Bike Gear Ratio Calculator Formula
Understand the math behind the bike gear ratio calculator. Each variable explained with a worked example.
Formulas Used
Gear Ratio
ratio = round(gear_ratio * 100) / 100Development
development = round(development_m * 100) / 100Gear Inches
gear_inches = round(gear_ratio * wheel_diameter_mm / 25.4 * 100) / 100Speed at 90 RPM
speed_at_90rpm = round(development_m * 90 * 60 / 1000 * 0.621371 * 100) / 100Variables
| Variable | Description | Default |
|---|---|---|
chainring_teeth | Chainring Teeth (front)(teeth) | 50 |
cog_teeth | Cog Teeth (rear)(teeth) | 17 |
wheel_diameter_mm | Wheel Diameter(mm) | 700 |
gear_ratio | Derived value= chainring_teeth / cog_teeth | calculated |
development_m | Derived value= gear_ratio * wheel_diameter_mm * 3.14159 / 1000 | calculated |
How It Works
How Bike Gear Ratios Work
Formula
Gear Ratio = Chainring Teeth / Cog Teeth
Development = Gear Ratio x Wheel Circumference
Development tells you how far the bike travels per pedal revolution. Higher gear ratios mean more distance per revolution but require more force.
Gear Inches
Gear inches is a traditional measure: Gear Inches = Gear Ratio x Wheel Diameter (inches).
Worked Example
A road bike with a 50-tooth chainring, 17-tooth rear cog, and 700mm wheels.
- 01Gear ratio = 50 / 17 = 2.94
- 02Wheel circumference = 700 * 3.14159 = 2199 mm = 2.199 m
- 03Development = 2.94 * 2.199 = 6.47 m per pedal revolution
- 04At 90 RPM: 6.47 * 90 * 60 / 1000 = 34.93 km/h = 21.71 mph
Frequently Asked Questions
What gear ratio is best for climbing?
Lower ratios (below 1.5) are better for climbing. A 34/28 combination (ratio 1.21) is a common granny gear for steep hills.
What gear ratio is best for flat riding?
Ratios of 2.5-3.5 are typical for flat riding at moderate cadence. A 50/17 (2.94) is a common cruising gear for road bikes.
What does development mean?
Development is the distance your bike travels with one full pedal revolution. Higher development means more ground covered but more effort required.
Ready to run the numbers?
Open Bike Gear Ratio Calculator