Bike Gear Ratio Speed and Cadence Calculator

A bike gear ratio speed and cadence calculator estimates road speed from your selected chainring, rear sprocket, wheel circumference, and pedaling cadence. Enter those four values to get the gear ratio, speed in km/h, and speed in mph; the estimate assumes one wheel revolution per wheel circumference and no tire slip.

Formula

The calculator uses the standard gear-and-wheel relationship:

• Gear ratio = front chainring teeth ÷ rear sprocket teeth.
• Speed in km/h = cadence × gear ratio × wheel circumference × 60 ÷ 1,000.
• Speed in mph = cadence × gear ratio × wheel circumference × 60 ÷ 1,609.344.

Here, chainring and sprocket are tooth counts, cadence is in crank revolutions per minute (rpm), and wheel circumference is in metres. The factor 60 changes minutes to hours; the final divisors convert metres to kilometres or miles. A 50-tooth chainring and 17-tooth sprocket give a ratio of about 2.94:1: the rear wheel turns about 2.94 times per crank revolution, assuming a conventional direct-drive bicycle drivetrain.

Worked Example

Using the calculator’s defaults: a 50-tooth chainring, 17-tooth sprocket, 2.105 m wheel circumference, and 90 rpm cadence.

1. Gear ratio = 50 ÷ 17 = 2.941176…
2. Wheel revolutions per minute = 90 × 2.941176… = 264.705882…
3. Distance per minute = 264.705882… × 2.105 = 557.205882… m/min.
4. Speed = 557.205882… × 60 ÷ 1,000 = 33.432352… km/h, or 33.43 km/h.
5. In miles per hour, 557.205882… × 60 ÷ 1,609.344 = 20.77 mph.

What Gear Ratio, Speed, and Cadence Mean

Gear ratio describes how many times the wheel rotates for each crank revolution. A larger ratio—created by a bigger front chainring, smaller rear sprocket, or both—travels farther per pedal turn. It is commonly called a harder gear because more pedal force is generally needed to accelerate or climb at a given cadence. A smaller ratio gives less distance per turn and is commonly easier to pedal uphill.

Cadence is how quickly the cranks turn, measured in rpm. In a fixed gear, increasing cadence raises calculated speed in direct proportion: double cadence, and the theoretical speed doubles. Changing gears changes the distance covered per crank turn, allowing a rider to alter cadence and effort at a given road speed. The calculator reports a mathematical estimate, not a recommended cadence or a prediction of how fast a particular person can ride.

How to Choose the Inputs

• Count the teeth: Use the number printed or specified for the chainring and the selected rear sprocket. On a multi-sprocket cassette, each sprocket produces a different ratio.
• Use actual wheel circumference: The value is the distance the tire travels in one full wheel rotation. Measure the inflated tire on the bike for a useful estimate, or use a reliable circumference value for the tire and rim combination.
• Enter cadence in rpm: Use crank revolutions per minute, not wheel revolutions. A cadence sensor normally reports crank cadence.
• Keep units consistent: This calculator expects circumference in metres. If you have inches, multiply by 0.0254; if you have feet, multiply by 0.3048. For example, 82.9 inches is about 2.105 m.

Wheel size labels alone can be misleading because tires with the same nominal rim size can have different inflated diameters. Circumference measured with the actual tire, pressure, and load is more relevant than a generic label. Entering a more accurate circumference improves the estimate, though it cannot remove all real-world variation.

Comparing Gears and Understanding Limits

To compare two gear combinations at the same cadence and wheel size, compare their chainring-to-sprocket ratios. For example, 50/17 is about 2.94, while 34/28 is about 1.21. At 90 rpm with a 2.105 m circumference, the first combination estimates 33.43 km/h and the second about 13.73 km/h. This illustrates the difference in distance per pedal turn; it does not mean one gear is universally better. Terrain, rider strength, desired cadence, and available gearing all matter.

The calculation assumes a steady cadence and an idealized drivetrain. It does not account for aerodynamic drag, wind, gradient, road surface, tire deformation, drivetrain losses, acceleration, or coasting. Those factors affect actual speed. It also assumes a conventional bicycle where crank turns drive the rear wheel through the stated tooth counts; unusual transmissions or internal gearing may need an effective ratio rather than a simple chainring-to-sprocket calculation.

Displayed results are rounded to two decimal places. Intermediate calculations use the entered values, not the rounded figures shown. Small differences from a bike computer or GPS reading are expected, especially if the entered wheel circumference is approximate or cadence changes during the measurement.

Common Errors and Misconceptions

• Reversing the tooth counts: Divide chainring teeth by rear sprocket teeth, not the other way around. A 50/17 ratio is greater than 1.
• Confusing gear ratio with speed: A ratio alone cannot give road speed. You also need cadence and wheel circumference.
• Using wheel rpm as cadence: Crank rpm is multiplied by the gear ratio to estimate wheel rpm.
• Assuming the estimate is a speed guarantee: It is a kinematic calculation, not a model of how much power is required or whether a rider can sustain that pace.
• Mixing circumference units: Entering 82.9 as though it were metres instead of inches would make the result vastly too high.

Frequently Asked Questions

How do I calculate bike speed from cadence and gearing?
Divide chainring teeth by rear sprocket teeth, multiply by cadence in rpm and wheel circumference in metres, then multiply by 60. Divide by 1,000 for km/h or 1,609.344 for mph.

Does a bigger gear ratio make a bike faster?
At the same cadence and wheel circumference, a bigger ratio gives a higher theoretical speed. In practice, a harder gear may be difficult to turn, and wind, slope, and rider effort affect actual speed.

What wheel circumference should I enter?
Use the rolling circumference of the inflated tire mounted on the bike. A measured value is preferable to a nominal wheel-size label because tire dimensions vary.

Why does my bike computer show a different speed?
Its wheel setting, cadence, or timing may differ from your inputs. Actual cadence can vary, and the calculation omits tire and drivetrain effects, terrain, wind, and GPS or sensor measurement variation.

Can I compare gears without knowing cadence?
Yes. Compare chainring teeth divided by sprocket teeth to see distance per crank turn relative to another gear. To calculate road speed, you still need cadence and wheel circumference.

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in chainring | Front chainring teeth | teeth | 50 | 0
in sprocket | Rear sprocket teeth | teeth | 17 | 0
in wheel | Wheel circumference | m | 2.105 | 0
in cadence | Cadence | rpm | 90 | 0
out speed | Estimated speed | km/h | cadence * chainring / sprocket * wheel * 60 / 1000 | 2
out mph | Estimated speed | mph | cadence * chainring / sprocket * wheel * 60 / 1609.344 | 2
out ratio | Gear ratio | :1 | chainring / sprocket | 2