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Gearbox Ratio Calculator

Reviewed by the CalculatorHub team, edited by James Graham. Based on AGMA gear standards. Last verified 14 June 2026.

This calculator determines output shaft speed, output torque, and power loss for a gearbox given the input shaft conditions, gear ratio, and mechanical efficiency. Use it to size gearboxes in drive train design, select speed reducers for conveyor and pump applications, or verify that a selected gearbox has sufficient rated output torque for your load.

Input shaft rotational speed (e.g. motor speed at rated load)
Full load torque from the motor or prime mover
GR = Input speed / Output speed. GR greater than 1 reduces speed, GR less than 1 increases speed.
Per-stage efficiencies compound multiplicatively for multi-stage units
-- RPM
-- N.m
-- kW
-- kW

Gearbox calculation formulas

N2 = N1 / GR
T2 = T1 × GR × eta
Pin = T1 × omega1 = T1 × 2 × pi × N1 / 60
Pout = Pin × eta
Ploss = Pin - Pout

Where: N = speed (RPM), T = torque (N.m), GR = gear ratio (input/output), eta = mechanical efficiency (decimal fraction), omega = angular velocity (rad/s).

Gearbox selection checklist

  • Required output torque must be less than the gearbox rated output torque divided by the service factor (AGMA 9005).
  • Check overhung load (radial force on output shaft) if using a sprocket, pulley, or gear on the output shaft.
  • Confirm the gearbox is rated for the duty cycle (continuous, intermittent, or reversing) and ambient temperature.
  • Worm gears are self-locking at high ratios; this can be beneficial (e.g. hoists) or a concern (prevent back-driving in emergency).

Gearbox ratio calculator: frequently asked questions

How does a gearbox affect speed and torque?

A gearbox (speed reducer) multiplies torque and reduces speed by the gear ratio. Output speed N2 = N1 / GR. Output torque T2 = T1 * GR * eta, where GR is the gear ratio (input speed / output speed) and eta is the gearbox mechanical efficiency (typically 95% to 98% for helical gears per stage). Power out = Power in * eta.

What is gear ratio?

Gear ratio (GR) = Input speed (RPM) / Output speed (RPM) = Number of teeth on driven gear / Number of teeth on driving gear. A GR greater than 1 is a speed reducer (most gearboxes). A GR less than 1 is a speed increaser (used in wind turbines and some pump drives).

What are typical gearbox efficiencies?

Single-stage helical gearbox: 97 to 99%. Worm gearbox (high ratio): 50 to 90% depending on ratio (lower for ratios above 20:1). Bevel gearbox: 97 to 99%. Planetary gearbox: 96 to 99% per stage. Efficiency per stage compounds: a 3-stage helical gearbox at 98% per stage has overall efficiency of 0.98^3 = 94.1%.

How do I calculate output shaft torque?

Output torque T2 = T1 * GR * eta, where T1 is input torque (N.m), GR is gear ratio, and eta is gearbox efficiency. Alternatively, T2 = P_in * eta / omega_out, where omega_out = 2 * pi * N2 / 60 is the output angular velocity. For a motor producing 98.8 N.m at 1,450 RPM through a 5:1 gearbox at 97% efficiency: T2 = 98.8 * 5 * 0.97 = 479.2 N.m at 290 RPM.

What is service factor in gearbox selection?

Service factor (SF) is a multiplier applied to the calculated load to account for shock, overload, and duty cycle. AGMA 9005-F16 provides service factor guidelines. For a motor driving a conveyor (uniform load, 8-10 hours per day), SF = 1.0. For a crusher or ball mill (heavy shock, 24 hours per day), SF = 2.0 to 2.5. The gearbox rated torque must exceed T_design * SF.

Official sources

  • AGMA 9005-F16 Industrial Gear Lubrication: AGMA 9005-F16.
  • AGMA 6013-B16 Standard for Industrial Enclosed Gear Drives: AGMA 6013-B16.

Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.