Gearbox Ratio Calculator

Work out gearbox ratio instantly with clear inputs, formula shown and shareable results.

About Gearbox Ratio Calculator

Gear stages multiply, so the overall ratio is the product of the stage ratios. Speed divides by that ratio and torque multiplies by it. Each meshing stage costs about 2-3 percent in efficiency, so a two-stage helical gearbox delivers roughly 94 percent of the ideal output torque.

Formulas

Overall ratio

i total = i1 x i2; output rpm = input rpm / i total

Torque

Output torque = input torque x i total x stage efficiencies

Worked example

With the default values below, Gearbox Ratio Calculator returns Overall ratio of 14.000. Change any input above and every figure here is recalculated in your browser.

Inputs used

First stage ratiominimum 0.1: 3.5
Second stage ratiominimum 0.1: 4
Input speedminimum 1 rpm: 1,750 rpm
Input torqueminimum 0.1 lb·ft: 30 lb·ft

Results

Overall ratio: 14.000
Output speed: 125.00 rpm
Ideal output torque: 420.0 lb·ft
Output torque after losses: 395.2 lb·ft

Step by step

1Values used
2Overall ratio
3Torque
4Overall ratio= 14.000
5Output speed= 125.00 rpm
6Ideal output torque= 420.0 lb·ft
7Output torque after losses= 395.2 lb·ft

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is the Gearbox Ratio figure worked out?

i total = i1 x i2; output rpm = input rpm / i total. Gear stages multiply, so the overall ratio is the product of the stage ratios.

Why split a large reduction into stages?

A single mesh above about 7:1 needs a very large gear and gives poor tooth contact. Splitting the ratio keeps both gears compact and efficiency high.

What values do I need to enter?

This calculator takes 4 inputs: First stage ratio, Second stage ratio, Input speed, Input torque. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.