Horsepower to Kilowatts Converter

What is horsepower, and which definition does this use?

Horsepower is a unit of power, the rate of doing work. It was coined by James Watt to sell steam engines against horses, and it stuck. The problem: "horsepower" now means several different things, and they are not equal. This converter uses mechanical (imperial) horsepower, the US automotive standard, defined as 550 foot-pounds per second, the value listed in NIST Special Publication 811:

• 550 ft·lbf/s = 745.699872 watts
• So 1 hp = 0.745699872 kW
• And 1 kW = 1.34102 hp

The formulas:

• kW = hp × 0.7457
• hp = kW × 1.34102

Worked example: a 200 hp engine makes 200 × 0.7457 = 149.14 kW. A 150 hp engine makes 150 × 0.7457 = 111.85 kW. Since 1 hp is a bit under 0.75 kW, a quick mental rule is "multiply hp by three-quarters."

To see the definition in action: 550 ft·lbf/s means lifting 550 pounds one foot every second, or 33,000 pounds one foot per minute, the figure Watt used. Multiply by 0.7457 and you have the SI power in kilowatts that most of the world prints on its spec sheets.

The three horsepower you will meet

If you compare a US spec sheet (hp) with a European one (kW or PS), you must know which horsepower you have:

• Mechanical / imperial hp = 550 ft·lbf/s = 745.700 W = 0.7457 kW. The US and UK automotive standard. This converter.
• Metric hp (PS, Pferdestärke) = 75 kgf·m/s = 735.499 W = 0.7355 kW. About 1.4% smaller than mechanical hp, used across continental Europe.
• Electrical hp = exactly 746 W. Used for electric motors in North America.

Two more you rarely need but occasionally see: boiler hp (9,809.5 W = 9.81 kW, for steam boilers) and drawbar hp (tractor pulling power). Because PS is smaller than hp, the same physical engine often shows a slightly higher PS number than its hp number, a 300 hp engine is about 304 PS. That 1.4% gap is why spec-sheet comparisons across the Atlantic can disagree without either side being wrong, the ISO 80000 quantities and units standard is what keeps the definitions of power and its units consistent across national spec sheets.

HP to kW reference table

Multiply hp by 0.7457 for any row. The kW column is what most of the world's car specs print.

Numeric check: 100 hp = 74.57 kW (100 × 0.7457) and 300 hp = 223.71 kW (300 × 0.7457). To get PS, multiply hp by 1.0139; to get hp from PS, multiply PS by 0.9863.

The table also shows why the two units are never far apart in magnitude: a 100 hp engine is 74.6 kW, so the kW figure is always roughly three-quarters of the hp figure. That ratio is the fastest way to sanity-check any conversion.

Where hp and kW show up: cars, motors and appliances

Cars: US window stickers and magazines quote hp; European, Japanese and Chinese specs quote kW. An American reading "150 kW" on a rental car abroad is looking at about 201 hp. Electric vehicles complicate this further, they are often rated in kW (and sometimes in both), so a "150 kW motor" is a 201 hp motor by the mechanical definition.

Electric motors: nameplate ratings use kW (or electrical hp = 746 W). A 10 kW motor is about 13.4 mechanical hp. When you size a motor against a gas engine, compare in the same unit or you will be off by ~1.4% on the mechanical-vs-metric question and more if someone used electrical hp.

Generators and appliances: a "5 hp" shop compressor draws about 3.7 kW of shaft power, though it pulls more electrical power from the wall due to motor inefficiency, which is why a 5 hp motor on 240 V can trip a 30 A breaker. The kW number is the physical power; the electrical draw is larger.

Common hp-to-kW mistakes

Four things to watch:

• Using 0.7355 (metric) instead of 0.7457 (mechanical). The 1.4% gap is small but real; use the right factor for the horsepower you have. This converter uses mechanical hp.
• Mixing electrical hp (746 W) with mechanical hp (745.7 W). They are within 0.04%, practically the same, but the boiler hp (9.81 kW) is wildly different and must never be swapped in.
• Multiplying when you should divide. hp → kW multiplies by 0.7457 (smaller number); kW → hp multiplies by 1.34102 (bigger number).
• Forgetting that hp is power, not torque. Torque (ft·lb, N·m) is a different quantity, see the N·m to ft·lb converter. Engines are rated in both, and they are not interchangeable.

Rule of thumb: 1 hp ≈ 0.75 kW and 1 kW ≈ 1.34 hp. Good enough for a spec-sheet sanity check in either direction.

One more: do not convert a torque figure with the power factor. Torque and power are different quantities, so a 400 on a torque spec is not a 400 on a power spec, convert each with its own factor.

Horsepower and kW in electric and hybrid cars

Electric vehicles have turned the hp/kW conversion into a daily need, because EV motors are rated in kW while American shoppers still think in horsepower:

• A 150 kW motor produces 150 × 1.34102 = 201 hp.
• A 250 kW motor is 250 × 1.34102 = 335 hp.
• A 100 kW motor is 100 × 1.34102 = 134 hp, about a compact car.

Note that EV marketing often quotes peak power (brief, battery-limited) while a gasoline engine's rating is sustained at a given rpm, so a 335 hp EV motor can feel quicker off the line than a 335 hp V6 yet taper sooner at speed. Hybrids compound the confusion by listing a gasoline engine in hp and an electric motor in kW in the same brochure, convert both to the same unit before adding them, because you cannot sum a hp figure and a kW figure.

Charging rates use kW too, but that is power delivered to the battery, not motor output: a 150 kW DC fast charger refills energy, while the 150 kW motor spends it. Keeping the two straight is why an EV spec sheet lists "motor power" and "charge power" in the same kW unit with completely different meanings.

A quick mental method for hp to kW

Two shortcuts cover most horsepower conversions you will do on the fly:

• hp to kW: multiply by three-quarters (0.75). A 200 hp engine is 200 × 0.75 = 150 kW (exact 149.14).
• kW to hp: multiply by 1.34. A 110 kW car is 110 × 1.34 = 147 hp (exact 147.51).

The 0.75 rule is low by about 0.6% because the true factor is 0.7457, so it drifts slightly at high numbers: 500 hp is 375 kW by the rule but 372.85 kW exactly, a 2 kW error. Worked example: a generator rated 50 kW is 50 × 1.34102 = 67.05 hp, so a "75 hp" generator and a 50 kW generator are close but not identical. For an engine swap, a racing-class limit or a warranty claim, use the exact 0.7457 and 1.34102 factors; for a spec-sheet glance the three-quarters rule is close enough.