Bike Gear Ratio Calculator

Overview: what this calculator does

This Bike Gear Ratio Calculator helps you translate chainring and cassette choices into numbers you can feel on the road. Enter the number of teeth for the chainring and the cog, plus the wheel diameter in the unit you have on hand, and the calculator produces three key outputs.

The calculator returns the unitless gear ratio, the more tangible gear inches, and the real-world distance per pedal rotation — development in meters. These values let you compare setups, predict cadence at a given speed, and match gearing to terrain and fitness.

Everything updates in real time so you can tweak ring and cog sizes, or change wheel units — including a dedicated feet + inches input — without losing focus or re-entering data. The layout is designed for clarity so you always understand what changed and why.

Formulas and how they work

At its core the calculator uses three simple, proven formulas that cycling coaches and bike mechanics rely on to talk about gearing. Each formula is derived from geometry and standard unit conversions to ensure accurate, interpretable results.

You do not need to memorize everything, but seeing the formulas makes it easier to trust the results and to check any manual computation. Below are the exact expressions used by the calculator for clarity and reproducibility.

// Gear Ratio
gearRatio = chainringTeeth / cogTeeth

// Gear Inches
gearInches = wheelDiameterInInches * gearRatio

// Development (meters per crank rev)
developmentMeters = gearInches * π / 39.3701

The first formula is dimensionless and is the simplest expression of how teeth count drives mechanical advantage. The second converts that advantage into a wheel-equivalent diameter; the third turns circumference into linear distance in meters.

Units, ft + in behaviour and why precision matters

Wheels and rims are measured in a variety of units depending on region, manufacturer, and era. The calculator accepts inches, centimeters, meters, and a special compound mode: feet + inches.

When you select the feet + inches mode the single main input hides and two fields appear: one for feet and one for inches. This lets you enter legacy or workshop measurements that are often reported as 5' 8" style values without rounding errors.

Small differences in wheel diameter, especially tire height, change gear inches and development noticeably. Precision in wheel measurement removes guesswork and produces results that match what you feel on climbs and flats.

How to use the calculator (step-by-step)

1. Enter the chainring teeth count as an integer (e.g., 34). The calculator treats this as a plain number with no unit selection required. 2. Enter the cog or sprocket teeth as an integer (e.g., 16). This is also a plain numeric input.

3. Enter the wheel diameter in the unit you have: choose inches, cm, m, ft, or ft + in. If you choose ft + in, the main input hides and you will see two fields that span the full width, matching the dropdown height for perfect alignment.

4. Results appear instantly: gear ratio, gear inches, and development in meters. Use the results to compare gearing changes, predict speed at a cadence, or plan a cassette/chainring swap.

Five practical examples

Table 1 — Common chainring / cog pairs

This table lists frequent pairings and their simple ratio to help you pick a cassette or chainring for a purpose. Use the numbers to compare mechanical advantage and to plan swaps quickly.

Interpreting the tables and results

Ratios are a simple quick filter. They tell you whether a gear feels "hard" or "easy" before you translate to speed or cadence. Combine ratio with wheel size to get gear inches and development which are more practical for riding.

Use the calculator to test scenarios: reduce a chainring by 2 teeth and note development changes, or move to a larger wheel size and observe the speed shift at a given cadence. These small adjustments add up on long rides and steep climbs.

Table 2 — Typical wheel sizes & conversions

Wheel labels can be confusing. This table helps you convert nominal labels to practical diameters for the calculator. Use the correct diameter including tire to achieve results that match road feel.

Practical use cases where these numbers matter

Training: match development to race targets so you practice at realistic speeds for a chosen cadence. Gear selection: pick chainring and cassette combos for topography — longer development for flats, shorter for hills.

Bike builds: when swapping wheel sizes or tires you must re-evaluate gearing; the calculator makes this comparison instant. Diagnostics: unexpected cadence or difficulty might be explained by tiny changes to tire height or a mis-specified wheel diameter.

Table 3 — Suggested combos by terrain

Below are widely used starting points. Numbers are conservative suggestions — always test on real terrain and refine. Each row lists a purpose, a recommended chainring/cog pairing and a short justification.

Practical tips and best practices

• Measure wheel diameter including tire mounted and inflated for realistic results. Nominal wheel labels can mislead if tire height varies.

• Use the feet + inches mode for workshop tapes or older frames where measurements are stored that way. It avoids rounding errors. • If you test on the road, record cadence and speed to cross-check the predicted development and tune the calculator’s input.

• Small chainring/cog changes produce noticeable differences on climbs; plan swaps conservatively and test before committing to a new cassette. • When comparing bikes, normalize to the same wheel diameter and then compare development to get an apples-to-apples view.

Frequently asked questions

Below are commonly asked questions and expert answers that users find helpful when deciding chainring and cassette choices. If your question is not listed, try the step-by-step guide earlier or consult a bike mechanic for fit-specific advice.

Closing thoughts

Numbers are tools: they do not replace feel, but they make feeling more predictable. Use the calculator to make small, informed changes rather than wholesale swaps without testing.

Whether you are optimizing a commuter, pacing for a race, or building a touring bike, understanding gear ratio, gear inches and development simplifies decision making. Start with the suggestions above, test on the road, and refine until your gearing matches your goals.