Bicycle Gear Ratio Speed Calculator and Formula

A bicycle gear ratio speed calculator estimates how fast a bike travels for a chosen chainring, rear sprocket, wheel circumference, and pedaling cadence. Enter the tooth counts and wheel circumference in metres, plus cadence in revolutions per minute (rpm), to get the gear ratio, speed in km/h, and speed in mph.

Formula

The calculator uses the standard gearing-and-cadence relationship:

Gear ratio = front chainring teeth ÷ rear sprocket teeth

Speed (km/h) = gear ratio × wheel circumference (m) × cadence (rpm) × 60 ÷ 1,000

Speed (mph) = speed (km/h) × 0.621371

• Front chainring teeth is the number of teeth on the crank-side chainring, counted in teeth.
• Rear sprocket teeth is the number of teeth on the selected rear cog, counted in teeth.
• Wheel circumference is the distance the tire travels in one complete wheel revolution, in metres. It is often close to, but not exactly, the tire’s nominal diameter multiplied by pi.
• Cadence is crank revolutions per minute. One crank revolution advances the chain by the chainring’s tooth count, which turns the rear wheel according to the sprocket ratio.
• 60 ÷ 1,000 converts metres per minute into kilometres per hour.

The calculation assumes the chain is engaged normally and that one crank revolution advances the bike by one gear ratio’s worth of wheel revolutions. It does not account for tire deformation, drivetrain losses, wind, slope, or the rider’s power.

Worked Example

Using the calculator’s default values—40 teeth at the front, 20 teeth at the rear, a 2.10 m wheel circumference, and 80 rpm cadence:

1. Gear ratio = 40 ÷ 20 = 2.00:1.
2. Wheel revolutions per crank revolution = 2.00.
3. Distance per crank revolution = 2.00 × 2.10 m = 4.20 m.
4. Distance per minute = 4.20 m × 80 = 336 m/min.
5. Speed = 336 × 60 ÷ 1,000 = 20.16 km/h.
6. In miles per hour: 20.16 × 0.621371 = 12.5268…, which rounds to 12.53 mph.

The calculator rounds the displayed speed and ratio to two decimal places. Actual riding speed can differ from this idealized estimate.

How to Use the Calculator

Choose the gear you want to assess, then enter its front chainring and rear sprocket tooth counts. On a multi-chainring bike, use the chainring currently selected; for a cassette or freewheel, use the particular rear sprocket. Enter wheel circumference in metres and a cadence in rpm. The results show the estimated speed at that cadence, not a speed the bike will necessarily reach.

For a more representative wheel measurement, mark the tire and floor at the same point, roll the bike straight for one full wheel revolution with the rider aboard, and measure the distance between floor marks. Keep the units consistent: the calculator expects metres, not centimetres, inches, or millimetres. For example, 210 cm must be entered as 2.10 m.

What the Ratio Tells You

A higher ratio means more distance traveled for each crank revolution. In the example, a 2.00:1 ratio turns the rear wheel twice for each crank revolution. A lower ratio turns the wheel fewer times per crank revolution, generally making it easier to pedal up a steep hill at a given cadence, while a higher ratio can support more speed on flat or downhill terrain if the rider can produce enough force.

Gear ratio alone is not a complete measure of how easy a gear feels. Wheel size matters: the same ratio on a larger-circumference wheel covers more ground per crank revolution. A useful comparison across different wheel sizes is gear inches, calculated as wheel diameter in inches multiplied by the gear ratio, or metres of development, calculated as circumference in metres multiplied by the ratio. These describe distance per crank revolution in different forms; neither predicts speed without cadence.

Accuracy, Assumptions, and Common Mistakes

• Using tire diameter as circumference: The formula needs distance around the tire, not across it. If you measure diameter, multiply by pi as an approximation, or measure a complete roll for better practical accuracy.
• Mixing units: Enter circumference in metres. A value of 2.10 means 2.10 m, not 2.10 cm. The output includes both metric speed and its mph conversion.
• Confusing cadence with wheel rpm: Cadence is crank rpm. The gear ratio converts crank rotations into rear-wheel rotations.
• Treating the estimate as a guarantee: The calculator reports the speed implied by the selected gearing and cadence. It does not determine whether a rider can sustain that cadence or overcome wind, rolling resistance, or a climb.
• Rounding too early: Keep the full circumference and intermediate values when doing a manual comparison. The calculator rounds only the displayed results.

Real-world wheel circumference changes slightly with tire size, inflation, load, and measurement method. Drivetrain efficiency and tire slip can also affect motion, although their impact is not represented in this basic kinematic calculation. Cadence may vary continuously while riding, so the displayed speed is a snapshot for the entered value.

Frequently Asked Questions

How do I calculate bicycle speed from gear ratio and cadence?
Multiply the front-to-rear tooth ratio by wheel circumference and cadence. For km/h, multiply the resulting metres per minute by 60 and divide by 1,000. For mph, convert km/h by multiplying by 0.621371.

Does a bigger rear sprocket make the bike faster?
Not at the same cadence. A bigger rear sprocket lowers the front-to-rear ratio, so the bike travels a shorter distance per crank revolution. It usually makes pedaling easier, which can help on climbs.

How do I find my bicycle wheel circumference?
Measure one full tire roll on the ground, ideally with the bike loaded as it is when ridden. Alternatively, use a reliable tire-and-rim sizing reference or estimate from measured tire diameter, recognizing that the diameter method is less precise.

Why is my actual speed different from the calculator?
The calculator assumes the entered cadence and circumference remain exact and ignores resistance and drivetrain losses. Your measured speed also depends on wind, gradient, tire characteristics, rider position, and how accurately cadence and wheel size were entered.

Can the same gear ratio give different speeds?
Yes. At the same cadence, a larger wheel circumference gives a higher calculated speed. Even with the same wheel and gear, changing cadence changes speed in direct proportion.

Related

• 5K Mile Split Calculator: Pace and Finish Time
• 5K Race Pace Calculator: Find Your Minutes per Mile
• 1RM Max Calculator: Estimate Your One-Rep Max

in chainring | Front chainring teeth | teeth | 40 | 0
in sprocket | Rear sprocket teeth | teeth | 20 | 0
in wheel | Wheel circumference | m | 2.10 | 0
in cadence | Cadence | rpm | 80 | 0
out speed | Estimated speed | km/h | chainring / sprocket * wheel * cadence * 60 / 1000 | 2
out ratio | Gear ratio | :1 | chainring / sprocket | 2
out mph | Estimated speed | mph | speed * 0.621371 | 2