🏎️ Motor Sizing

Written and maintained by the PhDino author · Last reviewed 21 September 2026 · Checked against 3 independent reference calculations · how PhDino checks its numbers

Choosing an adequately rated motor for a known mechanical load, with margin for starting and real-world losses.

A motor has to supply more power at its shaft than the machine strictly needs, for two reasons: a service factor (margin for occasional overload, starting transients, or voltage variation) and the losses in whatever connects the motor to the machine — belts, gears, couplings — which waste part of the shaft power before it reaches the load. A motor's nameplate horsepower is its shaft OUTPUT, so the motor's own electrical efficiency does not change the rating you need; it only changes how much electrical power the motor draws to deliver it.

Motors are also only manufactured in standard ratings (…, 3, 5, 7.5, 10, 15, 20 HP and so on), so the calculated requirement gets rounded up to the next available rating. Undersizing a motor causes it to run hot, trip overloads, or fail early under real operating conditions; oversizing wastes energy and money and can also hurt power factor at partial load — sizing is a balance, not just "bigger is safer."

Key formula

Required shaft HP = (Machine HP × Service Factor) / Drive Efficiency
Motor rating = the next standard size at or above the required shaft HP

Variables

Service Factor

design margin above the calculated load, often 1.15 for general-purpose service and higher for shock or heavy-start loads

Drive Efficiency

fraction of motor shaft power that reaches the machine after belt, gear and coupling losses (a direct-coupled load is close to 100%)

How to use the Motor Sizing calculator

Use this to turn the power a machine needs into the standard motor rating to buy. It applies a service factor for the type of duty and allows for the losses between the motor and the machine, then rounds up to the next standard size. It is meant for the steady running requirement of pumps, fans, conveyors and similar drives.

The load figure you enter is what the driven machine needs at its own shaft, in horsepower. If you have the requirement in kilowatts, divide by 0.746 to get horsepower. Always round to the next larger standard rating, never down.

Power the Driven Machine Needs (HP)

The power the driven machine takes at its input shaft when running normally, from the machine's data sheet or a load calculation. Leave off any safety margin here; that comes from the service factor.

Service Factor

A margin for the character of the load: about 1.0 to 1.15 for smooth, continuous loads such as a fan, 1.25 for moderate shock or frequent starting, and 1.5 or more for heavy shock loads such as crushers. This is a factor on the load, not the motor's nameplate service factor.

Drive Efficiency (belts, gears, couplings) (%)

The efficiency of everything between the motor shaft and the machine: belts, gears and couplings. A direct coupling is close to 100%, a V-belt drive around 90 to 95%, and a worm gearbox can be far lower. It is not the motor's own efficiency.

Worked example: a belt conveyor

A conveyor needs 3.2 HP at its head shaft when carrying its normal load. It starts loaded and sees moderate shock, so a service factor of 1.25 is chosen. A V-belt drive with 92% efficiency connects the motor. Which motor rating does it need?

Worked by hand:

1. Apply the service factor. 3.2 HP × 1.25 = 4.00 HP.
2. Allow for the drive. The belts waste part of the motor's output, so the shaft must supply more: 4.00 ÷ 92% = 4.35 HP.
3. Pick a standard rating. Standard sizes run 1.5, 2, 3, 5, 7.5, 10 HP and so on, so 4.35 HP rounds up to 5.0 HP.
4. Electrical check. A 5.0 HP motor of about 89% efficiency draws 5.0 × 0.746 ÷ 0.89 = 4.19 kW from the supply.

Buy a 5 HP motor. Rounding up costs a little in efficiency at partial load but it is the price of margin, whereas the next size down would run every day above its rating. If the same conveyor needed 4.0 HP, the requirement would be 5.43 HP and the next standard size would jump to 7.5 HP, which shows how coarse the standard steps are.

Reading the result: what happens after the horsepower

The rating is only the start. The supply wiring and the protective devices are sized from the motor's nameplate current and the applicable electrical code, not from horsepower, and a separate electrical calculation gives the conductor size and the voltage drop for the run.

• Starting can demand more than running. A high-inertia load, such as a large fan or a loaded conveyor, may need several times the running torque to get moving, and a motor that runs the load happily can stall on start.
• Standard motors are rated for a 40°C ambient and altitudes up to about 3,300 ft (1,000 m). Hot rooms, dusty enclosures and high sites need a larger frame or a derating.
• Oversizing is not free. A lightly loaded motor has a poorer power factor and lower efficiency than one working near its rating, so extra margin beyond the service factor costs energy every hour it runs.
• A variable-frequency drive changes starting and speed control, and the drive and the motor must be matched to each other.

Notes & limitations

• Standard motor ratings jump in steps (1.5, 2, 3, 5, 7.5, 10 HP and so on) — always round the calculated requirement UP, never down, to the next standard size.
• Starting torque requirements (especially for high-inertia loads) can require a larger motor than steady-state running power alone would suggest — this calculator only covers the steady-state sizing.

Common mistakes

• Dividing by the motor's own efficiency. A motor's rating is its shaft output, so its efficiency affects only how much electricity it draws, not the horsepower to buy. Only losses after the motor shaft belong in the drive-efficiency box.
• Confusing the load's service factor with the motor's nameplate service factor. They are different numbers with different jobs: one describes the duty, the other is extra overload capacity built into the motor.
• Rounding down to the nearest size because the calculated value is only a little above it. The margin exists to be used, and a motor that runs above its rating continuously overheats and fails early.
• Sizing for average power on a load that peaks. A crusher or a reciprocating compressor needs a motor selected for its peak and its starting torque, not its mean.
• Mixing horsepower and kilowatts. One horsepower is 0.746 kW, so a 5.5 kW requirement is 7.4 HP, not 5.5.

Frequently asked questions

Papers worth reading

Increased efficiency versus increased reliability Bonnett, A. H. & Yung, C. (2008), IEEE Industry Applications Magazine. Looks at whether reaching premium motor efficiency forces compromises in reliability and other performance, which matters when you choose a motor class as well as a size.