Bicycle Gear Ratio Calculator

How it is calculated

Three values are derived in sequence. The ratio is R = Tchainring / Tcog, with both tooth counts as plain numbers. Gear inches are GI = R × D, where D is the overall wheel diameter in inches, tyre included. Development, the distance rolled per crank revolution, is M = GI × 0.0254 × π in metres.

The engine keeps the unrounded ratio for the later steps and only rounds each displayed figure to two decimals. The wheel and cog must be greater than zero, otherwise nothing is returned; a chainring of 0 simply gives zeros. The wheel diameter is read in inches, so convert a metric size before entering it.

Common mistakes

• Entering the wheel size in millimetres, such as 622 for a 700c rim. The field is in inches, so 622 would give a gear of thousands of inches. Use the overall diameter with the tyre, roughly 27 for a typical 700c wheel.
• Swapping chainring and cog. The front chainring is the numerator; reversing them gives the inverse ratio, a number below 1 that describes a very easy gear.
• Using the rim diameter instead of the tyre's outside diameter. The tyre adds to the rolling diameter, which changes the gear inches and the distance per revolution.
• Comparing development figures between bikes with different tyre pressures or sizes as if they were exact. The result is a theoretical rolling distance.
• Entering the cassette's total tooth count or the number of gears instead of the teeth on the single cog you are riding in.