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.

If you have a computer model that's not listed, here, most likely it uses one of the 6 calibration schemes shown. Remove the batteries, wait a few minutes and reinstall them. A calibration number will usually appear. This default value will normally be for a tire in the size range of normal full-sized tires, and if you examine the chart, you should be able to figure out which calibration group to use.

Most manufacturers use the same calibration formula for all models, so if your make is listed, but not your model, try the formula listed for other models of the same brand.

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 a 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:

• Roll-out Test for Super Accuracy
• Choosing a computer for accuracy
• Deriving tire diameter from ISO/ETRTO numbers
• Using a measured course to fine-tune your setting

This site also has pages on:

• Cyclecomputer Installation Tips
• Cyclecomputer Troubleshooting

The calibration charts are to a large extent based on instruction sheets provided with various cyclecomputers. Different manufacturers have used different brands of tires to calibrate, so there are some areas where there is slight inconsistency in values between one group and another.

If you require greater accuracy than this chart provides, do a rollout test or measured distance test.

The charts don't list all possible tire sizes, but do list the most popular ones. If your marked tire size falls between two sizes shown on the chart, interpolate the appropriate calibration number between those above and below, or for greater accuracy, do a roll-out test.

If you have a computer model that's not listed, here, most likely it uses one of the 6 calibration schemes shown in the chart. Remove the batteries, wait a few seconds and reinstall them. A calibration number will usually appear. This default value will normally be for a tire in the size range of normal full-sized tires, and if you examine the chart, you should be able to figure out which calibration group to use.

A little bit of experimentation should show you which button does what. Often switching from mode to mode, or entering "set" mode is accomplished by holding one of the buttons for several seconds.

Instruction sheets for most of the newer models are online. They can be found with an Internet search on the model number, along with the word "manual" or "instructions".

If you have information on any newer models or others that we might have missed, please let us know, so that we can update this listing.

The values on the chart will generally give a value accurate to within one or two percent, which is good enough for most cyclists, and is more accurate than most automobile odometers. If you require more accuracy, you can do a "roll-out" test.

Unless you want to count "miles" ridden on a stationary trainer, it is best if you measure the roll-out of the front wheel and mount the computer sensor there. The rear wheel "creeps" along the road surface as you pedal, and can skid during braking, so it gives a less-accurate readout.

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. Run the test on a paved surface: most floors are slipperier, and that will affect the reading too. You could scoot alongside a wall, or have an assistant hold the bicycle upright for you.

You may use the valve stem as a reference, starting the roll with the valve right over a perpendicular line, 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.

Use an accurate, metal tape measure to measure the rollout. You may measure for one wheel revolution, or for even greater accuracy, for three or four -- whatever your tape measure can span -- and divide by the number of revolutions.

If your tape measure is divided in inches, multiply the measured circumference by 2.54 for centimeters or 25.4 for millimeters. For cyclecomputers that require a radius value, divide the result by 6.2832 (2 x π) to get the radius.

Once you have measured the rolling circumference, use the formula indicated to find the calibration number for the cyclecomputer involved.

I the geratest accuracy is important to you, keep the claibrated tire at the pressure you used for the roll-out test.

Feel free to send us the results, giving the following information:

Information on less-common tires and tire sizes is especially valuable.

You will need to round the result of a roll-out test to the nearest whole number. If accuracy is important, for example, to measure out a route for a bike club ride, choose a computer calibrated in millimeters or in 100ths of an inch -- and which displays hundredths of a kilometer or mile. You might carry a small digital voice recorder on a lanyard around your neck, turn it on and record the bicycle computer's reading for each turn along the ride, then enter the distances into a spreadsheet later to construct your cue sheet.

Most computers measure natively in kilometers, and are calibrated in centimeters or millimeters. The exact value to convert to miles is 1/1.609344, but some computers use 1/1.6, more than 0.5% different.

Some computers' conversion factors could be determined; that is indicated in the charts for each calibration scheme. The computers in Group E use no internal conversion -- you set them for miles by adjusting the calibration number. These types of computers are preferable when accurate measurement in miles is important.

If you have access to a measured course, you can fine-tune your setting 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. It is best to use a designated measured course, and/or to measure over a distance of 10 miles or more to reduce round-off error and to avoid using inaccurately-placed mileposts. (The highway department generally avoids placing them in the middle of people's driveways...)

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 28 mm on a rim with a bead seat diameter of 622 mm

To 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.)

Global Positioning System (GPS) units are becoming increasingly popular for use on bicycles. GPS units calculate location from data sent by earth satellites. Location and direction of travel are displayed on a screen, and data can be downloaded to a computer. A GPS system is very handy to lay out a route or to keep track or where you have been. Many popular bicycle routes are being published on the Internet in a standard GPS format. You can download them into a GPS unit and have it announce each turn to you.

A GPS unit is very accurate about location, but less so about distance, because it records a waypoint only once every couple of seconds -- sometimes longer, if it can't "see" the satellites due to vegetation, tall buildings, an underpass etc. A GPS unit will typically record a route with a lot of tight curves as slightly shorter than your actual ride.

A GPS unit can sense elevation; few bicycle computers do. But a GPS unit tends to record slightly more elevation gain than elevation loss, as you will notice when returning to the same starting location. Why? The GPS unit records more waypoints when you are climbing slowly, and so will be likely to record ones nearer the top and bottom of a climb.

More-sophisticated GPS software could compensate somewhat for these anomalies, for example by using curve fitting instead of drawing straight lines. A GPS unit with a wheel sensor could keep track of distance when it didn't see the satellites. Real soon now...

A GPS unit with a permanenetly-installed, internal, rechargeable battery is preferable over one with AA cells, which can lose contact due to vibration on a bicycle and dump your data.

A bicycle computer will typically run for a year or more between battery changes. A GPS unit's radio receiver and screen eat power, so a GPS unit will run for only a few hours on its internal battery. It charges when connected to a computer's USB port or a car's cigarette-lighter adapter.

You might use an external battery, or a hub generator or solar panel (with electronics to regulate the output for the GPS unit). A USB port delivers 5 volts, and a cigarette lighter, 12 volts.

A GPS unit is more expensive than a bicycle computer, though price is coming down.

I would like to thank those who helped with updates and error spotting, including: John Allen, Miha Ambroz, Badarka *at* aol.com, DaveM10 *at* 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.