Gear Ratio Calculator

How to use

1. 1Pick a mode: single gear pair, a multi-stage gear train, or vehicle speed.
2. 2For a single pair, enter the driving gear teeth and the driven gear teeth to see the ratio, the reduced form, and whether it is a reduction or an overdrive.
3. 3Optionally add an input RPM and an input torque to read the output speed and output torque on the other side of the mesh.
4. 4For a gear train, add a stage for each mesh and enter both tooth counts per stage. The overall ratio and a per-stage breakdown update automatically.
5. 5For vehicle speed, enter the engine RPM, transmission gear ratio, final drive ratio, and tire diameter to get road speed, or switch to solve for the engine RPM at a target speed.
6. 6Press Copy summary to copy the full result, or copy any single value with its Copy button. Use Reset to clear the inputs.

About this tool

Gear Ratio Calculator works out the ratio between two meshing gears and turns that ratio into the numbers people actually need: output speed, output torque, mechanical advantage, and (for vehicles) road speed. A gear ratio is simply the driven (output) gear teeth divided by the driving (input) gear teeth, so a 13 tooth gear driving a 39 tooth gear gives a 3:1 ratio. From that single number everything else follows. The tool covers three jobs. The single pair mode takes the two tooth counts and reports the ratio (also reduced to its simplest whole-number form, for example 39:13 shown as 3:1), tells you whether the pair is a reduction, an overdrive, or direct drive, and optionally takes an input speed in RPM and an input torque in any unit to show the exact output speed and output torque on the other side. Output speed is the input RPM divided by the ratio, and output torque is the input torque multiplied by the ratio, which is the whole point of gearing: trade speed for torque or torque for speed. The gear train mode chains as many stages as you like, where each stage is one mesh of a driving gear and a driven gear. The overall ratio is the product of every stage ratio, and a step table shows the running speed and torque after each stage, which is how a real gearbox, a planetary stack, or a printer reduction drive multiplies a ratio across several meshes. It also reports the output rotation direction, because each external mesh reverses direction and idler gears change only direction, not the size of the ratio. The vehicle speed mode answers the classic drivetrain questions. Enter the engine RPM, the transmission gear ratio, the final drive (axle) ratio, and the tire diameter to get the road speed in both mph and km/h, or flip it around to find the engine RPM at a chosen cruising speed (the what RPM at 70 mph question). Road speed is the wheel speed times the tire circumference, and wheel speed is the engine RPM divided by the combined gear and final drive ratio. The torque figures are the ideal lossless result; a real gear set loses a small percentage to friction at each mesh, which the tool states plainly rather than pretending to model. It is useful for car and motorcycle drivetrains, bicycle and go-kart sprockets, 3D printer and CNC stepper gearing, robotics, woodworking lathes, and any project where one shaft drives another. Everything is computed in your browser, so the numbers you enter are never uploaded or stored.

Free to use. Works in your browser. No signup, no login.