Bike Gear Calculator

A bike gear calculator describes how far one turn of the pedals moves a bicycle. The chainring at the cranks and the cog on the rear hub set the gear ratio, and the wheel turns that ratio into distance. Four figures follow from those inputs: gear ratio, gear inches, development, and speed at a chosen cadence.

Chainring and cog are counted in teeth. Road cranksets commonly carry 50/34 or 52/36 chainrings, single-ring mountain bikes 30 to 36 teeth, and rear cassettes start at 10 or 11 teeth and run up to 34 or 36 on road bikes and 50 or more on wide-range mountain bike cassettes. Wheel size enters as an outer diameter in millimeters. Each preset is estimated from its ISO 5775 tire size: the bead seat diameter (622 mm for 700c and 29-inch rims, 584 mm for 650b or 27.5-inch, 559 mm for 26-inch mountain bike rims) plus twice the tire width, treating the height of the inflated tire as equal to its width. A wheel measured from the floor to the top of the tread, or a measured rollout divided by pi, is more exact and can be entered as a custom diameter. Cadence is crank revolutions per minute; relaxed riding sits around 60 to 80 RPM and trained road riders often hold 85 to 100.

Gear ratio is chainring teeth divided by cog teeth, the number of wheel turns per crank turn. Gear inches multiply that ratio by the wheel diameter in inches (millimeters divided by 25.4). The convention comes from high-wheel bicycles, so the figure is the diameter of a direct-drive wheel that would pedal the same. Development, also called rollout, multiplies the ratio by the wheel circumference, pi times the diameter, and gives the distance traveled per crank revolution in meters, or in feet at 3.28084 feet per meter. Speed is development times cadence: meters per revolution times revolutions per minute is meters per minute, which times 60 and divided by 1,000 is km/h, and km/h times 0.621371 is mph. Settings switches speed between mph and km/h and development between meters and feet, and adds a step-by-step view of each formula.

Gear inches and development measure the same thing on two scales, so either compares gears across bikes with different wheels: a 50/17 on a 700 x 25c road wheel (77.81 gear inches) and a 40/13 on a 26 x 2.0 mountain bike wheel (79.83) pedal almost alike. The speed figure is what the gear produces at that cadence, not a forecast of riding speed, which depends on power, gradient, wind, and rolling resistance. Load and tire pressure flatten the tire where it meets the road, so the true rolling diameter runs slightly below the preset estimate.

A 50-tooth chainring and a 17-tooth cog give a ratio of 50 / 17 = 2.9412. A 700 x 25c wheel is taken as 622 + 2 x 25 = 672 mm, which is 672 / 25.4 = 26.4567 inches, so the gear measures 2.9412 x 26.4567 = 77.81 gear inches. The circumference is pi x 0.672 = 2.1112 m, the development is 2.1112 x 2.9412 = 6.209 m per crank revolution, and at 90 RPM the bike covers 6.209 x 90 = 558.8 m per minute, which is 33.53 km/h or 20.83 mph.

Gear selection affects climbing ability, top-end speed, and pedaling comfort across every cycling discipline. Knowing the numeric relationship between chainring, cog, and wheel size helps riders, mechanics, and coaches make informed drivetrain choices.

• Road Cycling Optimization: A 53/11 against a 50/11 on 700 x 25c wheels shows what a compact crankset gives up at the top end: at 100 RPM the 53/11 reaches 61.03 km/h and the 50/11 57.58 km/h, a gap of 127.47 against 120.26 gear inches.
• Mountain Bike Climbing Gears: A 30-tooth chainring with a 51-tooth cog on 29 x 2.25 wheels works out to 17.04 gear inches and 1.36 m per crank revolution, low enough to keep turning the pedals at a walking pace of about 5.7 km/h at 70 RPM.
• Commuter Bike Setup: A single-speed gear has to cover flat roads and moderate hills with one ratio. A 44/16 on 700 x 32c tires develops 5.93 m per revolution, about 32 km/h at 90 RPM, which sets the top of the comfortable range before spinning out.
• Track and Velodrome Racing: Fixed-gear track bikes require precise ratio selection because there is no shifting. A 49/15 on a 700 x 23c tire gives 85.91 gear inches and 49.36 km/h at a race cadence of 120 RPM, a figure to settle before buying a chainring.
• Gravel and Bikepacking: Riders carrying touring weight need lower gearing than unburdened road setups. A 40/42 on 700 x 38c tires comes to 26.17 gear inches, a common target zone for loaded fire-road climbing.
• Youth and Fitness Cycling: Coaches setting up junior bikes can match gear inches to a young athlete's power output, ensuring the drivetrain does not over-gear small riders who pedal at lower cadences.
• Bicycle Retail and Fitting: Shop mechanics comparing cassette upgrades can show customers the exact speed and development changes between an 11-28 and an 11-34 cassette, making the recommendation concrete rather than abstract.