What this bike gear calculator compares
A front chainring and rear sprocket set a mechanical relationship between pedal turns and wheel turns. This tool combines their tooth counts with wheel circumference and cadence to show gear ratio, development, effective gear inches and theoretical speed.
It calculates one selected gear at a time. Enter another sprocket or chainring to compare combinations while keeping the same wheel measurement. The default 50-tooth front ring, 20-tooth rear sprocket, 2,100 mm circumference and 90 rpm are editable examples. The circumference is an example measurement, not a promise about a particular tyre size.
How to use the inputs
Count or read the teeth on the chainring and sprocket you want to compare. Use positive whole numbers: 50 and 20, rather than a cassette range such as 11–34. Enter wheel circumference in millimetres and cadence in revolutions per minute, then press Calculate.
The interface accepts tooth counts from 1 to 200, circumference from 100 to 5,000 mm and positive cadence up to 300 rpm. These are input guards for the arithmetic, not equipment recommendations. Fractional tooth counts, zero sprockets and non-finite values are rejected with an inline explanation.
The model assumes a conventional direct chain drive in the selected gear. An internal geared hub with another reduction or multiplication factor needs that factor included in a different model; this tool does not accept it. It also assumes the driven wheel rolls without slipping.
Gear ratio and development
Gear ratio = front chainring teeth ÷ rear sprocket teeth.
A ratio of 2.5 means the rear wheel makes 2.5 turns for each full crank revolution in this direct-drive model. The ratio has no unit. A larger front ring raises the ratio; a larger rear sprocket lowers it.
Development (metres per crank revolution) = gear ratio × wheel circumference in metres.
Development is also called rollout. Sheldon Brown's gearing explanation distinguishes this distance-based description from gear inches. Unlike tooth ratio alone, development changes when wheel circumference changes.
Worked example: 50 / 20
With a 50-tooth chainring and 20-tooth sprocket, the ratio is 50 ÷ 20 = 2.50. A circumference of 2,100 mm is 2.1 m. Development is therefore 2.5 × 2.1 = 5.25 m per crank revolution.
At 90 rpm, the wheel covers 5.25 × 90 metres per minute. Multiply by 60 and divide by 1,000 to get 28.35 km/h, or approximately 17.62 mph. This result describes the selected gearing turning at the specified cadence; it does not establish that you can sustain that speed on a road.
Worked example: a lower gear
Keep the same circumference and cadence, but enter 34 front teeth and 34 rear teeth. The ratio becomes 1.00, development becomes 2.10 m/rev, and theoretical speed becomes 11.34 km/h. The lower ratio covers less distance per pedal turn. The calculation does not decide whether that gear suits a particular rider or climb.
Where gear inches come from
Gear inches = gear ratio × effective wheel diameter in inches.
The effective diameter is wheel circumference divided by π, then divided by 25.4 to convert millimetres to inches. This conventional model expresses gearing as an equivalent wheel diameter, as described by Sheldon Brown. It is not the distance travelled with one pedal turn.
For the 50/20 and 2,100 mm example, effective diameter is approximately 26.32 inches, so gear inches are approximately 65.79. The calculation derives diameter from the supplied circumference instead of assuming that a wheel's familiar size label is its measured outside diameter.
Gain ratio is a separate measure that also includes crank length. It is not displayed here because this tool has no crank-length input. Comparing gear inches does not account for differences in crank length.
Measuring wheel circumference
Garmin's wheel circumference guidance allows a measured circumference rather than relying exclusively on a size table. Use a rollout measurement for your actual tyre and setup when precision matters.
On a suitable flat surface, mark a starting wheel position and measure the distance travelled by one complete wheel rotation. Keep the bicycle rolling straight and measure in millimetres. Repeating over several rotations and dividing by the number of turns can reduce the effect of a small marking error. Follow your device or tyre manufacturer's measurement guidance for load and pressure.
A nominal wheel label does not uniquely fix loaded rollout. Tyre dimensions and the actual setup matter. If the circumference input is 1% too large, the computed development and speed will also be 1% too large, because circumference enters both formulas as a direct multiplier.
Cadence speed versus actual ride speed
Theoretical speed is a mechanical consequence of gearing, wheel rollout and cadence. It contains no wind, drag, rider power, slope or rolling resistance. A hill can make the chosen cadence difficult to hold, and a descent can involve coasting rather than pedalling.
Use the average speed calculator for distance and time measured over a ride. Use the climb gradient calculator to interpret terrain. The W/kg tool explains a rider-power ratio, but does not supply the missing resistance model either.
When reading the climbing passages in our Tour route guide, think of gearing as one practical part of the riding context. This page does not specify what gearing any professional rider used.
Frequently asked questions
Why do two combinations give the same result?
Equal front-to-rear ratios produce equal development with the same circumference. A 50/25 and 34/17 both have a ratio of 2.00. Other drivetrain considerations are outside this calculation.
Can I enter the rim diameter instead of circumference?
No. The input needs the tyre's wheel rollout in millimetres. A rim dimension does not include the mounted tyre and is not the required circumference.
Does a higher gear make me faster?
At the same cadence, a higher ratio gives a greater theoretical speed. Whether you can maintain that cadence depends on the riding conditions and effort. This tool makes no performance promise.
Method and sources
The formulas combine tooth ratio, measured circumference and cadence. Sheldon Brown's technical reference supports development and gear inches, and Garmin's manual supports measuring circumference. Return to the cycling tools hub for the other calculators.