What is a Gear Ratio and Gear Inches?
Gears are used to increase force or speed, and the gear ratio is a fundamental unit of bicycle gearing. To calculate the gear ratio, divide the number of teeth on the front chainring by the number of teeth on the rear sprocket:
gear ratio = front chainring teeth ÷ rear sprocket teeth
For example, with 50 teeth in front and 25 in the back, the gear ratio is 2.0. A ratio of 1.0 means the wheel turns once per pedal revolution; a ratio of 2.0 means the wheel turns twice. Higher ratios mean more speed but more effort; lower ratios mean easier pedaling for climbing hills.
Gear Inches Explained
Gear inches is another relative measure of bicycle gearing. It corresponds to the diameter of the main wheel of an old-fashioned penny-farthing bicycle with equivalent gearing, and is calculated as:
gear inches = wheel diameter (in) × gear ratio
Gear inches are usually rounded to the nearest whole number. For instance, a 29" wheel with a 42/11 combination gives approximately 110 gear inches — a very high gear suitable for fast flat riding or descents.
How Does the Bike Gear Calculator Work?
Our bike gear calculator generates tables showing gear ratios and gear inches for a full range of chainring and cog combinations:
- Choose the chainring range — set the minimum and maximum teeth for the front chainring (e.g., 42 to 52).
- Choose the cog range — set the minimum and maximum teeth for the rear sprocket cassette (e.g., 11 to 28).
- Enter the wheel diameter (optional) — in inches (imperial) or millimeters (metric) to get the gear inches table.
- Select your unit system — imperial (inches) or metric (mm).
- The calculator displays two tables: Gear Ratio and Gear Inches (if wheel diameter is provided).
Common Wheel Sizes
| Wheel Type | Diameter (in) | ISO (mm) | Common Use |
|---|---|---|---|
| 26" | 26.0 | 559 mm | Older mountain bikes |
| 27.5" / 650b | 27.5 | 584 mm | Modern trail MTB |
| 700c / 28" | 27.5–28 | 622 mm | Road & gravel bikes |
| 29" | 29.0 | 622 mm | 29er mountain bikes |
American vs Metric Units
Imperial (American): wheel diameter is entered in inches. Gear inches output is in inches — the traditional unit used by cyclists in the USA and UK.
Metric: wheel diameter is entered in millimeters (ISO standard). The calculator converts to inches internally to compute gear inches, which remain in the traditional inch-based format for compatibility.
FAQs
What is a fixed gear bike?
A fixed-gear bike ("fixie") has a single gear with no freewheel mechanism. The pedals are always in motion when the bike moves — you cannot coast. Fixies are popular for their simplicity and direct feel, and their gear ratio is the only number that matters for gearing.
Which cog is gear 1 on a bike?
Gear 1 (the "lowest" or "easiest" gear) uses the largest cog on the rear cassette and the smallest chainring in front. This gives the lowest gear ratio, ideal for climbing steep hills or starting from a stop.
Why not just use chainring and cog teeth as a measure?
Because it is not immediately obvious whether 39/14 is the same as 53/19. Both equal a gear ratio of approximately 2.79 (or about 78 gear inches on a 28" wheel). A single number is far easier to compare and remember.
Do hub gears affect the calculation?
Yes. For bikes with internal hub gears (planetary gears), the gear ratio also depends on the internal hub ratio, which this calculator does not include. The tool is intended for derailleur-geared bicycles with external cassettes and chainrings.