How to Calculate Gearbox Output Torque and Speed (Practical Method)

Torque and speed are linked: a gearbox trades speed for torque, minus a small loss to friction. This article gives you the working formulas, one worked example, and the three errors that most often produce a gearbox that is either oversized or undersized.

The two formulas you actually need

Output speed is straightforward:

Output rpm = Motor rpm ÷ Ratio

Output torque needs the efficiency term, because the gearbox does not transmit power perfectly:

Output torque (Nm) = 9550 × Motor power (kW) × Efficiency ÷ Output rpm

The constant 9550 converts kW and rpm into Nm. For gearboxes, efficiency depends on the design — helical and bevel-helical units are typically in the high nineties per stage, while worm units lose considerably more, especially at high ratios. Always ask for the efficiency figure rather than assuming one; it changes the answer materially on worm drives.

A worked example

A 7.5 kW motor at 1450 rpm drives a 20:1 helical gear unit. Assume a gearbox efficiency of 0.96.

If that same output speed came from a worm gearbox at, say, an efficiency of 0.75, the available output torque at the same motor power would be noticeably lower — the motor is doing the same work, but more of it disappears as heat in the gearbox. That is why, on a worm drive, you sometimes need a larger frame simply to get the torque out, even though a helical unit of the same ratio could do the job with less input power. Efficiency is not a footnote; it is part of the calculation.

Error 1 — assuming the motor's rated power equals available power

Motor nameplate power is the mechanical power at the motor shaft under rated conditions. It is not the power at the gearbox output. Ignoring efficiency gives an optimistic torque figure that the unit cannot deliver continuously, and the shortfall shows up as overheating or premature wear — often in the first year.

Error 2 — ignoring starting torque

A squirrel-cage motor can develop well above its rated torque during starting, and a loaded machine requires more torque to start than to run. If you select purely on running torque, the gearbox may survive everyday operation and fail on the one day the machine starts fully loaded. This is why the service factor applied to power before selection matters more than any refinement of the running-torque figure. For advice on choosing that factor, see our selection guidance in the frequently asked questions.

Error 3 — treating ratio as a whole number only

Once you fix the output speed you need, the ratio is determined — but real gearboxes come in discrete ratios, and gear units are frequently combined with a belt or chain drive to fine-tune the final speed. Two practical consequences:

• If your required output speed falls between two available ratios, the question becomes whether to adjust upstream (pulley diameter, VFD set point) or accept the nearest ratio.
• On multi-stage units, the ratio distribution across stages affects load sharing and, on some designs, the practical limit for a given frame size.

For high-ratio duties the P series planetary gear reduction unit is often more efficient than stacking worm stages, while general-purpose duties are usually served best by a single-stage or two-stage R series helical gear unit. The full range is listed in our collections.

A short checklist before you send an enquiry

• Motor power, motor speed, and whether the motor is already fixed.
• Required output speed, or the driven machine's speed plus required ratio.
• Required output torque, and whether that figure is continuous or peak.
• Duty hours per day, starts per hour, and whether starts are loaded.
• Mounting position, ambient conditions and the shaft connection required.

Send those five items and a supplier can return a specific model with a torque figure that has been calculated rather than estimated.

Get advice from WGT

WGT WeiGao Transmission has manufactured gearboxes and gear reducers since 1988. Send your motor data, required output speed and torque through our contact page and we will return a calculated model recommendation with drawings and confirmed delivery time.

Frequently asked questions

What is the formula for gearbox output torque?
Output torque in Nm equals 9550 multiplied by motor power in kW, multiplied by gearbox efficiency, divided by output speed in rpm. The efficiency term is essential — omitting it overstates the available torque.

Do I calculate from torque or from power?
Either works if you know the other. Most selection tables are built on power plus ratio, so converting torque back to power (power = torque × rpm ÷ 9550) is often the quickest route to a catalogue page.

Why does the same motor give lower torque on a worm gearbox?
Because worm units have lower efficiency than helical or bevel-helical units, a larger share of the motor power is converted to heat inside the gearbox instead of being delivered to the output shaft.