BMX Gear Ratio Calculator
Enter the front sprocket, rear driver and wheel size and get the gear ratio, gear inches, rollout, development and speed at your cadence, with a gear chart.
About 20 for a 20 in BMX; measure for accuracy.
The result updates as you type.
25/9 gearing
55.6 gear inches · ratio 2.778
174.5 in rollout per pedal turn · 4.43 m development · gain ratio 4.03 · 14.9 mph (23.9 km/h) at 90 rpm
What this page cannot tell you
Which gearing suits your riding or track; rider strength and course decide that. The result is only as good as the wheel diameter you enter — tyre size and pressure change it.
Gear inches chart for a 20 in wheel
| Front \ rear | 8T | 9T | 10T | 11T | 12T | 13T | 14T |
|---|---|---|---|---|---|---|---|
| 25T | 62.5 | 55.6 | 50 | 45.5 | 41.7 | 38.5 | 35.7 |
| 26T | 65 | 57.8 | 52 | 47.3 | 43.3 | 40 | 37.1 |
| 27T | 67.5 | 60 | 54 | 49.1 | 45 | 41.5 | 38.6 |
| 28T | 70 | 62.2 | 56 | 50.9 | 46.7 | 43.1 | 40 |
| 30T | 75 | 66.7 | 60 | 54.5 | 50 | 46.2 | 42.9 |
| 33T | 82.5 | 73.3 | 66 | 60 | 55 | 50.8 | 47.1 |
| 36T | 90 | 80 | 72 | 65.5 | 60 | 55.4 | 51.4 |
How to work out BMX gearing
- Enter the sprocket. Front sprocket teeth, e.g. 25.
- Enter the driver. Rear cog or driver teeth, e.g. 9.
- Enter the wheel size. Diameter with the tyre, in inches.
- Add crank length and cadence. For gain ratio and speed.
- Compare setups. Read gear inches and check the chart.
10 situations where a BMX gear calculator helps
Matching a gear you like
Find other sprocket/driver combinations with the same gear inches.
Switching to a smaller sprocket
25/9 and 28/10 are close; see exactly how close.
Racing gear choice
Compare gear inches for different tracks.
Changing tyre size
A bigger tyre raises gear inches.
24 in cruisers
Enter the bigger wheel to see the difference.
Speed at a cadence
How fast a gear goes at 90 or 120 rpm.
Crank length changes
Gain ratio shows the effect of longer or shorter cranks.
Freecoaster and cassette drivers
Small drivers like 9T and 10T in one table.
Kids' bikes
Small wheels and gearing for young riders.
Fixed and single-speed bikes
The same maths works for any single gear.
Where the formulas come from
Sheldon Brown's 'Gain Ratios' article describes the gear inch system as the drive wheel diameter multiplied by the sprocket ratio, and development as 'wheel diameter in meters x front sprocket x pi / rear sprocket' — the distance travelled per pedal revolution. Rollout is the same distance in inches (gear inches × π). Brown's gain ratio divides the wheel radius by the crank length and multiplies by the sprocket ratio, so it accounts for crank length too.
Speed at a cadence is development × pedal revolutions per minute, converted to mph or km/h.
Limits: results depend on the wheel diameter you enter; tyre width and pressure change the real diameter, so measure it (or measure rollout directly) for accuracy. Which gear suits you is up to your riding. Everything runs in your browser.
Sources
- Sheldon Brown — Gain Ratios — Gear inches, development, gain ratio
Frequently asked questions
What is 25/9 in gear inches?
On a 20 in wheel, 25 ÷ 9 × 20 = 55.6 gear inches.
How do I calculate BMX gear ratio?
Divide the front sprocket teeth by the rear driver teeth: 25/9 = 2.78.
What is rollout?
The distance the bike moves per pedal turn: gear inches × π.
Is 28/10 the same as 25/9?
Close: 28/10 is 2.80 and 25/9 is 2.78 — about 0.4 gear inches apart on a 20 in wheel.
Does tyre size matter?
Yes — gear inches scale with the actual wheel diameter including the tyre.
What is gain ratio?
Sheldon Brown's measure that also includes crank length: wheel radius ÷ crank length × gear ratio.
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