Output Torque Calculator
Calculate output torque from power and speed, or through a gear reduction.
Output torque is the twisting force at a shaft's output, after any gear reduction. This calculator finds it from power and output speed — essential for sizing gearmotors, winches, and drive systems.
Output Torque Formula
Gear reduction trades speed for torque: halving the output speed roughly doubles the torque for the same power. That's why winches and industrial gearmotors run at low output RPM to produce huge torque.
How to Use This Calculator
- Enter power in horsepower.
- Enter output speed in RPM (after any gearbox).
- Read output torque in lb-ft and Nm.
Worked Example
How Gear Reduction Multiplies Torque
A gearbox trades rotational speed for torque while keeping power (minus small losses) roughly constant. Cut the output speed in half through a 2:1 reduction and the output torque nearly doubles. This inverse relationship is why the formula divides by output RPM — the slower the final shaft turns, the more torque it delivers. Winches, hoists, conveyors, and industrial gearmotors all exploit this to turn modest motor power into enormous low-speed force.
Sizing a Gearmotor or Drive
To pick a gearmotor, start from the torque your load actually needs at its operating speed, then work back to required power. Always allow a service-factor margin for starting torque and shock loads, which can briefly exceed running torque several times over. Output torque in lb-ft converts to Nm by multiplying by 1.3558.
Working From the Gear Ratio
If you know the motor speed and the reduction ratio rather than the output speed, work forward:
Output torque ≈ motor torque × ratio × efficiency
| Ratio | Output RPM (1,750 rpm motor) | Torque multiplier | Output torque, 5 HP |
|---|---|---|---|
| 5:1 | 350 | ~4.7× | 75 lb-ft |
| 10:1 | 175 | ~9.2× | 150 lb-ft |
| 20:1 | 87.5 | ~18× | 300 lb-ft |
| 40:1 | 43.8 | ~34× | 600 lb-ft |
| 60:1 | 29.2 | ~48× | 900 lb-ft |
The multiplier is always slightly below the ratio, because each stage loses a little to friction. Helical and spur boxes typically run 95–98% efficient per stage; worm gearboxes are far worse, often 50–90% depending on ratio, with the highest ratios the least efficient.
Gearbox Efficiency Is Not a Rounding Error
| Gearbox type | Typical efficiency | Notes |
|---|---|---|
| Spur / helical, single stage | 95–98% | Losses compound per stage |
| Planetary | 94–98% | Compact, high torque density |
| Bevel | 93–97% | For right-angle drives |
| Worm, low ratio | 75–90% | Right-angle, quiet, cheap |
| Worm, high ratio (50:1+) | 50–75% | Often self-locking — which is sometimes the point |
A 60:1 worm box at 60% efficiency delivers barely more than a third of the torque a naive ratio calculation predicts. That single factor causes more undersized-drive failures than any other assumption in this calculation.
Worm drives being self-locking at high ratios is a genuine design feature: a hoist or gate opener that cannot back-drive under load does not need a brake. You pay for that safety in efficiency.
Service Factor: Sizing for the Worst Moment
Running torque is not the number that breaks gearboxes. Starting torque, jams and shock loads are, and they can briefly exceed running torque several times over.
Industrial practice is to multiply the calculated running torque by a service factor before selecting a unit:
| Duty | Service factor | Examples |
|---|---|---|
| Uniform load, under 10 hr/day | 1.0 | Light conveyor, fan |
| Moderate shock, 10–24 hr/day | 1.25–1.5 | Mixer, loaded conveyor |
| Heavy shock or frequent starts | 1.75–2.0 | Crusher, hoist, reversing drive |
| Severe shock, jam risk | 2.0–3.0 | Shredder, rock breaker |
Output Torque for 10 HP by Speed
| Output RPM | Torque (lb-ft) | Torque (Nm) |
|---|---|---|
| 50 | 1,050 | 1,424 |
| 100 | 525 | 712 |
| 250 | 210 | 285 |
| 500 | 105 | 142 |
Frequently Asked Questions
Use Torque (lb-ft) = (HP × 5252) ÷ output RPM. The output RPM is the speed after any gear reduction.
Gear reduction increases torque in proportion to the reduction ratio. A 10:1 reduction multiplies torque by about 10 (minus gearbox losses) while dividing speed by 10.
Because for a fixed power, torque and speed trade off inversely. Lower output speed concentrates the same power into more twisting force.
Multiply pound-feet by 1.3558 to get newton-meters.
No — it gives the ideal output torque. Multiply by the gearbox efficiency (often 0.9–0.98) for the real delivered torque.
Output rpm = motor rpm ÷ ratio, then output torque ≈ motor torque × ratio × efficiency. A 1,750 rpm motor through a 20:1 box turns at 87.5 rpm with roughly eighteen times the torque.
Worm drives are far less efficient than gear types — often 50–75% at ratios above 50:1. A 60:1 worm box at 60% efficiency delivers barely a third of what a naive ratio calculation predicts.
1.0 for uniform light duty, 1.25–1.5 for moderate shock or long hours, 1.75–2.0 for hoists and frequent reversing, and 2.0–3.0 where a jam is possible. Multiply running torque by it before selecting a unit.
525 lb-ft (712 Nm) ideally, before gearbox losses. At 90% efficiency the delivered figure is nearer 473 lb-ft.
High-ratio worm drives cannot be back-driven by the load, which means a hoist or gate opener holds position without a brake. That safety is bought with low efficiency — it is a deliberate trade, not a fault.