What the inputs mean
- Chainring and cog: the teeth on the front ring and the rear cog you are riding in.
- Cadence: crank turns per minute (rpm), as your bike computer shows it.
- Tire size: the ETRTO size on the sidewall (28-622) or a marking such as 700x28c, 29x2.2 or 27.5x2.4. The tool estimates the outside diameter as bead seat diameter plus twice the width, so the result is approximate. The bike tire size chart explains the markings.
- Target speed: optional. The tool works backward to the cadence you need in that gear.
The formula
wheel circumference ≈ π × (bead seat diameter + 2 × tire width)
speed (mph) = cadence × chainring ÷ cog × circumference (in) × 60 ÷ 63,360
cadence = speed ÷ (chainring ÷ cog × circumference (in) × 60 ÷ 63,360)Every crank turn turns the rear wheel chainring ÷ cog times. Multiply by the wheel circumference to get distance per crank turn, then by cadence and 60 to get distance per hour. There are 63,360 inches in a mile.
Worked example
A 700x28c tire is 28-622, so its outside diameter is about 622 + 56 = 678 mm and its circumference about 2,130 mm (83.86 in). In a 50 × 17 gear at 90 rpm:
90 × 50 ÷ 17 × 83.86 × 60 ÷ 63,360 = 21.0 mph (33.8 km/h). To ride 20 mph in the same gear you need 20 ÷ 21.02 × 90 = 86 rpm. In the 34 × 17 you would need 126 rpm for the same speed, which is why you shift.
Reading the tables
The cadence table shows the chosen gear from 60 to 120 rpm: in 50 × 17 that runs from 14.0 to 28.0 mph. The grid shows every chainring and cog at your cadence, so you can see which gear holds a speed at a comfortable rpm. To compare whole drivetrains in gear inches, use the bike gear calculator. To see the power a speed takes, use the cycling power calculator. The same speed-from-gearing idea for motorcycles is in the motorcycle gear ratio calculator.
Why your computer may read differently
Real tires are rarely exactly bead seat plus twice the marked width, and they flatten under load. If your bike computer lets you enter a wheel circumference, measure it: mark the tire, roll one full turn with your weight on the bike, and measure the distance.
For the energy a ride burns, see Cycling calories on laptally.com.
Questions
How do you calculate bike speed from cadence?
Speed = cadence × chainring ÷ cog × wheel circumference. At 90 rpm in 50 × 17 on a 700x28c tire (about 2,130 mm around), that is 90 × 2.941 × 2.130 m = 564 m per minute, or 21.0 mph (33.8 km/h).
What cadence do I need for 20 mph?
It depends on the gear. On a 700x28c tire, 50 × 17 needs about 86 rpm and 34 × 17 needs about 126 rpm.
How fast is 50 × 11 at 90 rpm?
About 32.5 mph (52.3 km/h) on a 700x28c tire. At 120 rpm it is about 43.3 mph.
What speed does a 32 × 10 gear give on a 29er?
A 29x2.2 tire is about 734 mm tall (622 + 2 × 55.9). At 80 rpm, 32 × 10 gives about 22.0 mph and 32 × 50 about 4.4 mph.