Fixed Gear Calculator

Calculate your gear inches, skid patches, and speed ratios with our precision fixed gear calculator tool.

Common Cog Options Comparison (Using current Chainring)

Cog Size	Ratio	Gear Inches	Skid Patches	Speed @ 90 RPM

What is a Fixed Gear Calculator?

A fixed gear calculator is an essential tool for cyclists who ride track bikes or fixed-gear conversions. Unlike geared bicycles, a fixed-gear bike has a single, constant ratio. Choosing the right chainring and cog combination determines how easy it is to start from a stop, how fast you can go at a specific cadence, and how long your rear tire will last if you perform skids.

Cyclists use a fixed gear calculator to balance two opposing needs: acceleration and top-end speed. A “low” gear ratio allows for quick starts and easy hill climbing but limits top speed because you can only spin your legs so fast. A “high” gear ratio allows for higher speeds but makes starting and climbing significantly harder.

Common misconceptions about the fixed gear calculator involve ignoring tire width or wheel size. Since the actual distance traveled per pedal stroke depends on the outer diameter of the tire, a 23mm tire and a 32mm tire will produce different results even with the same chainring and cog.

Fixed Gear Calculator Formula and Mathematical Explanation

The mathematics behind a fixed gear calculator relies on basic mechanical ratios and geometry. To understand how your bike performs, we look at several key metrics.

1. Gear Ratio

The most fundamental calculation. It represents how many times the rear wheel turns for every single rotation of the pedals.

Formula: Gear Ratio = Chainring Teeth / Cog Teeth

2. Gear Inches

This traditional measurement mimics the diameter of the large front wheel of an old-fashioned “Penny Farthing” bicycle. It provides a standard way to compare gears across different wheel sizes.

Formula: Gear Inches = (Chainring / Cog) × Total Wheel Diameter (inches)

3. Skid Patches

On a fixed gear bike, skidding (locking the rear wheel) often happens at the same point in the pedal stroke. Skid patches represent the number of unique points on the tire that will touch the ground when skidding.

Formula: Skid Patches = Cog / GCD(Chainring, Cog)

Variables Table

Variable	Meaning	Unit	Typical Range
Chainring	Teeth on the front sprocket	Count	42 – 52
Cog	Teeth on the rear sprocket	Count	13 – 19
Cadence	Pedal rotations per minute	RPM	70 – 110
ISO Diameter	The rim’s bead seat diameter	mm	559 – 630

Practical Examples (Real-World Use Cases)

Example 1: The Urban Commuter

A rider uses a 46-tooth chainring and a 16-tooth cog on a standard 700c wheel with 25mm tires. Entering these into the fixed gear calculator reveals a gear ratio of 2.88 and approximately 78.5 gear inches. While this is great for flat cities, the fixed gear calculator shows they only have 8 skid patches. If they want to preserve their tires, they might consider switching to a 17-tooth cog to get 17 skid patches.

Example 2: The Track Racer

A track cyclist at a velodrome might use a 50-tooth chainring and a 14-tooth cog. The fixed gear calculator indicates a massive 96.5 gear inches. This setup requires significant strength to accelerate but allows the rider to maintain speeds over 45 km/h at a manageable cadence of 100 RPM.

How to Use This Fixed Gear Calculator

1. Enter Chainring: Input the number of teeth on your front sprocket (e.g., 48).
2. Enter Cog: Input the teeth on your rear cog (e.g., 17).
3. Select Tire/Rim: Choose your wheel size. Most road bikes are 700c. Enter the tire width (e.g., 28mm) for higher precision.
4. Set Cadence: Input your average pedaling speed. 90 RPM is the gold standard for efficiency.
5. Analyze Results: Look at the Skid Patches. If the number is low (e.g., 1 or 2), your tire will wear out very quickly in specific spots.

Key Factors That Affect Fixed Gear Calculator Results

• Chainring/Cog Ratio: The core factor. A higher ratio means more speed per stroke but more resistance.
• Tire Volume: A 32mm tire has a larger circumference than a 23mm tire, effectively “upping” your gear ratio.
• Mechanical Advantage: Crank arm length (e.g., 165mm vs 175mm) affects how the fixed gear calculator results feel to your legs, though it doesn’t change the ratio math.
• Cadence Consistency: Your ability to maintain high RPM (leg speed) determines your functional speed range.
• GCD (Greatest Common Divisor): This mathematical factor determines skid patches. Prime numbers for cogs (like 17 or 19) usually yield better skid patch counts.
• Terrain: While the fixed gear calculator gives you the math, your local hills determine if that math is sustainable for your fitness level.

Frequently Asked Questions (FAQ)

What is a good gear ratio for beginners?

For most urban environments, a gear ratio between 2.6 and 2.8 (around 70-75 gear inches) is recommended. A 46×17 or 48×18 setup is a perfect starting point.

Why do skid patches matter?

If you have 1 skid patch, every time you skid, the same spot on your tire hits the pavement. With 17 patches, the wear is distributed across 17 different spots, making your tire last 17 times longer.

Does crank length affect the fixed gear calculator?

Crank length doesn’t change the Gear Inches or Ratio, but shorter cranks (165mm) are common on fixed gears to provide better ground clearance in corners and allow for higher cadences.

Is a higher gear inch always better?

No. Higher gear inches are harder on your knees and make it difficult to stop. Pro track racers use high gear inches, but commuters should stay lower for safety and joint health.

What is “Development”?

Development is the distance the bike travels forward with one full revolution of the pedals, usually measured in meters.

Can I use this for a bike with an internal gear hub?

Yes, but you would need to multiply the final ratio by the internal hub’s gear ratio for that specific gear.

What is the “Magic Ratio”?

In fixed gear conversions without a chain tensioner, the magic ratio is a chainring/cog combo that fits perfectly with your frame’s chainstay length without leaving slack in the chain.

How do 650c wheels change the result?

Smaller wheels (650c) result in lower gear inches for the same tooth count compared to 700c wheels. The fixed gear calculator accounts for this automatically.