Bike Gear Ratio Calculator

Gear inches, development and cadence

A bike gear comes down to one ratio: chainring teeth divided by cog teeth. A 50-tooth ring on a 17-tooth cog gives 2.94, so the wheel turns 2.94 times for each pedal turn. Gear inches multiply that ratio by the wheel diameter, so you can compare bikes with different wheels. The formula is wheel diameter times chainring, divided by cog. Wikipedia lists the same form, with the example of 26 x 48 / 34 giving about 37 gear inches.

Development is the distance you roll per pedal turn. It is gear inches times pi, then converted to feet. The default of 77.4 gear inches gives about 20.3 feet per turn. Speed follows from cadence: feet per turn times turns per minute times 60, divided by 5,280 feet per mile. At 90 rpm that is about 20.7 miles per hour on flat ground with no slip.

Use the tool to choose a gear. Racers often sit near 90 to 100 rpm, and tourists spin lower on climbs. If you want 15 mph at 80 rpm, you need about 18.3 feet of development, or roughly 70 gear inches on a 700c wheel. Low gears for hills sit under 40 gear inches. A single-speed or fixed bike commonly uses 65 to 75 gear inches in flat cities.

Wheel size changes the answer, so pick your tire from the list or enter a measured diameter. A fatter tire has a larger rolling diameter and raises every gear a little. Fit matters too, so also check your bike frame size, and use the bike tire size converter to match rim and tire labels. For hill work, run your easiest ring and cog here and check that you can still turn the pedals. If you ride loaded on long tours, choose a lowest gear that feels almost too easy at the start, because fatigue makes it feel right by the third day.

Where these numbers come from

The formulas and default values in this calculator are based on:

• Wikipedia, Gear inches (en.wikipedia.org/wiki/Gear_inches), which gives gear inches as wheel diameter x chainring / cog and the 26 x 48 / 34 example
• Wikipedia, Bicycle gearing conventions for development. Note: listed tire diameters are typical rolling diameters, not exact

This is a planning estimate, not medical, legal or immigration advice. Fees, allowances, limits and entry rules change, so confirm the figures with your airline, cruise line, customs agency or the official government source before you book or travel, and follow their rules if they differ from this page.