Read the side of your tire for its circumference. Look for numbers such as 26 or 700; the former is a common tire size for mountain bikes, while the latter is common to road bikes. Most road bike wheel sizes start with 700, but other common sizes for mountain bikes include 29, 24, 16 and 12. Compare this number to the chart on page 15 in the Sigma manual (see link in References section) to determine tire size.
Stand your wheel up on the ground. Measure the height of the wheel from top to bottom with your millimeter tape. Multiply the height by 3.14 on your calculator. Input this number as the wheel size in your Sigma computer.
Lay the millimeter tape on the ground. Place your wheel at the end of the millimeter tape with the valve stem at the bottom of the wheel. The valve should point 90 degrees upward. Roll the wheel along the tape one full revolution until the valve once again points upward. Measure the number of millimeters the wheel covered in that roll, and input that number as the wheel circumference into your computer.