Fixed Gear Calculator: Gear Ratio & Speed

A fixed gear calculator works out your gear ratio, gear inches, and expected speed at a given cadence for a fixed-gear or single-speed bicycle. You enter your chainring teeth, cog teeth, wheel size, and pedal cadence, and it returns the gear ratio (chainring divided by cog), the distance traveled per pedal revolution, and the speed in km/h or mph. This is the same math track cyclists and messengers use to pick a gear that suits flat roads, hills, or a velodrome.

Formula

The core formulas are simple ratios. The gear ratio is the number of chainring teeth divided by the number of cog teeth:

Gear ratio = T_chainring / T_cog

Gear inches (a traditional measure) multiplies that ratio by the wheel diameter in inches:

Gear inches = (T_chainring / T_cog) × D_wheel

Where D_wheel is the outside diameter of the tire in inches (not just the rim). For speed, you multiply the gear inches by π to get the distance per crank revolution, then multiply by cadence (revolutions per minute) and convert to km/h:

Speed (km/h) = (T_chainring / T_cog) × D_wheel × π × cadence × 60 × 0.0254 / 1000

All lengths are in inches, so the 0.0254 factor converts inches to meters, and the 60 converts minutes to hours.

Worked Example

Using the defaults from the calculator: 46-tooth chainring, 16-tooth cog, 27-inch wheel, 90 rpm cadence.

1. Gear ratio = 46 / 16 = 2.875
2. Gear inches = 2.875 × 27 = 77.6 inches
3. Distance per crank revolution = 77.6 × π = 243.8 inches = 6.19 meters
4. Per minute at 90 rpm = 6.19 × 90 = 557.3 meters
5. Per hour = 557.3 × 60 = 33,437 meters = 33.4 km/h

The calculator shows 33.4 km/h, matching the manual calculation.

How to Choose a Fixed Gear Gear Ratio

Your ideal ratio depends on terrain, fitness, and whether you ride on the road or track. Here are general guidelines:

• Flat city riding: 46/16 (2.875) or 48/17 (2.82) gives a comfortable cruising speed around 25–30 km/h at a moderate cadence.
• Hilly areas: A lower ratio like 42/18 (2.33) or 46/19 (2.42) makes climbing easier, but you will spin out on descents.
• Velodrome or racing: Track sprinters often use 49/15 (3.27) or higher, while endurance riders might use 48/16 (3.0).
• Skid stops: Some fixed-gear riders choose ratios that allow a skid stop without pedaling, but this is a niche concern; most riders just brake normally.

Cadence is the key variable. A typical recreational rider pedals at 60–80 rpm; a fit cyclist might sustain 90–100 rpm. If you prefer a fast spin, choose a lower gear; if you like to push hard, choose a higher gear.

Common Mistakes and Misconceptions

One frequent error is confusing wheel diameter with rim diameter. The wheel diameter in the formula is the tire’s outer diameter, not the bead seat diameter (e.g., 700c is not 700 mm). A 700c wheel with a 25mm tire is about 27 inches (686 mm) in diameter. Use the actual tire diameter for accurate speed.

Another mistake is mixing units. If you enter wheel diameter in centimeters, the speed formula will be off by a factor of 2.54. Always convert to inches or use consistent metric values (wheel diameter in meters).

Also, gear inches is a legacy measurement from the penny-farthing era; it does not directly tell you speed. Two bikes with different wheel sizes but the same gear inches will feel similar, but the actual speed at a given cadence depends on the wheel circumference.

How to Read the Results

The gear ratio tells you how many times the rear wheel turns per pedal revolution. A ratio of 2.875 means the wheel spins 2.875 times for each crank turn. Higher ratios are harder to pedal but give more speed per revolution.

Gear inches is a relative measure of how “hard� a gear feels. A 70-inch gear is typical for city riding; 90+ inches is very high and hard to accelerate; 50 inches is low and good for climbing.

The speed output is theoretical and assumes no wind resistance, rolling resistance, or drivetrain losses. In practice, you will be slower than the calculator suggests, especially at higher speeds.

Limitations of the Calculator

This calculator ignores several real-world factors:

• Tire width and pressure: A wider tire or lower pressure has a larger effective diameter, which changes speed slightly.
• Cadence variation: Your actual cadence fluctuates, so the speed is an average.
• Drivetrain efficiency: Chains and hubs lose a few percent of power, but this does not affect the speed calculation because speed is purely geometric.
• Skid patches: The number of skid patches (wear spots on a tire) depends on the gear ratio in a more complex way. For 46/16, the ratio reduces to 23/8, so there are 8 skid patches. This is not shown here but is a common fixed-gear question.

For a truly accurate speed, use a GPS or a cycle computer. The calculator is best for comparing gear options, not for precise speed predictions.

Frequently Asked Questions

What is a good fixed gear ratio for a beginner?

Most beginners do well with a ratio around 2.7 to 2.9, such as 46/17 or 48/17. That gives a moderate pedaling effort at city speeds without being too spinny or too hard.

How do I calculate skid patches?

Skid patches are the number of distinct positions where the rear tire can lock up. For a gear ratio T/C, reduce the fraction to its lowest terms; the denominator is the number of skid patches (if the numerator is even, you get the denominator; if odd, you get twice the denominator). For 46/16, reduced to 23/8, you get 8 skid patches.

Does wheel size affect gear ratio?

No, the gear ratio is purely the teeth ratio. But wheel size affects gear inches and speed. A larger wheel covers more ground per revolution, so for the same gear ratio, a 29-inch wheel gives a higher speed than a 26-inch wheel at the same cadence.

What cadence should I aim for?

Most recreational fixed-gear riders pedal at 70–90 rpm. Track sprinters may hit 120+ rpm in a sprint. A cadence of 90 rpm is a good target for flat riding; adjust your gear so you can maintain that without straining.

Can I use this calculator for a single-speed bike?

Yes, the math is identical for any bike with one gear. The only difference is that a freewheel allows you to coast, but the gear ratio and speed calculations are the same.

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in t | Chainring teeth | teeth | 46 | 20
in c | Cog teeth | teeth | 16 | 8
in w | Wheel diameter (inches) | in | 27 | 10
in cad | Cadence | rpm | 90 | 30
out ratio | Gear ratio | - | t / c | 2
out inches | Gear inches | in | (t / c) * w | 1
out speed | Speed | km/h | ((t / c) * w * 3.14159 * cad * 60) / 1000000 * 0.0254 * 3600 / 1.60934 | 1