Gear Ratio Calculator
Calculate gear ratio, output speed and torque for a gear pair or a gearbox.
Inputs
Formula
- Ratio = driven teeth ÷ driver teeth
- Output RPM = input ÷ ratio
- Output torque = input torque × ratio × efficiency
Gear ratio is the tooth count of the driven gear divided by the driver. A ratio above 1 reduces speed and multiplies torque; below 1 it does the opposite. Real gearboxes lose a little in friction, so 97% efficiency per stage is assumed here.
Step-by-step calculation
- 1
Ratio
driven ÷ driver teeth
3
- 2
Output speed
input ÷ ratio
480 RPM
Worked example
- 120-tooth driver and 60-tooth driven → ratio 3:1.
- 21,440 RPM in gives 480 RPM out.
- 310 Nm in gives about 29 Nm out after losses.
Tips
- Keep the tooth counts non-integer multiples (e.g. 21:60) so the same teeth do not always mesh — it wears more evenly.
- Multi-stage gearboxes multiply their ratios: 3:1 then 4:1 gives 12:1.
Frequently asked questions
What does a 3:1 gear ratio mean?
The input turns three times for every one turn of the output, giving three times the torque.
Can I use diameters instead of teeth?
Yes — for meshing gears the pitch diameters give the same ratio.
Does gearing change power?
No, only the balance of speed and torque. Power out equals power in minus friction losses.
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