Gear Inches Calculator

How it is calculated

Gear inches is GI = (chainring teeth ÷ cog teeth) × wheel diameter, with the wheel diameter in inches, measured with the tyre inflated. The tool also gives the distance rolled per full pedal turn: rollout = GI × π × 0.0254 in metres, because 0.0254 m is one inch and GI × π is the distance around the effective wheel. Gear inches is shown with one decimal and the rollout with two. Tooth counts must be whole numbers from 1 to 200, and the wheel diameter must be between 5 and 60 inches; anything else, such as 50.5 teeth or a diameter typed in millimetres, shows no result. Only one chainring and one cog are used per calculation, so to compare a full cassette you run each cog in turn. The result does not depend on crank length or pedalling speed, and it ignores tyre squash and drivetrain losses.

Common mistakes

• Entering the wheel diameter in millimetres (such as 622 for a 700c rim) or in centimetres. The field needs inches between 5 and 60, so 700 or 622 gives no result.
• Using the rim size instead of the diameter with the tyre on. The tyre adds to the diameter, so a bare rim value gives a lower number than what you ride.
• Swapping chainring and cog. A 16-tooth ring with a 50-tooth cog gives a tiny ratio, not the gear you have.
• Typing a fractional tooth count or a total for a double or triple crankset. Teeth must be whole numbers for one ring and one cog at a time.
• Comparing a number from a single-speed with a hub-gear bike without allowing for the internal ratio, which the tool does not know about.