Gear Inch Ratio Calculator & Formula

This calculator works out the gear inch ratio for a bicycle drivetrain. You enter your chainring tooth count, rear cog tooth count, and wheel diameter in inches; it returns the gear in inches and the development in meters per pedal revolution. The gear inch value tells you how far the bike moves, in equivalent wheel inches, for each pedal rotation.

Formula

The gear inch ratio is calculated as:

Gear inches = (Chainring teeth ÷ Rear cog teeth) × Wheel diameter (inches)

Where:

• Chainring teeth (t) – number of teeth on the front chainring (unitless).
• Rear cog teeth (c) – number of teeth on the rear cassette sprocket (unitless).
• Wheel diameter (w) – the outer diameter of the wheel including the tire, measured in inches.

The development (distance per pedal revolution) is derived from the same ratio multiplied by the wheel circumference:

Development (m) = (t ÷ c) × w × 0.0254 × π

The factor 0.0254 converts inches to meters, and π gives the circumference from the diameter.

Worked Example

Using the default values from the calculator: a 50-tooth chainring, a 12-tooth cog, and a 27.5-inch wheel diameter.

1. Compute the gear ratio: 50 ÷ 12 = 4.1667
2. Multiply by wheel diameter: 4.1667 × 27.5 = 114.58 inches
3. So the gear inches = 114.6 (rounded to one decimal).
4. For development: 4.1667 × 27.5 × 0.0254 × 3.1416 = 9.14 meters per pedal revolution.

The calculator shows 114.6 gear inches and 9.14 m development, matching the manual calculation.

What Gear Inches Actually Mean

Gear inches is a legacy measurement that dates back to the penny-farthing era. A gear of 114.6 inches means the bike behaves as if you were riding a direct-drive bicycle with a wheel that is 114.6 inches in diameter. In practical terms, it tells you how hard each pedal stroke is and how far you travel per revolution.

Higher gear inches = harder pedaling but more speed per stroke. Lower gear inches = easier pedaling, better for climbing, but less speed. For comparison:

• Mountain bike climbing gear: 20–35 gear inches
• Road bike cruising gear: 70–100 gear inches
• Track sprint gear: 100–120 gear inches

These ranges are rough guides; actual comfort depends on your strength, cadence, and terrain.

How to Choose the Right Gear Ratio

Choosing a gear ratio is about matching your cadence (pedal revolutions per minute) to your speed and terrain. Here’s a practical approach:

1. Determine your target speed and cadence. Most recreational riders pedal at 60–80 rpm; racers often use 90–100 rpm.
2. Calculate the gear inches you need using the formula: speed (mph) × 336 ÷ cadence (rpm) = gear inches (for a given wheel size).
3. Select a chainring/cog combination that gives that gear inch value.
4. Test on a hill and on flat ground. If you spin out on descents, go higher; if you struggle on climbs, go lower.

For example, at 80 rpm and 20 mph on a 27.5-inch wheel, you need about 84 gear inches. A 50/16 combination gives 85.9, which is close.

Common Mistakes and Misconceptions

One frequent error is mixing up wheel diameter with wheel radius or using the rim size instead of the actual tire diameter. The wheel diameter must include the tire, not just the rim bead seat diameter. For instance, a “27.5-inch� mountain bike wheel with a 2.2-inch tire actually measures about 27.5 inches, but a 29er with a 2.0-inch tire is close to 29 inches. Always measure the true outer diameter or use a tire size chart.

Another misconception is that gear inches are the same as “gear ratio.� The gear ratio is just teeth ÷ teeth (e.g., 50/12 = 4.17). Gear inches multiply that by wheel diameter. Two bikes with the same gear ratio but different wheel sizes will have different gear inches and feel different.

Also, remember that gear inches ignore drivetrain losses, tire pressure, and rider weight. They are a comparative tool, not an absolute measure of effort.

Gear Inches vs. Meters of Development

While gear inches are common in English-speaking countries, many European cyclists use meters of development (distance per pedal revolution). The conversion is simple: 1 gear inch = 0.0254 × π meters of development, or roughly 0.08 meters. The calculator provides both so you can compare with any source.

For example, a 100-gear-inch bike travels about 7.98 meters per pedal revolution. A 50-gear-inch bike travels about 3.99 meters. This metric is more intuitive for understanding speed at a given cadence.

Limitations of the Formula

The gear inch formula assumes a rigid wheel and no tire deformation. In reality, tire deflection under load reduces effective diameter slightly, but the difference is negligible for most calculations. The formula also doesn’t account for crank length, which affects torque but not gear inches.

For bikes with internal hub gears, the calculation is more complex because the hub has an internal ratio. You would multiply the chainring/cog ratio by the hub gear ratio and then by wheel diameter. For example, a 50/16 chainring (3.125) with a 1.5 hub ratio gives 4.69, times 27.5 = 129 gear inches.

Finally, gear inches are a static measure. They don’t tell you about acceleration, momentum, or how the bike feels when climbing. For that, you need to consider rider weight, gradient, and power output.

Frequently Asked Questions

How do I measure wheel diameter for the calculator?

Measure the outer diameter of your wheel including the tire. Place the wheel upright, measure from the ground to the top of the tire, or use the tire size printed on the sidewall (e.g., 700x25c is about 27.5 inches, 29×2.1 is about 29 inches). For accuracy, measure directly with a tape measure.

What is a good gear inch for a beginner?

For flat terrain, a gear inch range of 55–75 is comfortable for most beginners. That allows a moderate cadence without excessive strain. For climbing, you’ll want lower, around 30–40 gear inches. Start with a mid-range gear and adjust based on your leg strength and fitness.

Does crank length affect gear inches?

No, gear inches only depend on chainring, cog, and wheel diameter. Crank length affects how much torque you apply but not the distance traveled per pedal revolution. Longer cranks make pedaling easier on joints but don’t change the gear ratio.

How do I convert gear inches to gear ratio?

Divide gear inches by wheel diameter. For example, 114.6 gear inches ÷ 27.5 inches = 4.17, which is the chainring/cog ratio. Conversely, multiply gear ratio by wheel diameter to get gear inches.

What is the difference between gear inches and speed?

Gear inches is a static measure of mechanical advantage. Speed depends on gear inches and cadence. Speed (mph) = gear inches × cadence (rpm) ÷ 336. So a 100-gear-inch bike at 90 rpm goes 26.8 mph, while at 60 rpm it goes 17.9 mph.

Related

• Bike Gear Ratio Calculator: How to Use It
• Bike Gear Range Calculator: How to Use It
• Bike Gear Ratio Calculator: How to Compare

in t | Chainring teeth | teeth | 50 | 0
in c | Rear cog teeth | teeth | 12 | 0
in w | Wheel diameter | in | 27.5 | 0
out gear | Gear inches | in | t / c * w | 1
out dev | Development | m | (t / c) * w * 0.0254 * pi | 2