How BMX gearing is calculated
Compare gearing using the chainring teeth, rear sprocket teeth and wheel diameter. The ratio tells you how many times the wheel turns for each turn of the cranks. Gear inches and rollout include wheel size, so they also let you compare bikes with different wheels.
Gear ratio, gear inches and rollout
- Gear ratio: chainring teeth ÷ sprocket teeth.
- Gear inches: wheel diameter in inches × gear ratio.
- Rollout: wheel circumference × gear ratio. This is the distance traveled for one crank revolution.
For a 44-tooth chainring, a 16-tooth sprocket and a 20-inch wheel, the ratio is 2.75 and the gearing is 55 gear inches. The rollout is about 172.8 inches, or 4.39 meters. At 100 crank revolutions per minute, that gives a calculated speed of about 26.3 km/h.
Comparing combinations
A larger chainring or smaller rear sprocket raises the ratio. Each pedal turn covers more distance, but requires more force for the same force at the wheel. A smaller chainring or larger rear sprocket lowers the ratio.
Different tooth counts can produce similar ratios. For example, 44/16 is 2.75 and 30/11 is about 2.727. Use the nearby-combinations table to compare the difference before changing parts. Check that the chainring, sprocket and chain are compatible with your bike.
Wheel size matters
The diameter entered here should include the tire. A nominal 20-inch wheel can have a different rolling diameter depending on the rim and tire. For a closer rollout estimate, measure the distance the bike travels in one wheel revolution and use that circumference to work out the diameter.
Choosing a gear
Compare the calculator results with your current setup, then test changes on the track. Cadence, rider strength, starts, turns and straight lengths all affect which ratio feels usable. The speed figure assumes the cadence stays constant; it does not model acceleration, wind, rolling resistance or coasting.
