How to calculate bike gear ratio: formula, calculator and examples

Before fitting a new cassette or crankset, it’s worth knowing exactly what gear ratio you’re already running. The same compact chainring can feel like plenty on a gentle gran fondo and fall short on one with steep ramps: the difference comes down to the exact combination of chainring, cog and wheel. Below you’ll find the formula to calculate it by hand and a calculator that turns the result into a cassette or crankset recommendation for your route profile.

How to calculate bike gear ratio (the formula)

Gear ratio compares how many times the rear wheel turns for every full turn of the pedals:

Gear ratio = chainring teeth ÷ cog teeth

Example: a 50-tooth chainring with a 34-tooth cog gives 50 ÷ 34 = 1.47. The higher the ratio, the harder each pedal stroke pushes and the faster you go for the same cadence.

To get a real-world distance instead of an abstract ratio, multiply it by your wheel’s circumference. This is what’s usually called development, and it’s the figure that actually tells you how a gear will feel on a climb:

Development (m) = (chainring teeth ÷ cog teeth) × wheel circumference (m)

A standard 700x25c wheel has a circumference of about 2.105 m. So a 50/34 combination on that wheel gives:

(50 ÷ 34) × 2.105 = 3.10 m per pedal revolution

The lower that number, the easier the gear is to spin on a steep ramp. Instead of doing this by hand for every combination you’re considering, plug your own numbers into the calculator below.

Calculate your gear ratio

How to read the result

Gear development is the distance the bike travels per full pedal revolution. A short development (below 5.4 metres) makes the steepest ramps much easier, at the cost of running out of gear on descents or fast flat sections. A long development (above 7 metres) is fast on flat ground but demands more force on climbs.

• Below 5.4 m → very steep climbs, like Larra-Larrau or the Angliru. Here a wide-range cassette like the SRAM Force AXS 10-36 makes sense.
• Between 5.4 and 6.2 m → classic long gran fondo climbs. The Ultegra 11-34 cassette or the 50/34 compact crankset give you just the right margin.
• Between 6.2 and 7 m → balanced profile. A 52/36 semi-compact crankset makes better use of flat sections without suffering too much on the climb.
• Above 7 m → rolling or flat routes, where a close-ratio cassette like the 11-28 keeps your cadence useful.

Why chainring or cassette alone isn’t enough

Gear ratio depends on the combination of both, not either one in isolation. A 50/34 compact chainring paired with an 11-25 cassette behaves very differently from that same chainring with an 11-34: the largest cog is what really decides whether you’ll suffer in the final kilometres of a climb. That’s why the calculator asks for all three figures (chainring, cog and wheel) instead of relying on the groupset’s marketing name.

If you want to go deeper on choosing a cassette and chainring for long climbs, the gearing guide for mountain gran fondos compares wide-range options in detail.