Fixed Gear Ratio Calculator: Find Your Gear Inches

A fixed gear ratio calculator works out the relationship between a bicycle’s front chainring and rear sprocket, then shows the resulting ratio and, when wheel size is included, gear inches. Enter the tooth counts and wheel diameter to compare setups and understand how far the bike travels per crank revolution.

Formula

Gear ratio = front chainring teeth ÷ rear sprocket teeth. For a fixed-gear bicycle, this is the number of rear-wheel turns for one crank revolution, assuming no chain slip. The rear wheel and crank are mechanically connected through the chain, so the gear ratio describes their rotation relationship.

Gear inches = wheel diameter × gear ratio. This traditional comparison figure estimates the effective diameter of a direct-drive wheel-and-gear combination. Rollout = π × wheel diameter × gear ratio, the approximate distance traveled by the wheel for one complete crank revolution.

• chainring: front chainring tooth count, in teeth.
• cog: rear sprocket tooth count, in teeth.
• wheel: loaded or nominal wheel diameter, in inches for this calculator.
• π: approximately 3.14159, used to convert wheel diameter to circumference.

The ratio itself has no physical unit; it is often written as a number followed by “:1.” The calculator uses inches for wheel diameter and reports gear inches and rollout in inches. For metric values, multiply inches by 2.54 to get centimeters, or by 0.0254 to get meters. Tooth counts are whole numbers; the rear cog must have at least one tooth.

Worked Example

Using the calculator’s default values—48 teeth in front, 17 teeth at the rear, and a 27-inch wheel:

1. Gear ratio = 48 ÷ 17 = 2.8235…, which rounds to 2.82:1.
2. Gear inches = 27 × (48 ÷ 17) = 76.235…, which rounds to 76.2 inches.
3. Rollout = π × 27 × (48 ÷ 17) = 239.50…, which rounds to 239.5 inches.

That rollout is about 6.08 meters per crank revolution. The result is an estimate because the entered wheel diameter may not match the tire’s actual rolling circumference under load.

How to Read a Fixed-Gear Ratio

A ratio of 48/17 is commonly stated as 2.82:1. In practical terms, the rear wheel rotates about 2.82 times for every full crank revolution. A larger ratio—created by a bigger chainring or smaller rear cog—moves the bicycle farther per pedal turn, but requires more force to start or climb. A smaller ratio makes it easier to turn the pedals, while the wheel covers less distance per revolution.

Gear inches and rollout help compare setups that use different wheel sizes. Two bicycles can share the same tooth-count ratio but have different effective gearing if their wheel diameters differ. Conversely, different tooth combinations can produce nearly the same gear inches or rollout. The ratio is the mechanical relationship; gear inches and rollout bring wheel size into the comparison.

These numbers do not directly state a rider’s speed. Speed also depends on cadence, tire circumference, and how consistently the rider turns the cranks. With a fixed gear, the pedals turn whenever the rear wheel turns, so coasting without turning the pedals is not possible while the drivetrain remains engaged.

Choosing Tooth Counts and Wheel Size

Use the calculator to compare options before changing parts. For example, a 46-tooth chainring and 16-tooth cog give a ratio of 2.875:1. With a 27-inch wheel, that is about 77.6 gear inches and 244.0 inches of rollout. Compared with the default 48/17 combination, it is slightly taller gearing: the wheel travels farther per pedal revolution, and the pedals take more effort to turn under the same conditions.

• Choose the chainring and cog tooth counts you want to compare; count the teeth or use clearly labeled component specifications.
• Enter the same type of wheel diameter for each comparison. A tire-size label is not always the exact outside diameter.
• Compare ratio for drivetrain rotation, and gear inches or rollout when wheel size matters.
• Consider the riding conditions and your ability to start, climb, and control the bicycle; the calculation alone cannot choose a suitable setup.

There is no universally best ratio. Terrain, rider preference, cadence, tire size, and the bicycle’s intended use all affect what feels appropriate. A calculator provides a consistent way to compare the mechanical effect, not a guarantee of comfort or performance.

Common Mistakes and Limits

Do not reverse the tooth-count division. Front teeth go in the numerator and rear teeth in the denominator. Dividing 17 by 48 would describe the reciprocal, not the usual bicycle gear ratio. Also avoid comparing gear-inch figures calculated with different wheel-diameter units: this calculator expects inches, not millimeters or centimeters.

Wheel diameter is an approximation. Tire width, pressure, rim dimensions, and load can change the actual rolling circumference. For greater precision, measure the distance traveled in one wheel revolution with the rider and normal tire pressure aboard, then use that measured circumference for rollout. If circumference is measured in inches, rollout per crank revolution is circumference × chainring teeth ÷ rear cog teeth. Gear inches can then be estimated as rollout ÷ π.

Finally, rounding can make close combinations appear identical or slightly different. The displayed ratio is rounded to two decimal places, while gear inches and rollout are rounded to one decimal place. Use the unrounded tooth-count relationship for calculations, and treat small differences in displayed results accordingly.

Frequently Asked Questions

How do you calculate a fixed-gear ratio?
Divide the number of teeth on the front chainring by the number of teeth on the rear cog. For 48 and 17 teeth, the ratio is 48 ÷ 17, or about 2.82:1.

What does a 48/17 gear ratio mean?
It means the rear wheel makes about 2.82 revolutions for every full crank revolution, assuming the drivetrain is engaged and the chain does not slip. It does not, by itself, specify a riding speed.

What are gear inches?
Gear inches multiply wheel diameter in inches by the gear ratio. They are a traditional way to compare effective gearing across bicycles with different wheel sizes.

Is a higher fixed-gear ratio harder to pedal?
Generally, yes. A higher ratio covers more distance per crank revolution and requires more torque at the pedals for a given wheel force. It can make starting and climbing more demanding.

How do I convert rollout from inches to meters?
Multiply the rollout in inches by 0.0254. For example, 239.5 inches is approximately 6.08 meters.

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in chainring | Front chainring teeth | teeth | 48 | 0
in cog | Rear sprocket teeth | teeth | 17 | 1
in wheel | Wheel diameter | in | 27 | 0
out ratio | Gear ratio | :1 | chainring / cog | 2
out gear_inches | Gear inches | in | wheel * ratio | 1
out rollout | Rollout per crank revolution | in | pi * wheel * ratio | 1