formula for the circumference of a circle as:

C = 2(pi)R

pi = 3.142
R = radius.

If you measure diameter instead of radius, then use the formula C = piD

Since pi is a ratio, circumference will end up being in whatever unit you measure the radius or diameter in.

Do not forget to take your measurements from the center of the hub to the edge of the tire itself (for radius), or from the outer edge of the tire, across the center of the hub, then to the outer edge of the opposing outer edge of the tire (for diameter) - and not simply the edge of the rim.

Or if you do not want to mess around with math, just use a quick reference table. It will be accurate enough for what you want to find out.

My GPS bike computer uses magic to guess my wheel size by counting revolutions and comparing that to the speed as measured by the GPS. It guessed correctly, even in spite of the error in GPS signals and possible errors in circumference tables. Nifty.