Bike Gear Calculator

Frequently Asked Questions

What are gear ratio, gain ratio, and gear inches?

Gear ratio = chainring teeth ÷ cog teeth (e.g. 50 ÷ 25 = 2.0). Gear inches = gear ratio × wheel diameter inches (e.g. 2.0 × 27 = 54 in). Gain ratio (Sheldon Brown) divides gear inches by crank length to account for leverage.

How do I find road speed at a given cadence?

Speed = cadence (rpm) × gear ratio × wheel circumference. At 90 rpm in 50×11 on a 2,096 mm tire: 90 × (50/11) × 2.096 ÷ 1000 × 60 ≈ 51 km/h.

What does “1×” vs “2×” mean for gearing?

1× (single chainring) trades top-end or low-end range for simpler shifting and lighter weight. A 1×12 with 10–52T cassette spans 5.2×; a typical road 2×11 (50/34 × 11–32) spans 4.3× but with much smaller steps between ratios.

What gear range do I need for steep climbs?

A useful guide: low-gear inches ≤ rider weight (kg) at 5% grade for comfortable spinning. A 75 kg rider tackling 10%+ climbs typically wants a low gear under 30 gear inches (e.g. 34×34 ≈ 27 in).

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How This Calculator Works

This tool turns a bike's chainring & cog choice into four practical numbers: gear ratio, development (metres travelled per pedal stroke), gear inches, and speed at a chosen cadence. You can punch in two setups side by side - useful for comparing a road compact 50/34 to a 1× gravel 42T, or for checking what an 11–28 cassette gives you versus an 11–34. Wheel circumference is picked from a dropdown of common tyre sizes; the underlying maths only needs one circumference and one cadence to deliver everything else.

Formula / Method

Gear ratio = chainring teeth ÷ cog teeth (e.g. 50/15 = 3.33). Development = ratio × wheel circumference (m); at 50/15 with a 2.111 m 700×25 wheel that's 7.03 m per pedal stroke. Speed = development × cadence (rev/min) × 60 ÷ 1000 km/h; multiply by 0.6214 for mph. Gear inches = ratio × wheel diameter (inches), the historical penny-farthing equivalent - the diameter of the giant front wheel needed to give the same one-pedal travel. Gain ratio (Sheldon Brown) = ratio × (wheel radius ÷ crank length) and is the only metric that fairly compares bikes with different crank lengths, since longer cranks give more leverage at the same gear.

Worked Example

Setup A: 50T chainring, 15T cog, 700×25 wheel (circumference 2.111 m), pedalling 90 rpm. Ratio = 3.33; development = 3.33 × 2.111 = 7.03 m/rev; speed = 7.03 × 90 × 60 ÷ 1000 = 37.96 km/h (23.6 mph); gear inches = 3.33 × 26.5 ≈ 88.3 in. Setup B: 34T chainring, 28T cog, same wheel and cadence. Ratio = 1.214; development = 2.56 m/rev; speed = 13.83 km/h (8.6 mph); gear inches = 32.2 in - perfect for a steep grade where you want spinnability over speed. The ratio between A and B is 2.74×, which is exactly how much steeper a climb you can take in B without changing cadence.

Common Mistakes

Forgetting that 700×25 measured circumference is not the nominal "700 mm" - actual rolling circumference under load runs ~2,095–2,160 mm depending on width and pressure.

Comparing gear inches across crank lengths without converting to gain ratio - a 170 mm crank at 88 gear inches feels different to a 175 mm crank at the same number.

Picking a gear too tall for your terrain - modern climbing wisdom is "easier than you think you need"; bailout gears below 1.0 ratio are normal on gravel bikes.

Ignoring cadence sweet spot; most riders are most efficient at 85–100 rpm, so target gears that hit that range at your typical speeds.

Using the calculator with bent cassettes or stretched chains - effective ratio drifts and shifting suffers.

Tips & FAQ

How do I pick a climbing gear? Aim for a 1:1 ratio or lower on anything > 10% grade if you climb seated. Why are pros on 11–30 cassettes now? Modern grand tour stages with 25%+ ramps demand it; the old 11–23 race cassette is essentially extinct for mountain stages. Track bikes? Fixed-gear track racing typically uses 88–94 gear inches - chosen to spin 110–130 rpm at 60+ km/h. 1× vs 2×? 1× loses ~7–9% efficiency at extreme cross-chains and tighter ratio jumps; 2× keeps closer ratios but adds a front derailleur. Cassette range vs jumps? An 11–28 has 1-tooth jumps low; 11–34 has 2–4 tooth jumps that you'll feel when chasing a paceline.

Frequently Asked Questions

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