It is the circumference of the wheel in millimeters. You can measure this yourself as each wheel can vary slightly depending upon the tire used, air pressure, tread wear, and rider weight. You can use the "roll out" method: put the valve in the six o'clock position, roll the bike forward in a straight line until the valve is back to the six o'clock position and measure the length between the two points. If you measure in inches you can multiply by 2.54 to get centimeters or 25.4 to get millimeters. For more detailed instruction read Sheldon Brown's great webpage on the subject here: http://www.sheldonbrown.com/cyclecomputer-calibration.html