Bicycle Gear Inches Calculator

Gear inches from chainring, cog, and wheel size.

Inputs

Chainring teeth

Rear cog teeth

Wheel diameter (in)

Crank length (mm)

Cadence (RPM)

Tab to move between fields

Results

Gear inches

79.4

Gear ratio2.94

Development (m)6.34

Gain ratio5.85

Speed (mph)21.3

Speed (km/h)34.2

How to Use This Calculator

Enter Chainring teeth, Rear cog teeth, and Wheel diameter (in).
Set Crank length (mm) and Cadence (RPM).
Review the Gear inches result.
Use Gear ratio and Development (m) to inform your decision.

How the result changes with Rear cog teeth

Rear cog teeth	Gear inches
13	103.8
24	56.3
37	36.5
48	28.1

What each input means

Chainring teeth: Number of teeth on the front chainring.
Rear cog teeth: Number of teeth on the rear sprocket.
Wheel diameter (in): Overall wheel + tire diameter in inches. 700×25c is ~26.3 in, 700×32c is ~27 in.
Crank length (mm): Crank arm length in mm (typically 165-175 mm).
Cadence (RPM): Pedaling cadence in revolutions per minute.

What each result means

Gear inches: Gear inches: effective wheel diameter multiplied by gear ratio.
Gear ratio: Chainring teeth divided by cog teeth.
Development (m): Distance traveled per crank revolution in meters.
Gain ratio: Sheldon Brown gain ratio — accounts for crank length.
Speed (mph): Speed at the given cadence in miles per hour.
Speed (km/h): Speed at the given cadence in kilometers per hour.

How this is calculated

Worked example, using the default values

Identify Input Parameters
4 parameters
Chainring teeth = 50, Rear cog teeth = 17, Wheel diameter (in) = 27, Crank length (mm) = 172.5 = 5 input(s) provided
Calculate Gear inches
Gear inches = wheelDiameter * gearRatio
79.4 = 79.4
Calculate Gear ratio
Gear ratio = chainring / cog
2.941 = 2.941
Calculate Development
Development = developmentInches * 0.0254
6.34 = 6.34

Engine last updated July 28, 2026. Checked against 2 independently-derived tests — how we verify calculators.

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