HP to kW Calculator

The formula#

P(kW) = HP × 0.7457

One mechanical horsepower is 745.7 W, which almost everyone rounds to 746. The rounding is a 0.04 % error and it has never mattered.

There are two horsepowers, and that one does matter#

Mechanical horsepower — the imperial one — is 745.7 W. It comes from James Watt's figure of 33,000 foot-pounds per minute, which is where the horse in the name came from.

Metric horsepower — written PS or CV — is 735.5 W, defined as 75 kgf·m/s.

They are 1.4 % apart, and European and Indian motor practice has long quoted the metric one. So a pump listed as 5 HP is 3.729 kW in one catalogue and 3.677 kW in another, and neither is wrong.

That difference is invisible in a rule of thumb and awkward when somebody is checking a delivered nameplate against a purchase order. The calculator shows both so you can see which one your figure matches.

The mistake that undersizes cables#

Here is the one that costs real money.

A motor's HP rating is its mechanical output — the power at the shaft. It is not what the motor draws from the supply. The motor has losses, so the electrical input is always higher:

Input (kW) = Shaft (kW) / efficiency

Take that 5 HP motor at 88 % efficiency:

Shaft output = 5 × 0.7457 = 3.73 kW
Electrical input = 3.73 / 0.88 = 4.24 kW

4.24 kW, not 3.73 kW. Fourteen percent higher. Size a cable or a breaker from the 3.73 and you have undersized it by that margin before you have applied a single derating factor — and derating is what makes an already-tight cable fail. The cable sizing calculator works from current, and the current follows the input power, not the output.

And a third term for current#

To get from horsepower to amps you need power factor as well, because the motor draws reactive current to magnetise its core:

I = HP × 746 / (√3 × V × PF × η)

At 415 V three-phase, 0.85 power factor and 88 % efficiency, that 5 HP motor draws about 6.9 A. The HP to amps calculator does this directly, and adds the starting current — around six times full load on a direct-on-line start, which is what the protection has to survive rather than what the cable has to carry.

Quick reference#

Efficiency varies with size and with load: a small motor is worse than a large one, and any motor running lightly loaded is worse than the same motor at its rated point. Take the figure from the nameplate where you have it.

Which figure to specify#

If you are buying, specify in kW. It is the SI unit, IS and IEC motor standards use it, and it removes the metric-versus-mechanical ambiguity entirely. Horsepower survives in pumps, compressors and vehicle engines, and there is no harm in it — as long as everyone in the conversation means the same 746.