Electric Motor Torque Calculator (Volts, Amps, RPM)

Electric Motor Torque Calculator

Work out the full-load torque of an electric motor straight from its electrical nameplate. Enter supply type, line voltage, full-load amps, power factor and efficiency, then either type the full-load RPM or var the tool build it from poles, frequency and slip. Get torque in newton-metres and pound-feet plus mechanical power and synchronous speed.

🎯Real Motor Nameplate Presets

🔌Electrical Inputs

Supply type

Sets the power formula and whether power factor is used.

Line voltage (V)

Line-to-line volts for AC, or terminal volts for DC.

Line current, full-load (A)

Full-load amps (FLA) read from the motor nameplate.

Power factor (0 to 1)

AC only. Ignored for DC, where it is treated as 1.

Efficiency (%)

Shaft output over electrical input, usually 82 to 96 pct.

Speed source

Choose how the shaft speed used for torque is found.

Full-load speed (RPM)

Nameplate running speed. Used when speed source is direct.

Number of poles

Even pole count that sets synchronous speed.

Supply frequency (Hz)

Mains frequency, 60 Hz in North America, 50 Hz elsewhere.

Slip (%)

Drop below sync speed at load, typically 2 to 5 pct.

Full-load torque 0 Nm newton-metres at the shaft

Torque in pound-feet 0 ft-lb same torque, imperial units

Mechanical power 0 kW shaft output

Synchronous speed 0 RPM no-load magnetic speed

🔢Formula Constants

9.548860 / 2π Nm factor

1.732√3 for 3-phase

1.35582Nm per ft-lb

745.7watts per hp

📏Synchronous Speed by Poles

Poles	Sync RPM at 60 Hz	Sync RPM at 50 Hz	Rev per Second 60 Hz
2	3600	3000	60.0
4	1800	1500	30.0
6	1200	1000	20.0
8	900	750	15.0
10	720	600	12.0
12	600	500	10.0
16	450	375	7.5

📋Typical Power Factor and Efficiency by Size

Motor Size	Typical Power Factor	Typical Efficiency	Common Use
Under 1 hp	0.72 to 0.80	75 to 82 pct	Small fans, pumps
1 to 5 hp	0.80 to 0.86	82 to 88 pct	Conveyors, blowers
5 to 20 hp	0.84 to 0.88	87 to 91 pct	Compressors, mills
20 to 50 hp	0.86 to 0.90	90 to 93 pct	Large pumps
50 to 200 hp	0.87 to 0.91	93 to 95 pct	Industrial drives
Over 200 hp	0.88 to 0.92	94 to 96 pct	Crushers, cooling

⚙NEMA Design Torque Classes

NEMA Design	Starting Torque	Slip at Full Load	Typical Load
Design A	100 to 200 pct	0.5 to 3 pct	High-inertia, low slip
Design B	100 to 200 pct	Under 5 pct	General purpose
Design C	200 to 250 pct	Under 5 pct	Conveyors, crushers
Design D	Up to 275 pct	5 to 13 pct	Punch press, hoists
Design E	75 to 190 pct	Under 3 pct	Premium efficiency

🗃Poles, Speed and Use Comparison Grid

Poles	Sync RPM 60 Hz	Sync RPM 50 Hz	Typical Full-Load RPM	Torque vs 2-Pole	Common Application
2	3600	3000	3450 to 3500	1.0x (baseline)	Pumps, high-speed fans
4	1800	1500	1725 to 1760	About 2.0x	General industrial
6	1200	1000	1140 to 1170	About 3.0x	Compressors, mixers
8	900	750	850 to 870	About 4.0x	Conveyors, agitators
10	720	600	680 to 700	About 5.0x	Ball mills, kilns
12	600	500	560 to 580	About 6.0x	Low-speed crushers

📑How the Torque Is Built

3-phase input powerP = √3 × V × I × PF = 1.732 × line volts × full-load amps × power factor, giving real electrical watts drawn.

1-phase input powerP = V × I × PF. A single-phase supply drops the √3 factor but still uses power factor.

DC input powerP = V × I. Direct current has no power factor, so the two terminal quantities multiply directly.

Mechanical powerP_mech = P × efficiency. Multiply electrical input by efficiency as a decimal to get shaft output in watts.

Synchronous speedN_sync = 120 × frequency / poles. A 4-pole motor on 60 Hz spins its field at 1800 RPM.

Full-load speedN = N_sync × (1 − slip / 100) when computed, or the value you type. Slip makes the shaft lag the field.

Shaft torqueT = 9.5488 × P_mech / N. The 9.5488 is 60 / (2π), converting watts and RPM into newton-metres.

Unit conversionsT_ftlb = T / 1.35582 and hp = P_mech / 745.7, giving pound-feet and horsepower.

💡Motor Torque Tips

Use full-load amps, not locked-rotor: The nameplate FLA gives running torque. Starting current can be 6 to 8 times higher, so a 6.5 A motor may pull 40 to 52 A for a second on start. Feeding locked-rotor amps into this tool overstates continuous torque by several times.

Mind slip when you skip the RPM field: A 4-pole 60 Hz motor syncs at 1800 RPM but runs near 1750 at load, about 2.8 pct slip. Using 1800 instead of 1750 understates torque by roughly 3 pct, since torque scales as power divided by actual shaft speed.

When you read the nameplate on a motor, what do you see? You see the amperage and volts. What about the torque? Torque is the twisting force that moves your equipment around. And this calculator gets it for you. It take the values from the nameplate into pounds-feet or newton-metres of torque. It also provide the synchronous speed and the shaft power.

So what’s first? Power. How many amps and volts is coming in? Motor type matter here. For three phase, this is multiplied by approximately 1.732. For single phase it’s your power factor. And for DC motors, it’s voltage times current. This calculator will selects the appropriate equation for you. If you pick the wrong one, you’re off to a bad start.

How to Calculate Motor Torque

The second factor are efficiency. Not all of the power drawn by a motor gets used. Windings lose some power as heat. Drag and friction losses occurs. What we care about is how much power makes it to the shaft. That’s called the nameplate efficiency. So if your motor draws 4000 watts with 90 percent efficiency then you’re getting 3600 watts on the shaft. You would of calculate torque based off that assumption and miss five to twenty percent, which can cause the drive to fail when loaded.

The last component of the equation is speed. Power divided by speed equal torque. When a motor is loaded it will slow down. This is called slip, which is true for an AC motor. An example would be that a 4-pole motor operating off 60 hertz only operates at approximately 1750 RPM as opposed to 1800 RPM. This synchronous speed makes the torque too low. If possible, use the actual running speed.

The other constant is: Watts = (RPM/9.5488) X Newton-Metres. So it do the math for you in units. Divide the result by 1.35582 for pound-feet. And that’s how we know slow motors is more torquey. A two-pole motor spinning at 3600 RPM has only half as much torque than an eight-pole motor running at 900 RPM. Both would be rated the same for power.

If the name plate is illegible, reference tables shows the usual efficiencies and power factors. The NEMA design chart clarifies expected slip and starting torque. These details makes the calculator more useful when sizing.

Two practices will ensure accuracy. First, use full-load amps. Do not use the locked-rotor current because it is much higher. Second, use the true running speed. That’s the one that accounts for slip. If you do this you’ll have a good torque number to work with. From there you can verify your gear ratios or size couplings. This makes electrical data turn into mechanical answers.