Bike gear ratio calculator
Work out gear inches, gain ratio, development and speed for any gear. Change your chainring, cog, wheel and tyre and the numbers update instantly.
Your setup
This gear
- Gear inches
- 78.5
- Gain ratio
- 5.87
- Development
- 6.26 m
- Speed @ 90 rpm
- 33.8 km/h21.0 mph
50T chainring across a typical cassette
Gear inches and speed at 90 rpm for each rear cog.
| Rear cog | Gear inches | Speed @ 90 rpm |
|---|---|---|
| 11T | 121.3 | 52.3 km/h |
| 13T | 102.7 | 44.2 km/h |
| 15T | 89.0 | 38.3 km/h |
| 17T | 78.5 | 33.8 km/h |
| 19T | 70.2 | 30.3 km/h |
| 21T | 63.6 | 27.4 km/h |
| 24T | 55.6 | 24.0 km/h |
| 28T | 47.7 | 20.5 km/h |
| 32T | 41.7 | 18.0 km/h |
Wheel size (ETRTO) reference
Wheel names are confusing because several refer to the same rim. What matters for gearing is the ETRTO bead-seat diameter (BSD) in millimetres. For example 700c, 29in and 28in are all the same 622 mm rim.
| Wheel name | ETRTO BSD | Typically found on |
|---|---|---|
| 700c / 29in | 622 mm | Road, gravel, hybrid, 29er MTB |
| 650b / 27.5in | 584 mm | Gravel and trail MTB |
| 26in | 559 mm | Older MTB, some folding bikes |
| 27in | 630 mm | Vintage road bikes |
| 650c | 571 mm | Small road / older triathlon |
| 24in | 507 mm | Junior bikes, jump bikes |
| 20in | 406 mm | BMX and most folding bikes |
| 16in | 349 mm | Brompton |
Gear calculator FAQs
- What are gear inches?
- Gear inches express how far the bike travels for one pedal turn, normalised as the diameter (in inches) of a direct-drive wheel that would give the same distance. Higher = a harder, faster gear; lower = an easier climbing gear. A typical road bike spans roughly 30 (easy) to 120 (fast) gear inches.
- What is a gain ratio and why use it instead of gear inches?
- Gain ratio, devised by Sheldon Brown, also accounts for crank length: it is the wheel radius (including tyre) divided by the crank length, times the chainring/cog ratio. Because it factors in the lever you pedal with, it compares fairly across different wheel and crank sizes. Most road gears fall between about 2.0 and 9.0.
- Does tyre size change my gearing?
- Slightly. A wider tyre increases the rolling diameter, which raises gear inches and speed a little for the same gear. Going from a 25 mm to a 32 mm tyre on a 700c wheel adds roughly 1 percent to your effective gearing.
- How do I work out my top speed in a gear?
- Multiply the development (metres per pedal revolution) by your cadence (pedal revolutions per minute), then convert. This calculator does it for you and shows km/h and mph at the cadence you set.
- What is ETRTO and the bead-seat diameter (BSD)?
- ETRTO (ISO 5775) is the international tyre and rim sizing standard. The bead-seat diameter is the rim diameter where the tyre sits, in millimetres. For example 700c, 29in and 28in are all the same 622 mm rim. BSD is the number this calculator uses for accurate sizing.