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Gear ratio calculator

Compute the gear ratio from the number of teeth on the driving and driven gears.

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Need Gear ratio (:1)? The Gear ratio calculator derives it from Driving gear teeth, Driven gear teeth in one step. For instance, with Driving gear teeth = 15 and Driven gear teeth = 45 it returns Gear ratio (:1) = 3.

How to use it

  1. Enter your values: Driving gear teeth, Driven gear teeth.
  2. Read the result instantly: Gear ratio (:1).

Frequently asked questions

How does the Gear ratio calculator work?

It takes Driving gear teeth and Driven gear teeth and derives Gear ratio (:1) from them. The calculation is live as you type, so the result updates on every change.

Which values does the calculator ask for?

2 values: Driving gear teeth and Driven gear teeth. Nothing else is required — no account, no file upload.

What does a typical calculation look like?

With Driving gear teeth = 15 and Driven gear teeth = 45, the calculator returns Gear ratio (:1) = 3. Those figures come from running this exact tool, so you can reproduce them by entering the same values.

What does it give for smaller values?

Scaled down to Driving gear teeth = 8 and Driven gear teeth = 23, Gear ratio (:1) comes out at 2.875. The relationship is worth checking at both ends before you rely on a single result.

When would I actually use this?

Reading a spec sheet: converting between units, comparing power-to-weight across two cars, and sanity-checking an acceleration claim.

What is the most common mistake?

Comparing horsepower figures from different standards. Metric PS, SAE and DIN horsepower differ by a few percent, and a kerb weight quoted without fluids and driver understates the real mass.

Where do the figures come from, and how current are they?

Unit conversions are exact by definition. Acceleration estimates from power and weight ignore traction, gearing and aerodynamics, so they set a floor rather than a prediction.

Further reading

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GuideGear Ratios, and Why Your Speedometer Lies After New TyresEngine rpm reaches the road through one unbroken multiplication chain, and the last link is the tyre. Change the tyre's rolling circumference and the speedometer goes wrong by exactly that percentage — in a direction the law cares about.ExplainerRolling Circumference Decides More Than You ThinkThe speedometer is the famous consequence, and the least expensive one. The same number sets your odometer and therefore your lease bill, your effective final drive and cruising rpm, and what the ABS believes about wheel slip.ExplainerGear Inches, Development, and Why Cyclists Argue About Ratios50/11 means nothing until you attach a wheel. Gear inches and metres of development, derived and converted, a full table for a real 12-speed drivetrain, and why the gear step between two sprockets decides whether you can hold a cadence.How-toBelt Length Between Two Pulleys: The Geometry, and Why the Answer Is Not What You OrderTwice the centre distance plus both circumferences is 36 % too long. Here is the real formula, how the two straight runs and the two arcs actually add up, how far the standard approximation drifts, and what to do with a figure that is not a catalogue length.ExplainerTorque vs Horsepower: What Each One Actually DoesTorque is the twisting force an engine makes; horsepower is how fast it does work. Learn the hp = torque × rpm ÷ 5252 link and why both numbers matter.ExplainerHow Mileage Reimbursement Works: Rates, Rules and RecordsMileage reimbursement pays a set rate per mile driven for work. Learn how the rate is set, how the IRS, HMRC and URSSAF differ, and how to keep a compliant log.