kW to HP Calculator

The formula#

HP = kW / 0.7457   =   kW × 1.341

For metric horsepower — PS or CV, common on Indian and European motor plates — the divisor is 0.7355, so one kilowatt is 1.360 PS.

The two differ by 1.4 %. It is invisible in an estimate and awkward when a delivered nameplate is being checked against an order.

The trap: input kW is not output HP#

This is the direction where the conversion goes wrong most often.

A motor's HP is shaft output. A kW figure may be either output or input, and which one you have changes the answer:

• A motor plate marked 3.7 kW is stating shaft output. That is 5.0 HP, and the conversion is direct.
• A 4.24 kW figure measured at the supply, or read from a bill, is input. Convert that straight to HP and you get 5.7 HP for a motor that is a 5 HP machine — because you have converted the losses into horsepower too.

To go from measured input to shaft horsepower, take the efficiency out first:

HP = (kW input × efficiency) / 0.7457

Standard motor sizes, both ways#

IS and IEC motors are built to a preferred series in kW, and the HP equivalents are the familiar numbers because the series was chosen to land near them:

Note that none of them land exactly. A "10 HP" motor is a 7.5 kW machine delivering 10.06 HP, and a "3 HP" motor delivers 2.95. The kW figure is the real rating; the HP is the traditional label rounded to it.

When it matters which you quote#

Specify in kW for anything you are buying, testing or protecting: it is what IS and IEC motor standards use, it removes the metric ambiguity, and it is the number the efficiency class is defined against.

Quote HP when you are talking to somebody who thinks in it — pump and compressor selection, vehicle engines, and most site conversations in India. There is nothing wrong with the unit. There is something wrong with two people using it to mean 745.7 and 735.5 without noticing.

For the current that motor will draw, the HP to amps calculator applies efficiency and power factor together, which is what actually sets the cable and the protection.

Why the plate and the invoice disagree#

A motor bought as 10 HP is frequently delivered with 7.5 kW on the plate, and both numbers are correct. 10 HP is 7.457 kW, and the manufacturer has rounded to the nearest standard metric rating. The motor is the one you ordered.

It becomes a problem when the rounding goes the other way. A 15 HP requirement converts to 11.19 kW, and the nearest standard sizes are 11 kW and 15 kW. An 11 kW motor is 14.75 HP — about 2 per cent short. On a fan or a pump that margin disappears into the service factor and nobody notices. On a load that genuinely needs its rated output continuously, a motor running permanently at 102 per cent of rating runs hot, and insulation life falls roughly by half for every 8 to 10 degrees of extra winding temperature.

The safe habit is to convert first and choose the frame afterwards, rather than assuming the nearest catalogue number is equivalent to the figure you calculated.

Service factor is not spare capacity#

Many motors carry a service factor of 1.15, which is often read as permission to run 15 per cent over the rating. It is not quite that.

The service factor describes a short-term overload the motor can survive without damage — it exists to absorb process variation, a stiff bearing, a voltage dip. Running there continuously is explicitly outside what the figure promises: the winding runs hotter, the power factor falls, efficiency drops, and the motor delivers the extra output while spending its design life to do it.

Two further points are easy to miss. A motor fed from a VFD generally has its service factor reduced to 1.0, because the drive already adds harmonic heating the factor was never intended to cover. And the service factor applies to the motor alone — the cable, the overload relay and the starter are all sized on the nameplate rating, so a motor running into its service factor is being protected by devices that were never told about it.