What are gear inches on a bike?

Gear inches is the diameter in inches of the wheel a penny farthing would need to give the same gearing, worked out as the gear ratio times the wheel diameter in inches. A higher number is a harder, faster gear and a lower number is an easier climbing gear. Road bikes typically run from about 30 gear inches in the lowest gear to over 100 in the highest.

How is a bike gear ratio calculated?

The gear ratio is the number of teeth on the front chainring divided by the number of teeth on the rear sprocket. A 50 tooth chainring with a 15 tooth sprocket gives a ratio of 3.33, meaning the rear wheel turns 3.33 times for every full turn of the pedals.

What is development or rollout?

Development, also called rollout, is the distance the bike travels for one full turn of the pedals. It equals the gear ratio times the wheel circumference. It is often the most intuitive way to compare gears because it is measured directly in metres, so a 7 metre gear moves you 7 metres per pedal stroke.

Bike Gear Calculator

Comparing bike gears is hard when all you have is tooth counts, because a 50/15 on one bike and a 34/11 on another can feel surprisingly similar. This calculator turns any combination of chainring, sprocket and wheel size into the three numbers cyclists actually use to compare gears: the gear ratio, the gear inches, and the metres of development. The gear ratio is simply the front teeth divided by the rear teeth, telling you how many wheel turns you get per pedal stroke. Gear inches is the traditional measure, the diameter of the direct-drive wheel that would give the same gearing, which lets you line up gears across bikes with different wheel sizes on a single scale. Development, or rollout, is the distance you roll forward with each full turn of the pedals, measured straight in metres, which many riders find the most intuitive of all. Enter your chainring and sprocket tooth counts and your wheel diameter in millimetres, including the tyre, and the calculator returns all three figures plus the speed you would reach at a steady 90 rpm. Use it to plan a gear range, to check the jumps between the sprockets on your cassette, or to compare a new setup against the one you ride now. Lower numbers are easier gears for climbing, and higher numbers are harder gears for speed on the flat and downhill.

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Chainring teeth

Sprocket teeth

Wheel diameter (with tyre)

88.2

gear inches for this gear

Gear ratio3.33

Development (rollout)7.04 m

Speed at 90 rpm38.0 km/h

A 50/15 gear on a 672 mm wheel is 88.2 gear inches, rolling out 7.04 m per pedal turn: a hard, fast gear suited to flat ground and descents.

Wheel diameter should include the tyre: about 672 mm for a 700x25c road wheel. Gear inches uses the wheel diameter and development uses the rolling circumference (diameter times pi).

How it works

The gear ratio is the chainring teeth divided by the sprocket teeth. Gear inches is that ratio multiplied by the wheel diameter in inches, so it puts every gear on the same scale regardless of wheel size. Development is the ratio multiplied by the wheel circumference, which is the diameter times pi, giving the metres travelled per pedal revolution. The speed at 90 rpm is the development times 90 revolutions per minute, converted to kilometres per hour, and it gives a quick sense of how fast the gear is at a typical cruising cadence.

Worked example

Take a 50 tooth chainring, a 15 tooth sprocket, and a 700x25c wheel with a diameter of 672 mm. The gear ratio is 50 / 15 = 3.33. In inches the wheel is 672 / 25.4 = 26.46, so the gear inches are 3.33 × 26.46 = 88.2. The wheel circumference is 672 × pi = 2111 mm, so the development is 3.33 × 2.111 m = 7.04 m per pedal turn. At 90 rpm that gear carries you along at 90 × 7.04 × 60 / 1000 = 38.0 km/h. That is a hard gear, the kind you would use pushing hard on the flat rather than climbing.

Related calculators

Bike Cadence and Speed Calculator: speed from cadence.
Gear Ratio Calculator: ratio, RPM and torque.
Bike Pace Calculator: speed, time and pace for a ride.
Cycling Power Calculator: power needed at a speed.
Bike Size Calculator: frame size from your height.

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