Fixed Gear Bike Calculator: Gear Ratio & Speed

A fixed gear bike calculator works out your gear ratio (chainring teeth ÷ cog teeth) and the speed you will travel at a given cadence, or the cadence needed for a target speed. You enter your front chainring size, rear cog size, wheel diameter, and either cadence or speed, and it returns the missing value plus your gear inches and skid patch count. This page explains the math, how to use the results, and what the numbers mean for your setup.

Formula

The core formula for gear ratio is simple:

Gear ratio = chainring teeth ÷ cog teeth

For example, a 46-tooth chainring with a 17-tooth cog gives 46 ÷ 17 = 2.71. This means one full pedal revolution turns the rear wheel 2.71 times.

To get gear inches (a traditional measure of how hard a gear feels), multiply the gear ratio by the wheel diameter in inches:

Gear inches = (chainring ÷ cog) × wheel diameter

For speed, you need the wheel circumference (π × diameter) and the cadence (pedal revolutions per minute). The speed in mph is:

Speed (mph) = (gear ratio × wheel diameter × π × cadence × 60) ÷ 63360 × 1.60934

Where 63360 converts inches to miles (5280 ft × 12 in) and 1.60934 converts miles to kilometers if you want km/h. The calculator above gives speed in mph directly.

Worked Example

Using the defaults: chainring = 46 teeth, cog = 17 teeth, wheel diameter = 27 inches, cadence = 90 rpm.

1. Gear ratio = 46 ÷ 17 = 2.7059
2. Gear inches = 2.7059 × 27 = 73.06 inches
3. Wheel circumference = π × 27 = 84.82 inches
4. Distance per pedal revolution = 2.7059 × 84.82 = 229.5 inches
5. Distance per minute = 229.5 × 90 = 20,655 inches
6. Distance per hour = 20,655 × 60 = 1,239,300 inches
7. Convert to miles: 1,239,300 ÷ 63360 = 19.56 mph
8. Convert to km/h: 19.56 × 1.60934 = 31.5 km/h

The calculator shows 73.1 gear inches and 19.6 mph (31.5 km/h) for these inputs.

What Gear Ratio Should You Use?

Your ideal gear ratio depends on your terrain, fitness, and riding style. Here are common starting points:

• Flat city riding: 46/17 (2.71) or 48/17 (2.82) – good acceleration and a comfortable top speed around 20 mph at 90 rpm.
• Hilly areas: 46/19 (2.42) or 44/18 (2.44) – easier climbing, lower top speed.
• Track racing: 48/15 (3.20) or 52/14 (3.71) – high speed, hard to accelerate from a stop.
• Fixed gear commuting: 46/16 (2.88) – a balance of speed and hill-climbing ability.

If you are new to fixed gear, start with a lower ratio (like 46/18) until you get used to the lack of freewheeling. You can always change the cog later.

How to Read the Results

The calculator gives you three outputs:

• Gear inches: A higher number means a harder gear (more speed per pedal stroke, but harder to accelerate). Typical fixed gear bikes range from 60 to 80 gear inches. Track bikes often exceed 90.
• Speed: This is your theoretical speed at the given cadence, assuming no wind resistance, rolling resistance, or drivetrain friction. Real-world speed will be slightly lower.
• Skid patches: This counts the number of distinct positions where the rear wheel can lock up when you skid (stop pedaling and lock the cranks). More patches mean your tire wears more evenly. The number is the denominator of the reduced fraction chainring/cog. For 46/17, the reduced fraction is 46/17 (already reduced), so there are 17 skid patches. For 46/16, reduced to 23/8, so 8 patches.

Common Mistakes and Misconceptions

One frequent error is using the wrong wheel diameter. The wheel diameter includes the tire, not just the rim. A 700c rim with a 25mm tire has a diameter of about 27 inches (622mm + 2×25mm = 672mm ≈ 26.5 inches). Use the actual outside diameter of your tire, which is often printed on the sidewall.

Another mistake is confusing gear ratio with gear inches. Gear ratio is just the teeth ratio; gear inches accounts for wheel size, giving a more comparable number across different bikes.

A common misconception is that a higher gear ratio always means faster. In reality, your cadence limit and leg strength determine your top speed. A gear that is too hard will slow you down because you cannot spin it efficiently.

Limitations of the Calculator

This calculator gives theoretical values. Actual speed depends on:

• Wind resistance (significant above 15 mph)
• Rolling resistance of tires
• Drivetrain friction
• Rider weight and aerodynamics
• Road gradient

For planning purposes, subtract 1-3 mph from the calculated speed for realistic conditions. The gear ratio and gear inches are exact and useful for comparing setups.

Fixed Gear vs. Single Speed

Fixed gear bikes have the cog threaded directly onto the rear hub, so the pedals always move when the wheel turns. You cannot coast. Single speed bikes have a freewheel, allowing you to stop pedaling while the bike rolls.

For fixed gear, the skid patch count matters for tire wear and for performing skid stops. For single speed, skid patches are irrelevant because you can coast and brake normally.

Gear ratio selection is similar for both, but fixed gear riders often choose a slightly lower ratio to make braking with your legs easier.

Frequently Asked Questions

What is a good gear ratio for a fixed gear bike?

A good starting ratio is 46/17 (2.71) for flat areas or 46/19 (2.42) for hills. Many commuters prefer 46/16 (2.88) for a balance of speed and acceleration. Track racers often use 48/15 (3.20) or higher.

How do I calculate my speed from cadence?

Multiply your gear ratio by the wheel circumference (π × diameter) to get distance per pedal revolution. Multiply by cadence (rpm) and 60 to get distance per hour. Convert to mph or km/h.

What are gear inches and why do they matter?

Gear inches are the equivalent wheel diameter if the bike were a direct-drive penny-farthing. It is a standard way to compare gear ratios across bikes with different wheel sizes. Higher gear inches mean a harder gear.

How many skid patches do I have?

Skid patches equal the number of distinct locking positions of the rear wheel. It is the denominator of the reduced fraction chainring/cog. For example, 46/17 has 17 patches, while 46/16 (reduced to 23/8) has 8 patches.

Does a bigger chainring make me faster?

Only if you can maintain the same cadence. A bigger chainring (or smaller cog) gives more speed per pedal stroke, but requires more force. If your cadence drops, your speed may not increase. Find a gear you can spin comfortably at your target speed.

Can I use this calculator for a single speed bike?

Yes, the gear ratio and speed calculations are identical. Ignore the skid patch output for single speed bikes, as you can coast and brake normally.

Related

• Bike Gear Ratio Speed Cadence Calculator
• Bike Size Calculator: Find Your Perfect Fit
• Bike Tire Size Calculator: Find Your Fit

in t | Chainring teeth | teeth | 46 | 20
in c | Cog teeth | teeth | 17 | 8
in w | Wheel diameter (with tire) | inches | 27 | 20
in cad | Cadence | rpm | 90 | 30
out gear_in | Gear inches | in | t / c * w | 1
out speed | Speed | mph | (t / c * w * pi * cad * 60) / 63360 * 1.60934 | 1
out skid | Skid patches (fixed gear) | patches | 1 | 0