Gear Ratio Calculator

Gear ratio calculator for tooth counts, speed reduction, and torque increase

This gear ratio calculator is built for one common, practical decision: you have two gears that mesh directly, and you want to know what that tooth count pairing does to speed and torque. People usually search for this when selecting sprockets for a chain drive, swapping pinion and spur gears in a gearbox, or checking whether a planned ratio will give enough torque without losing too much RPM.

The core idea is simple. The driving gear (the one attached to the motor or input shaft) turns the driven gear (the one attached to the output shaft). Tooth counts set the mechanical ratio. A larger driven gear relative to the driver reduces output speed but multiplies torque. A smaller driven gear increases output speed but reduces torque. This calculator keeps the intent narrow and clear: one gear pair, one ratio, with optional estimates for output RPM and output torque.

To use it, enter the number of teeth on the driving gear and the driven gear. That alone gives you the ratio and the speed change. If you also know the input speed (RPM), you will get an estimated output speed. If you know the input torque, you will get an estimated output torque. Finally, you can enter an efficiency percentage to reduce the torque gain to something more realistic when there are losses from friction, lubrication, bearings, chain or belt drag, or gear mesh losses.

Assumptions and how to use this calculator

• This calculator assumes a simple two gear mesh (one driver, one driven). It does not model multi-stage gear trains, compound gear sets, or planetary gearboxes.
• Tooth counts are assumed to be correct and compatible for meshing (same pitch or module). If the gears cannot physically mesh, the ratio math is still valid, but the setup is not buildable.
• Output RPM is calculated from tooth ratio only. It ignores slip and compliance, which can matter in belts and sometimes chains under load.
• Output torque uses a simple multiplier adjusted by efficiency. It does not include motor torque curves, stall behavior, thermal limits, or gearbox ratings.
• Direction change is not modeled. A single external gear mesh reverses rotation direction, but this page focuses on magnitude (ratio, speed, torque), not rotation direction or handedness.

Common questions

What does a “3:1” gear ratio mean in plain terms?

A 3:1 reduction means the input shaft turns three times for every one turn of the output shaft. In practice, output speed is about one third of input speed, and available torque at the output is about three times higher before losses.

Which gear is “driver” and which is “driven”?

The driving gear is connected to the power source (motor, engine, or upstream shaft). The driven gear is the one being turned and is connected to the output load. If you swap them, the ratio inverts and you get the opposite behavior.

Why is output torque not exactly the gear ratio times input torque?

Because real systems lose energy to friction and drag. Even with quality gears, bearings, and lubrication, you rarely get perfect transfer. That is why the calculator includes an efficiency field. If you do not know it, leaving it at 100% shows the ideal case, and you can then test more conservative values such as 95% or 90% to see a realistic range.

What if I do not know my input RPM or input torque?

You can still use this calculator. Tooth counts alone give the ratio, speed reduction factor, and torque multiplier factor. If you later find your motor RPM or torque from a datasheet, you can add those values to convert the factor into real output numbers.

Does this apply to sprockets and chain drives, or only gears?

It applies to sprockets and chain drives for ratio and speed, because the ratio is still based on tooth counts. The torque estimate is also useful, but efficiency can differ. Chain drives and belt drives often have different loss profiles than enclosed gear meshes, so use the efficiency input to reflect that.