Gear Ratio Calculator – Calculate Gear Train Ratio
Calculate gear ratios for single or multi-stage gear trains. Our calculator determines output speed and torque based on gear tooth counts.
Step 1: Enter the number of teeth on the driver gear (input) and driven gear (output) for each stage.
Step 2: Optionally enter input RPM and torque to calculate output values.
Step 3: Click Calculate to see the total gear ratio, output RPM, and output torque.
What Is Gear Ratio
Gear ratio is the relationship between the number of teeth on two gears. If a 10-tooth gear drives a 30-tooth gear, the ratio is 3:1. The output turns once for every 3 input turns. You gain torque but lose speed. This is why low gears feel powerful but slow.
Speed vs Torque Trade-off
Gears trade speed for torque (or vice versa). A 4:1 ratio means output torque is 4x input torque, but output speed is 1/4 of input speed. This is why bicycles have multiple gears - low gears for climbing (high torque), high gears for speed on flats.
Multi-Stage Gear Trains
When gears are stacked in series, multiply individual ratios. A 3:1 stage followed by a 4:1 stage gives 12:1 total. This is how transmissions achieve wide ratio spreads - multiple gear pairs working together.
| Application | Typical Ratio | Purpose |
|---|---|---|
| Car 1st gear | 3.5:1 | Maximum torque for starting |
| Car 5th gear | 0.8:1 | Overdrive for highway cruising |
| Bicycle low gear | 0.5:1 | Climbing steep hills |
| Bicycle high gear | 4:1 | Speed on flat terrain |
| Electric drill | 10:1 | High torque for drilling |
| Wind turbine gearbox | 50:1 | Slow blades to fast generator |
Ratios less than 1:1 are "overdrive" - output spins faster than input. Ratios greater than 1:1 are "reduction" - output spins slower with more torque.
How do you calculate gear ratio?
Divide driven gear teeth by driver gear teeth. A 40-tooth driven gear with a 10-tooth driver gives 40/10 = 4:1 ratio. The output turns once for every 4 input turns, with 4x the torque.
Does gear ratio affect horsepower?
No. Gears change torque and speed, but power (horsepower) stays the same (minus small friction losses). Power = Torque × Speed. If torque goes up 4x and speed goes down 4x, power is unchanged.
What is a 1:1 gear ratio?
A 1:1 ratio means input and output turn at the same speed with the same torque. This is called "direct drive." Many transmissions have 1:1 as one gear, usually 4th or 5th in manual transmissions.
How do I choose the right gear ratio?
Match the ratio to your needs. Need more torque? Use a higher ratio (like 4:1). Need more speed? Use a lower ratio or overdrive (like 0.8:1). For vehicles, consider engine power band, vehicle weight, and intended use.
What is the difference between gear ratio and final drive ratio?
Gear ratio is for one gear pair. Final drive ratio is the differential ratio in a car. Total ratio = transmission gear ratio × final drive ratio. A 3:1 gear in a car with 4:1 final drive gives 12:1 total reduction.
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