Gear Ratio Calculator

Divide the teeth on your front chainring by the teeth on your rear cog to get the gear ratio. With a 50-tooth chainring and a 25-tooth cog, 50 ÷ 25 = 2.0: the rear wheel turns twice for each complete pedal revolution while pedaling.

With the same effective wheel diameter, a lower ratio gives an easier climbing gear. You travel less distance per pedal revolution and need less force at the pedals for the same force at the tire, assuming the same crank length and similar drivetrain losses. A higher ratio gives more speed at the same cadence, but does not tell you whether you can sustain that speed.

Which is lower: 30/51 or 34/50?

A 30-tooth chainring with a 51-tooth cog gives 30 ÷ 51 ≈ 0.588. A 34-tooth chainring with a 50-tooth cog gives 34 ÷ 50 = 0.680. On the same effective wheel diameter, 30/51 is about 13.5% lower: (1 − (30/51) ÷ (34/50)) × 100 ≈ 13.5%. It travels about 13.5% less distance per pedal revolution, so it is the easier climbing gear under the same conditions. That is a gearing comparison, not a prediction of effort saved or a guarantee for any gradient.

How wheel size changes gear inches

For an assumed effective wheel diameter of 27 inches, a 50/25 gear gives 27 × (50 ÷ 25) = 54 gear inches. The diameter here is an example input, not a nominal tire size. To compare bikes with different tires, use their effective diameters or measured rolling circumferences rather than teeth ratios alone.

From measured rollout to speed

Suppose your fitted tire rolls 2.10 meters in one wheel revolution. In a 50/25 gear, development is 2.10 × (50 ÷ 25) = 4.20 meters per pedal revolution. At 90 RPM, that is 4.20 × 90 = 378 meters per minute, or 378 × 60 ÷ 1,000 = 22.68 km/h.

This is a calculated speed assuming continuous pedaling without tire slip. It does not predict whether you can hold that cadence into a headwind or up a climb. The 2.10-meter rollout is specific to this example; use your own measurement for your bike.

1. Enter your current chainring, cassette and wheel size. Note the lowest gear and where it feels too hard on your usual rides.
2. Compare a candidate setup using the same wheel measurement. Check its lowest gear, highest gear and the jumps between neighboring gears, not just its total range.
3. Check speed at a cadence you are comfortable riding. Use the speed and cadence calculator for a target speed, or the route gearing planner to compare modeled climbing scenarios on a route.

No single ratio suits every rider or climb. Before buying parts, check the manufacturer's compatibility requirements for your derailleur's maximum cog size and total capacity, freehub, chain and chainring. A ratio calculation does not confirm that those parts will fit or shift together.