Gear Ratio Calculator
Gears transmit rotational motion and torque between shafts. The mechanical advantage (or disadvantage) of a gear pair depends on the ratio of teeth on each gear. A simple gear pair consists of a drive gear (input) and a driven gear (output). If the driven gear has more teeth, it rotates more slowly but with greater torque. If it has fewer teeth, it rotates faster but with less torque. Complex mechanical systems like vehicle transmissions, industrial machinery, and bicycles often use multiple gear stages in series, where the output of one pair feeds into the input of the next. This calculator computes the gear ratio from the number of teeth on the drive and driven gears, determines the output RPM given an input speed, and calculates the torque multiplication factor. It also supports multi-stage gear trains, where the total ratio is the product of individual ratios. Enter the tooth count for each gear pair and the input RPM, and the calculator shows you the output speed and torque ratio instantly. Use this tool to understand transmission behavior, design mechanical systems, or calculate the effect of gear substitution on engine speed and power delivery.
Stage 1
Stage 2 (optional)
Stage 3 (optional)
How gear ratios work
A gear ratio is simply the ratio of driven teeth to drive teeth. If the drive gear has 20 teeth and the driven gear has 60 teeth, the ratio is 60/20 = 3:1. This means the drive gear must rotate 3 times for the driven gear to rotate once.
The gear ratio directly determines two things:
- Speed reduction: Output RPM = Input RPM / Gear Ratio. A 3:1 ratio reduces RPM by a factor of 3.
- Torque multiplication: Output Torque = Input Torque x Gear Ratio. A 3:1 ratio increases torque by a factor of 3.
When gear ratios are less than 1:1 (called overdrive), the output rotates faster than the input, but torque is reduced. In a multi-stage system, the total ratio is the product of all individual ratios. For example, stages of 3:1 and 2:1 together give 3 x 2 = 6:1 total.
Gear ratio calculator: frequently asked questions
What is a gear ratio?
A gear ratio is the ratio of the number of teeth on the driven (output) gear to the number of teeth on the drive (input) gear. For example, if the drive gear has 20 teeth and the driven gear has 60 teeth, the ratio is 60:20 or 3:1. This ratio tells you how many times the drive gear must rotate to make the driven gear rotate once.
How does a gear ratio affect speed and torque?
A high gear ratio (greater than 1:1) reduces output speed but increases torque. A low gear ratio (less than 1:1, called an overdrive) increases output speed but reduces torque. The torque ratio is always the inverse of the speed ratio. A 3:1 ratio reduces speed by a factor of 3 but multiplies torque by 3.
What is a multi-stage gear train?
A multi-stage gear train combines multiple sets of gears in series. The output of one gear pair becomes the input to the next. The total gear ratio is the product of all individual ratios. For example, three stages with ratios of 3:1, 2:1, and 1.5:1 give a total ratio of 3 x 2 x 1.5 = 9:1.
What is overdrive?
Overdrive is a gear configuration where the output rotates faster than the input, with a ratio less than 1:1. In a car transmission, overdrive (typically 0.7:1 to 0.85:1) reduces engine RPM at highway speeds, improving fuel economy. This is achieved by having the input gear larger than the output gear.
How do I find the number of teeth on my gears?
Check the manufacturer's specifications or documentation for your machinery, vehicle, or transmission. Many gears have the tooth count stamped or engraved on the gear body. If you cannot find the specification, count the teeth manually by marking one tooth and rotating the gear until you return to that mark.
Official sources
- American Society of Mechanical Engineers (ASME): Standards for gear tooth design and power transmission.
- ISO 6336: International standard for gear rating and strength calculation.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.