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Bicycle Gear Calculator

Calculate bicycle gear ratio, gear inches, development, and cadence-based speed from tooth counts and a measured loaded wheel rollout.

Content updated October 7, 2026 · Details

Published

Content updated

Recorded calculation check · Verification scope

Gear ratio

Inputs ↓
Drivetrain ratio
2.50:1
Gear inches
65.79 in
Development
5.25 m/rev
Speed (mph)
17.6 mph
Speed (km/h)
28.35 km/h

Theoretical no-slip rollout from your measured loaded wheel value. Tire slip, dynamic deformation, drivetrain loss, wind, grade, acceleration, and rider power are not modeled.

Wheel measurement
Accepted range: 100–10000 mm (0.1–10 m after conversion).
mm
rpm

Results update as you type.

Result links include your inputs in the URL. Anyone with the link can read them; avoid sharing sensitive values.

Compare scenarios

Save a result, change your inputs, then compare. Up to three saved scenarios for this visit.

Worked example

chainringTeeth: 50 · cogTeeth: 20 · measurementMode: circumference · wheelValue: 2100 · wheelUnit: mm · cadenceRpm: 90

Result: 2.50:1

Try it →

Calculate external gear ratio, gear inches, development, and theoretical speed from chainring and rear-cog tooth counts, cadence, and measured loaded wheel geometry.

For the former compact workflows, use the gear-ratio preset or cadence preset.

Measure the wheel rollout

Inflate the tyre to normal riding pressure. Mark the tyre and ground at the valve, load the bicycle as it is ridden, roll straight for one complete wheel revolution, and measure between the marks. Enter that rollout as circumference.

Use diameter mode only for a directly measured loaded outside/effective diameter. Do not enter a nominal wheel-family label.

Compare gearing at a fixed cadence

Measure the wheel rollout once and keep it fixed while comparing chainring and cog combinations. Keep cadence unchanged for the first comparison so the speed difference reflects gearing. Then test a different cadence for the chosen combination. These runs distinguish mechanical development from how quickly the rider turns the cranks; neither checks whether the parts physically fit together.

Method

For this direct external drivetrain, the gear ratio is chainring teeth divided by rear-cog teeth. Development is measured circumference multiplied by that ratio. Effective diameter is circumference divided by π, and gear inches is that diameter in inches multiplied by the ratio. Theoretical speed is development multiplied by cadence and converted from metres per minute to km/h and mph.

Calculations keep full precision until the displayed result. Enter valid numbers within the shown ranges. The calculator does not show a result when a value is missing, outside its range, or would make the calculation undefined.

Limits

The result is theoretical no-slip rollout. Tire slip, dynamic deformation, drivetrain loss, wind, grade, acceleration, and rider power are not modeled. It does not prescribe gearing, cadence, training, safety, or performance.

Sources

Compare Bicycle Gear Calculator with related tools

All comparisons →
Bicycle Gear vs Bicycle Power CalculatorCompare gear ratio, gear inches, development, and cadence-based speed against a mechanical power estimate from speed, grade, drag, and rolling resistance.Bicycle Gear vs Running Pace CalculatorCompare cycling speed derived from gearing, cadence, and a measured wheel against running pace, time, and distance in miles or kilometers.Bicycle Gear vs Swimming SpeedCompare two speed calculators from different sports: theoretical cycling speed from chainring and cog teeth, cadence, and measured wheel rollout versus average swimming speed, pace per 100 meters, and projected times.

Frequently asked questions

How is gear ratio calculated?

For this direct external drivetrain, gear ratio is chainring teeth divided by rear cog teeth. Development is measured wheel circumference multiplied by that ratio, and gear inches is the effective diameter in inches multiplied by the ratio.

How should I measure the wheel rollout?

Inflate the tire to normal riding pressure, mark the tire and ground at the valve, load the bicycle as it is ridden, roll straight for one complete wheel revolution, and enter the distance between the marks as circumference. Use diameter mode only for a directly measured loaded diameter, not a nominal wheel-family label.

What does the speed result represent?

Theoretical speed is development multiplied by cadence and converted from meters per minute to km/h and mph. It is a no-slip rollout result; tire slip, dynamic deformation, drivetrain loss, wind, grade, acceleration, and rider power are not modeled.

Sources

  • Schwalbe tire dimensions guidance

    primary · Jul 10, 2026

    Supports: Actual tire dimensions and circumference vary; measure actual rollout rather than infer it from a nominal label.

    — Tire manufacturer technical guidance

  • Wahoo manual rollout guidance

    primary · Jul 10, 2026

    Supports: Manual one-revolution rollout measurement and the approximate nature of tire-size charts.

    — Cycling equipment manufacturer support documentation, vUpdated 2025-05-16

  • CATEYE computer manual

    primary · Jul 10, 2026

    Supports: Mark-at-valve one-revolution tire circumference measurement.

    — Cycling computer manufacturer manual, vCC-PA500B_v3

  • Rohloff’s handbook

    primary · Jul 10, 2026

    Supports: Development equals wheel circumference multiplied by the declared drivetrain ratio.

    — Drivetrain manufacturer handbook

  • Guide for the Use of the International System of Units, Chapter 8

    primary · Jul 10, 2026

    Supports: Rotational-frequency dimensional arithmetic and SI unit practice.

    — National metrology institute

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