Sheldon Brown's
Most bicycle computers use one of six different calibration systems to allow the user to tell the computer what size wheel the bicycle uses.I have used the letters A through F to designate the six different systems in common use.
To find your calibration number, refer to the list below to find which is used by your computer. Then, use the chart to find the appropriate value for your tire size (and whether you want readings in miles or kilometers.)
If you click on the name of your cyclometer model, you will go to a specific chart for that calibration family.
In addition to the raw calibration numbers, I have on-line instructions for some models. If your computer model name is highlighted on its calibration chart, that is a link to the Calibration Procedure Instructions Page entry that applies to that model.
If you want to print out a general version of the Cyclometer Calibration Chart, click here.
These values will give a pretty good approximation, usually within 1-2%. If you have an tire size that is not listed, interpolate (split the difference) between the next larger and next smaller sizes listed.
For higher precision, refer to the sections on:
This site also has pages on:
| Tire Size | ISO | Group A | Group B | Group C | Group D | Group E | Group F |
|---|---|---|---|---|---|---|---|
| 700 X 44 | 44-622 | 87.55 | 236 | 222 | 354 | 1382 | 2224 |
| 700 X 38 | 38-622 | 85.82 | 231 | 218 | 347 | 1355 | 2180 |
| 700 X 35 | 35-622 | 84.21 | 230 | 217 | 345 | 1347 | 2168 |
| 700 X 32 | 32-622 | 83.22 | 227 | 216 | 342 | 1339 | 2155 |
| 700 X 28 | 28-622 | 82.55 | 225 | 214 | 336 | 1327 | 2136 |
| 700 X 25 | 25-622 | 82.12 | 223 | 211 | 335 | 1308 | 2105 |
| 700 X 23 | 23-622 | 81.56 | 222 | 210 | 333 | 1302 | 2097 |
| 700 X 20 | 20-622 | 81.02 | 221 | 209 | 332 | 1296 | 2086 |
| 27 X 1 3/8 | 35-630 | 85.08 | 232 | 217 | 345 | 1349 | 2169 |
| 27 X 1 1/4 | 32-630 | 84.33 | 230 | 216 | 343 | 1343 | 2161 |
| 27 X 1 1/8 | 28-630 | 83.58 | 228 | 216 | 342 | 1339 | 2155 |
| 27 X 1 | 25-630 | 82.91 | 226 | 215 | 340 | 1333 | 2145 |
| 26 X 2.125 | 54-559 | 82.12 | 225 | 207 | 330 | 1286 | 2070 |
| 26 X 1.9 | 47-559 | 80.63 | 220 | 206 | 324 | 1276 | 2055 |
| 26 X 1.5 | 38-559 | 77.71 | 212 | 199 | 312 | 1234 | 1985 |
| 26 X 1.25 | 32-559 | 77.44 | 206 | 195 | 311 | 1213 | 1953 |
| 26 X 1.0 | 25-559 | 75.31 | 205 | 191 | 305 | 1189 | 1913 |
| 26 x 1/650C | 25-571 | 76.85 | 206 | 195 | 311 | 1213 | 1952 |
| Tubular | Wide | 83.34 | 224 | 212 | 338 | 1316 | 2117 |
| Tubular | Narrow | 82.12 | 223 | 210 | 335 | 1308 | 2105 |
| 26 X 1 3/8 | 35-590 | 81.41 | 222 | 207 | 330 | 1288 | 2068 |
| 24 | Most | 75.43 | 205 | 192 | 305 | 1191 | 1916 |
| 24 x 1 | 25-520 | 69.01 | 188 | 175 | 279 | 1089 | 1753 |
| 20 X 1.75 | 44-406 | 60.15 | 158 | 150 | 254 | 927 | 1491 |
| 20 X 1 1/4 | 28-451 | 63.70 | 173 | 162 | 257 | 1005 | 1618 |
| 17 x 1 1/4 | 28-369 | 52.17 | 142 | 133 | 211 | 838 | 1325 |
| 16 x 1 3/8 | 35-349 | 50.47 | 137 | 128 | 204 | 797 | 1282 |
| Formulas: | Circum. inches | Circum. inches X 2.723 | Circum. cm | Radius mm | Circum. mm X .621 | Circum. mm |
If you have imformation on any newer models or others that I might have missed, please let me know, so that I can update later editions of this listing.
E-Mail: CaptBike@sheldonbrown.com
The values on the chart will generally give a value accurate to within one or two percent, which is more than good enough for the vast majority of cyclists, and is much more accurate than most automobile odometers. If you require more accuracy, you can do a "roll-out" test. Since the effective tire size is affected by tread thickness, tire pressure, and rider weight, the rolling circumference should be measured by rolling the bike with the rider aboard for one wheel revolution.You may use the valve stem as a reference, starting the roll with the valve right over a perpendicular line on the floor, and ending when the valve is back at its low point one revolution later.
Another approach is to put a small dot of paint on the tire and measure the distance between the marks that the paint prints on the road. With either approach, the rider must hold the handlebars absolutely straight while an assistant balances and pushes the bike. Otherwise, the wheel may not follow a straight path.
Once you have measured the rolling circumference, use the formula indicated to find the calibration number for the cyclecomputer involved. For cyclecomputers that require a radius value, divide the measured circumference by 6.283 (2x) to get the radius.
| A | Circumference in inches |
|---|---|
| B | Circumference in inches X 2.723 |
| C | Circumference in centimeters |
| D | Radius in millimeters |
| E | Circumference in millimeters / 1.6093 |
| F | Circumference in millimeters |
If you have access to a measured course, you can fine tune your settings by riding the known distance and checking the reading. Divide the actual distance by the cyclometer's mileage reading, then multiply your calibration number by the result to get a corrected calibration number.
| Actual Distance Cyclometer Reading | X Old Calibration Number = New Calibration Number |
|---|
The I.S.O. tire size consists of a tire width and a bead seat diameter. Both of these numbers are in millimeters. For example, a 28-622 (700 x 28C) tire has a nominal width of 28mm on a rim with a bead seat diameter of 622 mmTo get an approximate diameter (in mm) add the bead seat diameter to twice the tire width (since the tire comes into the diameter twice: 622 + (28 X 2) = 678. Multiply this by pi (3.142) to get the circumference in mm (F) 2130. Appropriate calculations will yield calibration numbers for computers in other groups.
(Thanks to Chris Ziolkowski for suggesting this.)
I would like to thank those who helped with updates and error spotting, including: Miha Ambroz, Badarka@aol.com, DaveM10@aol.com, Richard Drdul, Peter Epstein, John Everett, William Fallon, Rich Kim, Doug Milliken, Richard Nelson, Dave Poleshuck, Liam Relihan, Cliff Schlueter, Rich Shapiro, Emil Sit, Steven Sheffield, Adam Spiers, Rick Teichler, Jack Tingle, Kris Vlæminck and Desmond Walsh.
since December 26, 1995