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DIY Cost Tools

DIY Cost Tools

Gear Ratio Calculator

Calculate a simple driver-to-driven gear ratio, output speed, and output torque under an entered efficiency.

Use the units shown beside each field. Decimal and scientific notation are accepted for non-integer fields; blanks, commas, fractions and non-finite entries are rejected.

Reduction ratio3:1
Driven speed600 rpm
Ideal output torque150 lb·ft
Estimated output torque142.5 lb·ft

The displayed results apply to the default example. Enable JavaScript to calculate other inputs.

How to use this calculator

  1. Replace the example values with measurements or ratings for the exact vehicle, circuit, component or system.
  2. Keep every value in the labeled unit and use manufacturer data where available.
  3. Select Calculate, then review the result together with the limitations and safety notes.

Formula and method

Gear ratio = driven teeth ÷ driver teeth. Driven rpm = driver rpm ÷ ratio. Ideal output torque = input torque × ratio; estimated torque applies the efficiency.

Calculations run in your browser. Results are hidden whenever an input changes or validation fails, preventing an older result from being mistaken for a new calculation.

Worked example

Driver gear teeth = 20; Driven gear teeth = 60; Driver speed = 1800 rpm; Input torque = 50 lb·ft; Drivetrain efficiency = 95 %. The calculator returns Reduction ratio = 3:1; Driven speed = 600 rpm; Ideal output torque = 150 lb·ft; Estimated output torque = 142.5 lb·ft.

The worked example uses the same defaults shown in the interface, allowing the formula and displayed result to be checked together.

Limitations and safety

This idealized model ignores tooth geometry, bearing drag, lubrication, heat, shock loading and strength. Do not use it to rate gears or driveline components.

This is an educational planning model. Confirm ratings, tolerances, code requirements and safe procedures with manufacturers and qualified professionals before purchasing, modifying, wiring or operating equipment.

Frequently asked questions

What does a 3:1 ratio mean?

The driver turns three times for one driven-gear turn, ideally multiplying torque by three while reducing speed by three.

Why does the calculator reject my entry?

Each field shows its supported range. Whole-number fields reject decimals, and all fields reject blanks, words, comma-formatted numbers, fractions and non-finite values.

How should I use the displayed precision?

Keep extra digits while checking the calculation, then round to the precision supported by the measurements, component tolerances and published ratings.

Review notes and reference

Reviewed October 4, 2026. The formula and default example were checked together. The script was tested for defaults, input boundaries, invalid formats, condition-specific errors and recovery after an error. Reference: NHTSA vehicle and tire safety guidance. Explanations and examples are original.

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