kW to HP Converter

A kW to hp converter turns kilowatts into horsepower and back. The United States rates engines and motors in mechanical horsepower while most of the world uses kilowatts, so the same machine often carries two numbers that look unrelated. They are tied by one constant: 1 kW = 1.34102 hp and 1 hp = 0.74570 kW. A 110 kW electric motor therefore makes 147.51 hp, and a 300 hp engine makes 223.71 kW.

If you know torque and engine speed rather than power, the Horsepower Calculator works horsepower out from lb-ft and rpm, and the Watts to Amps Calculator turns a power rating into current at a given voltage. This page answers one question: which unit a power figure is written in.

The kW to hp formula and the four horsepower definitions

Every conversion on this page runs through watts. A kilowatt is exactly 1,000 watts and each kind of horsepower is a fixed number of watts, so a conversion is one multiplication and one division:

The definitions differ because horsepower was standardized separately in different countries and trades. The table gives each definition and its value in watts, based on NIST Special Publication 811, the federal guide to SI units and conversion factors.

Mechanical horsepower is exact once the foot (0.3048 m) and the pound-force (4.4482216152605 N) are fixed: 550 × 0.3048 × 4.4482216152605 = 745.69987158227022 W. Metric horsepower is exact as well, 75 × 9.80665 = 735.49875 W, because the kilogram-force is defined through standard gravity. The UK horsepower NIST lists (745.70 W) is the same mechanical unit, rounded.

Worked example: a 110 kW electric motor in horsepower

The fueleconomy.gov listing for the 2021 Nissan Leaf with the 40 kWh battery pack gives the motor as 110 kW. To express that in the units a US buyer expects:

So the motor counts as roughly 147 hp in American terms and roughly 150 PS in a European brochure, while the electric-horsepower figure lands 0.06 hp below the mechanical one. Enter 110 with kW to hp selected and the converter shows all three, plus 110,000 W and 11.21 boiler horsepower. The listing's 40 kWh battery figure is energy, not power; see kW versus kWh below.

Worked example: a spec sheet that says 140 kW (190 PS)

European brochures usually print power in kilowatts with metric horsepower in parentheses. Take a car listed at 140 kW (190 PS). There are two routes to US horsepower, and they do not agree to the decimal:

The 0.34 hp gap exists because the PS figure is itself a rounded conversion: 140 kW works out to 190.35 PS, printed as 190, and converting it again stacks a second rounding on the first. The kilowatt value is the one declared in type-approval paperwork under UN ECE Regulation No. 85, so start from it whenever both are shown. In reverse, 190 PS is 139.74 kW, which a brochure rounds back to 140 kW.

kW to hp conversion table

Every row was computed with the NIST factors and rounded to two decimals. The 37 kW row is the IEC size DOE pairs with a 50 hp motor, and the 110 kW row matches the EV example above.

For mental math, 3 kW is almost exactly 4 hp (4.02 hp), and 3 hp is a little over 2.2 kW (2.24 kW). Multiplying kilowatts by 4/3 lands about 0.6% low, close enough for a quick sanity check but not for a specification.

Electric motor sizes: hp to kW and nameplate equivalents

US motors come in horsepower steps (1, 1.5, 2, 3, 5, 7.5, 10 hp and up), IEC motors in kilowatt steps. The Department of Energy's motor efficiency standards, 10 CFR 431.25, pair each horsepower size with a standard kilowatt equivalent. The pairs are size classes, not exact conversions, as the last column shows.

The 125 hp row is the outlier: its paired IEC size, 90 kW, is 3.45% below an exact conversion. To place a kilowatt-rated motor on its horsepower tables, 10 CFR 431.25 converts with 1 kW = 1/0.746 hp, calculated to three decimal places, and a result at or above the midpoint between two listed sizes rounds up. An 18.5 kW motor converts to 24.799 hp, above the 22.5 hp midpoint between 20 and 25 hp, so it counts as 25 hp. A 90 kW motor converts to 120.643 hp and rounds up to the 125 hp class.

kW versus kWh: power is not energy

Kilowatts and kilowatt-hours share a prefix but measure different things. The US Energy Information Administration puts it plainly: one kilowatt is 1,000 watts, and one kWh is one kilowatt generated for one hour. Power is a rate; energy is the amount delivered over time.

An EV makes the difference concrete. The 2021 Leaf's 110 kW is how hard its motor can push; its 40 kWh pack is how much energy it carries. At a constant 110 kW the pack would last 40 ÷ 110 = 0.364 hours, about 22 minutes. Everyday driving uses far less than peak power, so fueleconomy.gov rates the car at 30 kWh per 100 miles, and it counts 33.7 kWh as one gallon of gasoline when computing MPGe.

Horsepower has an energy counterpart too, the horsepower-hour. A 5 hp motor running at full output for 3 hours delivers 5 × 0.7457 × 3 = 11.19 kWh of shaft work, and the electric meter records more than that because of motor losses.

Car spec sheets and EV ratings: SAE hp, kW and PS

US spec sheets generally quote SAE net horsepower (SAE J1349), and a rating advertised as certified has passed the SAE J2723 procedure. In Europe the declared figure is net power in kilowatts under UN ECE Regulation No. 85: power measured on a test bench at the end of the crankshaft or its equivalent, with the listed auxiliaries fitted. Many brochures add metric horsepower in parentheses, labeled PS, CV or ch depending on the language.

For electric drive trains, Regulation No. 85 records maximum net power and the maximum 30 minutes power, the highest net power the drive train can average over 30 minutes. The 30-minute figure cannot exceed the peak and is typically lower, so two documents can both be right and still differ. When a dual-motor EV lists a system output, convert that figure instead of adding the motors, because battery and inverter limits can cap the total.

How to use this converter

Pick a direction, type the power value (commas may separate thousands, as in 1,500) and choose the horsepower type. Mechanical fits almost every US engine, motor, pump and outboard; Metric (PS) fits European and Japanese spec sheets; Electric fits documents that state 746 W per horsepower; Boiler is only for steam boilers.

The result card shows the answer with the arithmetic behind it, the next table lists the same power in every unit, and the last table converts common ratings with your current settings so you can scan neighboring sizes. Two decimals suit a specification; four help when checking someone else's math.

Who this converter is for

Car shoppers use it to compare a US window sticker with a European or Japanese brochure, and EV owners to translate motor kilowatts into familiar horsepower. Electricians, HVAC technicians and plant engineers move between NEMA horsepower sizes and IEC kilowatt sizes when replacing a motor. Boat and generator owners check ratings quoted in either unit, facility staff read boiler capacities, and students check physics homework against the exact definitions.

Key terms

• Kilowatt (kW): 1,000 watts; one watt is one joule per second.
• Mechanical horsepower (hp): 550 foot-pounds-force per second, or 745.69987 W.
• Metric horsepower (PS, CV, ch): 75 kilogram-force meters per second, or 735.49875 W.
• Electric horsepower: exactly 746 W.
• Boiler horsepower: steam-raising capacity, 9,809.50 W in NIST SP 811.
• Kilowatt-hour (kWh): one kilowatt for one hour, exactly 3.6 million joules.
• Net and gross power: output measured with the required accessories fitted (SAE J1349, UN ECE) or without them (SAE J1995).
• Maximum 30 minutes power: the highest net power an electric drive train can average for 30 minutes.

What changes the result, and tips for clean conversions

Only the horsepower definition and the rounding can change a correct conversion. Other apparent changes are really different measurements.

• Horsepower type. PS instead of mechanical hp raises the number by 1.39%: 400 kW is 536.41 hp but 543.85 PS.
• Rounded factors. 500 kW × 1.34 gives 670 hp; the exact answer is 670.51 hp.
• Double rounding. Convert from the declared figure, not from one already converted and rounded.
• Gross versus net. Classic-car figures may be gross ratings, measured without accessories, and read higher than a net rating.
• Crank versus wheel. Spec sheets give crankshaft or motor output; a chassis dyno measures after drivetrain losses.
• Output versus input. A motor nameplate states shaft output; electrical input is higher by the motor's losses.

Limitations

This page converts units and nothing more. It cannot say how much electricity a motor draws, which depends on efficiency and load, it does not estimate wheel horsepower, and it does not adjust for test conditions, which differ between SAE net, gross and UN ECE ratings. Boiler horsepower needs a footnote: Virginia's boiler rules (16VAC25-40-10) define it as evaporating 34.5 pounds of water per hour from and at 212 °F, or 33,475 Btu per hour, which is about 9,810.6 W with the International Table Btu. The converter uses the NIST figure of 9,809.50 W; the 0.01% gap does not matter for sizing.

Related calculators

• To get horsepower from torque and rpm, or from a quarter-mile time, use the Horsepower Calculator.
• To find the current a load draws at a given voltage, use the Watts to Amps Calculator.
• For length, weight, volume and temperature conversions, use the Unit Converter.
• To work out torque from a force and a lever arm, use the Torque Calculator.
• To price the kWh a motor or appliance uses, use the Electricity Cost Calculator.
• For volts, amps, ohms and watts in one circuit, use the Ohm's Law Calculator.

Sources

• NIST Special Publication 811 (2008 edition), Guide for the Use of the International System of Units, Appendix B.8: the four horsepower factors, the foot, the kilogram-force and the kilowatt hour (NIST SP 811, Appendix B.8).
• 10 CFR 431.25, energy conservation standards for electric motors: standard kilowatt equivalents (Table 5), the 1 kW = 1/0.746 hp rule with midpoint rounding, and the 91.7% efficiency for a 10 hp, 4-pole enclosed motor (eCFR).
• U.S. Energy Information Administration, kW and kWh definitions (Measuring electricity).
• fueleconomy.gov (DOE and EPA), 2021 Nissan Leaf (40 kW-hr battery pack): 110 kW motor, 30 kWh per 100 miles, 33.7 kWh per gallon (vehicle data).
• UN ECE Regulation No. 85, net power and maximum 30 minutes power, OJ L 323, 7.11.2014 (EUR-Lex).
• SAE J2723 (revised October 2021), engine power and torque certification under SAE J1349 and J1995 (SAE Mobilus).
• Virginia Administrative Code 16VAC25-40-10, definition of one boiler horsepower (Virginia Law).