Gear Ratio Calculator
This gear ratio calculator turns two gears into a ratio. Enter the teeth on the drive gear and the driven gear, add an input RPM if you like, and you'll get the gear ratio, the output RPM, the torque multiplier, and whether it's a reduction or an overdrive. Everything updates as you type, so it's easy to plan a build, a bike, or a robot. It's free and runs in your browser, so nothing you enter leaves your device.
- Gear ratio
- Output RPM
- Torque change
- Reduction or overdrive
- Teeth count
Last updated June 17, 2026 Ratio, RPM, torque Reviewed by the Calcowa team
Enter drive and driven teeth above zero.
The drive gear is the one powering the system, and the driven gear is the one it turns. A ratio above 1 to 1 is a reduction; below 1 to 1 is an overdrive. The output RPM and torque use the same ratio.
How do you calculate a gear ratio?
A gear ratio compares how fast two meshed gears turn, and it comes straight from their teeth. You divide the teeth on the driven gear, the one being turned, by the teeth on the drive gear, the one doing the turning. So a 12 tooth drive gear spinning a 42 tooth driven gear gives 42 over 12, which is 3.5, written as 3.5 to 1. That single number tells you a lot. Because the ratio is above 1, it's a reduction: the output turns slower than the input, here at about a third of the speed, and in exchange it multiplies torque by roughly the same 3.5. Flip the gears so the big one drives the small one and the ratio drops below 1, an overdrive that spins faster but with less torque. Gears always trade speed for turning force, never giving both at once. This tool runs the division, applies the ratio to your input RPM for the output speed, and labels the result, so you'll see the whole trade the moment you type the two tooth counts.
Working it out, step by step
Here's the routine for a 12 tooth drive and a 42 tooth driven gear, and it's just three steps:
- 1
Count the teethDrive gear has 12 teeth, driven gear has 42.
- 2
Divide driven by drive42 over 12 is 3.5, so the ratio is 3.5 to 1.
- 3
Read the tradeOutput spins a third as fast, with about 3.5 times the torque.
Common gear ratios
Here's how a few tooth pairs shake out, and you'll see the pattern fast. It's always driven over drive, so you can match your own gears to it, and if yours doesn't fit, you've probably swapped the two counts.
| Drive | Driven | Ratio |
|---|---|---|
| 12 | 42 | 3.50 : 1 reduction |
| 15 | 45 | 3.00 : 1 reduction |
| 20 | 40 | 2.00 : 1 reduction |
| 30 | 30 | 1.00 : 1 direct |
| 40 | 20 | 0.50 : 1 overdrive |
Frequently asked questions
You enter the number of teeth on the drive gear and the driven gear, and it works out the ratio between them. It divides the driven teeth by the drive teeth to get the ratio, then uses that to find the output speed and the torque change. Add an input RPM and it shows the output RPM too. Everything runs in your browser, so you'll see the numbers update as you type, and nothing you enter leaves your device.
Gear ratio is the driven gear teeth divided by the drive gear teeth. The drive gear is the one turning the system, often on the motor, and the driven gear is the one it turns. So a 12 tooth drive gear turning a 42 tooth driven gear gives 42 divided by 12, which is 3.5, written as 3.5 to 1. That means the drive gear spins 3.5 times for each turn of the driven gear. The calculator does that division the moment you type both counts.
It comes down to whether the ratio is above or below one. A reduction ratio, above 1 to 1, slows the output down and multiplies torque, which is what you want for moving heavy loads or climbing. An overdrive ratio, below 1 to 1, speeds the output up and trades away torque, handy for cruising at low engine speed. A 1 to 1 ratio is direct drive, with no change. The tool labels which one you've got as you change the gears.
Gears trade speed for torque, so the two always move in opposite directions. A reduction ratio of 3.5 to 1 cuts the output speed to a third while multiplying torque by about 3.5, minus a little to friction. An overdrive does the reverse. That's why a low gear feels strong but slow and a high gear feels fast but weak. The output RPM and torque multiplier here show both sides of that trade for the ratio you enter.
There isn't one, since it depends entirely on the job. A rock crawler wants a deep reduction near 4 to 1 or more for low-speed pulling power, while a highway cruiser wants something close to or below 1 to 1 to keep engine speed down. Bikes, drills, and machines each have their own sweet spot. Use the output RPM and torque figures here to match the ratio to what you need, rather than chasing a single number.
Yes, it's completely free, with no sign-up, and it runs right in your browser, so nothing you enter leaves your device. Type the drive and driven teeth, add an input RPM if you want the output speed, and read the ratio in a tap. Bookmark it for builds, bike setups, robotics, or any project with gears, and you'll have the ratio, RPM, and torque change in one place.
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