Bike gear ratio calculator
Chainring, sprocket and cadence in; speed out. Useful for exactly one question that matters more than the rest: is your easiest gear low enough for the climbs you keep avoiding?
Gearing and cadence
What speed does this gear give me at this cadence?
2105 mm is a 700×25c tyre; 2136 mm is a 28c. Useful for one question above all: is your easiest gear low enough to hold 70 rpm on a 10% ramp?
The only gearing question that matters
Not "is my bike fast enough". It is: can I hold 70 rpm in my easiest gear on the steepest climb I actually ride? Below about 60 rpm on a long climb you stop spinning and start weightlifting, which burns matches, wrecks your knees and ends with a walk.
Put your smallest chainring and your biggest sprocket into the calculator above, then check the speed it gives you at 70 rpm against the speed you can realistically manage up that climb. If the gear is faster than you are, the gear is too big.
Wheel circumference, without measuring anything
| Tyre | Circumference |
|---|---|
| 700 × 23c | 2096 mm |
| 700 × 25c | 2105 mm |
| 700 × 28c | 2136 mm |
| 700 × 32c | 2155 mm |
| 650b × 47 | 2090 mm |
| 29 × 2.1" | 2290 mm |
Close enough for every purpose here. If you want it exact, mark the valve, roll the bike one full turn with your weight on it, and measure the distance.
Common setups, and who they suit
- 50/34 with 11–28 — the standard compact. Fine on rolling terrain, punishing above 10% if you are not light.
- 50/34 with 11–32 or 11–34 — the same bike made climbable. For most riders who are not racing, this is the upgrade that actually changes rides.
- 48/31 or 46/30 gravel setups — lower still, and no real loss unless you regularly ride above 45 km/h.
Gearing is the cheapest performance change on a bike, and the only one that works on the days your legs do not. It is also the one people put off longest, usually out of pride.
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