Bike Gear Ratio Calculator

Frequently Asked Questions

What is gear development and why does it matter?

Development is the distance your bike travels per complete pedal revolution, measured in metres. It accounts for both the gear ratio and the wheel circumference, making it the most accurate way to compare gearing across bikes with different wheel sizes. Higher development means more distance per pedal stroke.

What gear ratio should I use for climbing?

For most recreational riders on climbs steeper than 8-10%, a ratio of 1:1 or lower is recommended. That means 34T chainring with a 34T cog, for example. Steep mountain passes often demand 34/36 or even 34/42 on modern gravel and touring bikes.

How do I know if my cadence is in the right range?

Most riders are most efficient between 85 and 100 rpm. Use the speed output from this calculator to find a gear that keeps you in that rpm range at your typical riding speeds. At 90 rpm on a 50/15 setup with 700x25 tyres, you should be doing about 38 km/h.

What is the difference between gear inches and gain ratio?

Gear inches is a historical measure (wheel diameter in inches times the gear ratio) that works well for comparing bikes with identical crank lengths. Gain ratio (Sheldon Brown) also divides by crank length, giving a dimensionless number that fairly compares bikes with different cranks. Use gain ratio when comparing road and mountain bikes or bikes with 165 mm vs 175 mm cranks.

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Estimates for informational purposes only.

Important Disclaimer: Estimates for informational purposes only.

This calculator provides estimates for informational purposes only. Results are based on assumptions and may not reflect actual outcomes. Consult qualified professionals in relevant fields before making important decisions based on these results.

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How This Calculator Works

This tool turns a bike's chainring and 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 1x 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 math only needs one circumference and one cadence to deliver everything else.

Formulas and Methodology

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 is 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; gear inches ≈ 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; gear inches = 32.2 in - perfect for a steep grade where spinnability matters over speed. The ratio between A and B is 2.74x, which is exactly how much steeper a climb you can take in B without changing cadence.

Key Concepts: Gear Metrics Explained

Gear ratio: The simplest number, but meaningless across bikes with different wheel sizes.

Development: Metres per pedal revolution - the most universally useful metric for comparing gearing across any wheel size, including 700c, 650b, and 26 inch.

Gear inches: The old penny-farthing standard still used by British tradition. Intuitive once you know that 70+ gear inches is high gear and 30 gear inches is climbing territory.

Gain ratio: Sheldon Brown's metric that accounts for crank length. A 170 mm crank at 88 gear inches gives a different mechanical advantage than a 175 mm crank at the same gear inches.

Common Mistakes

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

Comparing gear inches across crank lengths without converting to gain ratio.

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 and FAQ

How do I pick a climbing gear? Aim for a 1:1 ratio or lower on anything above 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. 1x vs 2x? 1x loses 7-9% efficiency at extreme cross-chains and tighter ratio jumps; 2x 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 will feel when chasing a paceline.

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