Bmx Rollout Calculator

Calculate the exact distance your bike travels per crank revolution.

Chainring Teeth (Front)

Cog Teeth (Rear)

Tire Diameter (Inches) Standard 20″ tires are often 20.1″ to 20.4″

Rollout 56.12″

Gear Ratio 2.78

Gear Inches 55.90

Gain Ratio 3.80

What is BMX Rollout?

Rollout is the physical distance your bicycle travels with one full rotation of the pedals. Unlike gear ratio, which only looks at the relationship between your sprocket and your cog, rollout takes into account the circumference of your tire. This is the most accurate way for BMX riders to compare different setups, especially when switching between different tire widths or pressures.

Why it Matters for Performance

In BMX racing and street riding, finding the "magic gear" is crucial. A higher rollout number means a higher top speed but harder acceleration (harder to "get out of the hole"). A lower rollout number gives you explosive acceleration but can cause you to "spin out" at high speeds.

Park/Street: Riders often prefer a rollout between 55″ and 56″.
Racing: Pro racers often adjust rollout based on the length of the first straightaway.
Flatland: Usually requires a much lower rollout for technical control.

The Math Behind the Calculation

The calculation follows this physics-based formula:

                Rollout = (Chainring Teeth / Cog Teeth) × (Tire Diameter × π)
            

For example, if you run a 25/9 setup with a standard 20-inch tire (actual diameter approx 20.125″), the calculation is: (25 ÷ 9) × (20.125 × 3.14159) = 55.89 inches.

Common Tire Diameter References

Tire Type	Typical Diameter
20″ x 1.75″	19.90″
20″ x 2.125″	20.125″
20″ x 2.40″	20.40″

function calculateRollout() { var chainring = parseFloat(document.getElementById("chainring").value); var cog = parseFloat(document.getElementById("cog").value); var tireDia = parseFloat(document.getElementById("tireDiameter").value); // Validate inputs if (isNaN(chainring) || isNaN(cog) || isNaN(tireDia) || cog <= 0) { document.getElementById("rolloutResult").innerHTML = "0.00"; document.getElementById("ratioResult").innerHTML = "0.00"; document.getElementById("inchesResult").innerHTML = "0.00"; document.getElementById("gainResult").innerHTML = "0.00"; return; } // Calculations var ratio = chainring / cog; var circumference = tireDia * 3.14159265359; var rollout = ratio * circumference; var gearInches = ratio * tireDia; // Gain ratio assumes standard 175mm cranks (175mm = 6.89 inches) var crankLength = 6.89; var gainRatio = (tireDia / (2 * crankLength)) * ratio; // Display Results document.getElementById("rolloutResult").innerHTML = rollout.toFixed(2) + '"'; document.getElementById("ratioResult").innerHTML = ratio.toFixed(2); document.getElementById("inchesResult").innerHTML = gearInches.toFixed(2); document.getElementById("gainResult").innerHTML = gainRatio.toFixed(2); } // Initial calculation on load window.onload = calculateRollout;