GENERIC BIKE COMPUTER
WHEEL SIZE CALCULATOR


Hi! When you're done using this page, check out my new bicycle gear ratio comparison page! Thanks for visiting!

To use the calculator below, select a wheel and tire size from the table or enter the wheel and tire size in mm in the "Dia. (mm)" boxes. It will automatically calculate the estimated circumference in mm (used by most new cycle computers), cm (used by some cycle computers), inches (maybe used by some) and MPH (which is a number used by some Sigma computers which don't automatically convert mm when you want to view your speed in MPH instead of KPH).

Wheel Size
Size Dia. (mm)
Tire
Size Dia. (mm)
Estimated Wheel Diameter/Computer Setting
mm cm inches MPH*

* = Value to enter if you want to see MPH on Sigma Units without Auto Conversion.

Please note: Not all 24" wheels were created equal, try to find the ISO/ETRTO number and use that. Also of interest... 29er wheels are actually the same size as 700c which is nowhere near 29". I'm not sure what marketing genius decided to call them 29ers, but I would think 700c would have gained even quicker acceptance since it was a standard tire size already. I thought it was a totally different size.

The calculator above is based on the ISO/ETRTO (European Tyre and Rim Technical Organization) tire size which specifies tire size as a tire width and rim circumference in mm. A ISO tire size of 28-622 (which of course is backwards (b/c it's European) from how we would refer to a 700 x 28C tire) has a tire width of 28 mm and a rim diameter of 622 mm. To get the approximate diameter of the wheel with the tire on, you would take the diameter of tire x 2 (because it's both on the top and bottom) + the diameter at the rim * pi (3.14) to get the cimcuference. So in this example, it would be ((28 X 2) + 622) * 3.1415 = 2130. This is approximate and rarely matches the tire size listed in any manual, but it was the best I could come up with without reproducing everybody's table individually.


INCHES TO MM CONVERTER


If your wheel size isn't listed in in the drop down above, you can just enter the number into the Dia. (mm) field. If you have inches and need mm, enter them in the following box:

Inches mm

APPROXIMATING WHEEL SIZE USING LINEAR INTERPOLATION


The numbers in the calculator above rarely match what's in any manufacturer's manual, but will give you an approximate tire size. Should you be unable to locate the manual (most manufacturers have their manuals available online). but your manual does not have your exact tire size, enter two tire sizes closest to your tire size and this will calculate the approximate size using linear interpolation. For example, if you're manual had values for 26x2.1 with a setting of 1000 and 26x2.3 with a setting of 2000, but you had 26x2.2 tires, you'd need to set your computer to 1500 (half way between 1000 and 2000). Get it?

From The Manual Your
Tire
Smaller Larger
Tire Size
Setting

ROLLOUT TEST


Another way to set your wheel size without any calculations at all is to measure the distance you travel in one revolution of the tire with it inflated to the pressure you normally would use. This is easier said than done. Two suggested ways to do this are:

  1. Wall
    • Sit on your bike next to a wall.
    • Roll forward till your valve stem is at the bottom of the wheel.
    • Have a friend mark that spot with tape of chalk.
    • Roll forward one revolution of the wheel so the valve stem is at the bottom of the wheel again.
    • Mark that spot with tape of chalk.
    • Measure the distance between the two marks in mm. That's your number!
  2. Paint or Water
    • Put a blob of wet paint on the pavement.
    • Ride over it and keep riding.
    • Each time your tire hits the pavement again, it should leave a mark.
    • You should be able to measure the distance between marks (or between multiple marks for even greater accuracy).
    • Measure that distance in mm. That's your number!

MEASURED MILE


No matter how you set your wheel size, you can improve it's accuracy by riding a measured mile, then adjust your setting based on the following formula (if you computer doesn't read 1.0):

1.0 (the measured mile)
Cyclometer Reading
x Old Setting = New Setting

For example: If you set your 26x2.1 wheel size to: 2091 and rode the measured mile and came up short, maybe your computer read .95 miles, then you'd do:

1.0 (the measured mile)
0.95 (what your computer read)
x 2091 (old setting) = 2201 (new setting)

If you computer read too long, maybe 1.05, you'd still do the same formula and your setting would be:

1.0 (the measured mile)
1.05 (what your computer read)
x 2091 (old setting) = 1991 (new setting)