Horsepower to Kilowatt Converter

Convert horsepower (mechanical, metric, or boiler) to kilowatts and back, using exact conversion factors from NIST SP 811, ISO 80000-4, SAE J1349, DIN 66036, and the ASHRAE Handbook. This free online calculator is fast, accurate, and works in any modern browser, no signup, no ads, no tracking.

Introduction

Horsepower is one of the most recognisable units in the world, even though it has not been the official unit of power in any major country for over a century. Automobiles, lawn mowers, marine engines, electric motors, and even vacuum cleaners still publish their ratings in horsepower, while the international standard unit, the watt, quietly coexists in the same spec sheets. The relationship between the two is fixed by international definition, but the catch is that there is more than one kind of horsepower, and they differ by more than 13× between the two extremes.

This converter accepts an input in horsepower (hp) and instantly reports the equivalent in kilowatts (kW) and watts (W), with the three primary hp standards (mechanical, metric, and boiler) available as toggleable modes. The default is mechanical horsepower (hp(I)), the international standard used in US automotive and industrial ratings. The metric horsepower (PS / CV / hp(M)) is used in European automotive brochures, and the boiler horsepower (hp(S)) is the standard in steam-plant and HVAC engineering. Switching between them is a single click, and the displayed watts value updates immediately so you can see the small (~1.4%) but real gap between mechanical and metric, and the very large (~13×) gap between mechanical and boiler.

The default value of 1 hp is loaded on page load so you can see the conversion immediately, 1 hp(I) = exactly 0.7456998715822702 kW = 745.6998715822702 W. Change the input to any positive number and the kW and W fields update in real time, with bonus read-outs showing the same power expressed in the alternative hp standards so you can compare them side by side.

How to Use This Tool

1. Enter the power in horsepower in the input box. Any non-negative number works; decimals are accepted.
2. The kilowatts field updates instantly as you type. It reflects the active standard's exact conversion factor.
3. Toggle the standard at the top, Mechanical (imperial) hp, Metric hp (PS / CV / pk), Boiler hp, or Electrical hp, and the kW value re-derives automatically from the same hp value under the new standard's factor.
4. Read the watts, mechanical hp, boiler hp, and electrical hp bonus read-outs in the result box below to cross-check the same power in every standard.
5. To convert in the reverse direction, type a kW value into the right-hand field and the hp field updates under the active standard. Or click the ⇄ button to swap the two field contents.

Negative inputs are rejected (true power magnitude is non-negative in the steady-state sense). Non-numeric input produces no result. The standard buttons can be clicked at any time; the kW value is re-computed from whatever is currently in the hp field under the new standard's factor.

The Formulas

Mechanical (imperial) horsepower (hp(I)): Defined as exactly 745.6998715822702 watts, per NIST Special Publication 811 and ISO 80000-4:2006. The derivation is 1 hp(I) = 550 ft·lbf/s exactly, using the 1959 international yard and pound agreement (1 ft = 0.3048 m exactly, 1 lb = 0.45359237 kg exactly, standard gravity 9.80665 m/s² exactly). To convert mechanical hp to kilowatts:

kW = hp(I) × 0.7456998715822702

Metric horsepower (hp(M) / PS / CV / pk): Defined as exactly 735.49875 watts, per DIN 66036. The derivation is 1 hp(M) = 75 kgf·m/s exactly, using the BIPM standard gravity 9.80665 m/s² exactly. The unit is known as "Pferdestärke" (PS, German), "cheval-vapeur" (CV, French), or "paardekracht" (pk, Dutch). To convert metric hp to kilowatts:

kW = hp(M) × 0.73549875

Boiler horsepower (hp(S)): Defined as exactly 9 809.5 watts, per the ASHRAE Handbook. The derivation is 34.5 lb of water evaporated per hour at 212 °F, with 1 lb = 0.45359237 kg exactly. Boiler hp is roughly 13.15× mechanical hp, so a 100 hp(S) boiler is the same power output as a 1314.9 hp(I) mechanical engine. To convert boiler hp to kilowatts:

kW = hp(S) × 9.8095

Electrical horsepower (hp(E)): Defined as exactly 746 watts, used in some legacy electric-motor ratings in the United States. To convert electrical hp to kilowatts:

kW = hp(E) × 0.746

Reverse direction (kW to hp): Divide by the standard's factor. 1 kW = 1.341022108... hp(I) = 1.359621617... hp(M) = 0.101941995... hp(S) = 1.340482574... hp(E). For everyday mental arithmetic, 1 kW ≈ 1.34 hp(I) ≈ 1.36 PS.

Cross-conversion between standards: 1 hp(I) = 1.013869676... hp(M); 1 hp(M) = 0.98632 hp(I); 1 hp(S) = 13.154953... hp(I); 1 hp(I) = 0.0760173... hp(S). The mechanical-vs-metric gap is ~1.4%; the mechanical-vs-boiler gap is ~13×. Always check which standard your source is using before doing serious engineering work.

Worked Examples

Example 1, 1 hp(I) = 0.7456998715822702 kW exactly. This is the defining constant of the converter. 1 hp(I) × 0.7456998715822702 = 0.7456998715822702 kW, which is 745.6998715822702 W. Round-trip check: 0.7456998715822702 kW ÷ 0.7456998715822702 = 1 hp(I) exactly. This is the simplest sanity check for any horsepower-to-kW conversion.

Example 2, 150 hp(I) engine. A US-spec 150 hp engine produces 150 × 0.7456998715822702 = 111.8549807373405 kW. In mechanical hp: 150 hp(I) (identity). In metric PS: 150 × 0.7456998715822702 ÷ 0.73549875 = 152.080451 hp(M). This is the kind of cross-border conversion that shows up in car reviews, the same engine is rated 150 hp in the US and ~152 PS in Europe.

Example 3, 200 PS European car. A European-spec 200 PS car produces 200 × 0.73549875 = 147.09975 kW exactly. In mechanical hp: 147.09975 ÷ 0.7456998715822702 = 197.31138 hp(I). In boiler hp: 147.09975 ÷ 9.8095 = 14.996 hp(S). Notice the ~1.4% gap between mech and metric hp, never assume 1 PS = 1 hp.

Example 4, 1 kW electric motor. 1 kW ÷ 0.7456998715822702 = 1.341022108... hp(I). In metric PS: 1 kW ÷ 0.73549875 = 1.359621617... hp(M). In boiler hp: 1 kW ÷ 9.8095 = 0.101941995... hp(S). This is a useful rule of thumb: every kW is roughly 1.34 hp(I) or 1.36 PS.

Example 5, 100 hp(S) boiler. A 100 hp(S) steam boiler produces 100 × 9.8095 = 980.95 kW exactly. In mechanical hp: 980.95 ÷ 0.7456998715822702 = 1315.4938 hp(I). In metric PS: 980.95 ÷ 0.73549875 = 1333.728 hp(M). This is the kind of conversion that arises in HVAC and steam-plant engineering, boiler hp is a "rating" unit, not a "torque times speed" unit, which is why its W/hp ratio is so much higher than mechanical or metric hp.

Example 6, 250 W electric bike (EU legal limit). 250 W ÷ 0.7456998715822702 = 0.335 hp(I). In metric PS: 250 ÷ 0.73549875 = 0.340 PS. In boiler hp: 250 ÷ 9809.5 = 0.0000255 hp(S). The EU pedelec limit of 250 W is roughly one-third of a mechanical horsepower, a useful number for thinking about how much power the human body can produce while pedalling.

Example 7, Boiler rating round-trip. A boiler rated at 9.8095 kW is exactly 1 hp(S) (since 1 kW × 9.8095 = 9.8095 kW and 1 hp(S) = 9.8095 kW). This is the ASHRAE definition at work, the boiler hp is simply a 9.8095 kW rating with a legacy unit symbol.

Where It Shows Up

• Car engines, European manufacturers usually rate in PS or kW; US-market brochures use hp. The same engine is rated higher in PS than in hp, which matters when comparing cross-border.
• Electric motors, typically rated in kW or hp depending on the regional market. Industrial motors often list both. The 1 hp(I) = 0.7456998715822702 kW factor is the standard used in most motor nameplates.
• Boats and outboard motors, usually rated in hp, even in Europe. A 9.9 hp outboard is a common class (9.9 × 0.7456998715822702 = 7.38 kW).
• Generators, rated in kW (real output) and sometimes kVA (apparent power including reactive). A "5 kW generator" can briefly deliver more horsepower equivalent, but only briefly due to thermal limits.
• Steam boilers and HVAC, rated in boiler hp(S) in the US and in kW in Europe. The conversion is 1 hp(S) = 9.8095 kW exactly, so a 100 hp(S) boiler is a 980.95 kW boiler.
• Power tools, workshop tools are rated in watts (EU) or hp (US). A "1.5 hp" table saw might be 1.5 × 0.7456998715822702 = 1.118 kW = 1118.55 W.
• E-bikes and electric vehicles, motor power is often given in watts (250 W is the EU legal limit for pedelecs) or hp (for higher-powered EVs). A 300 kW EV motor is roughly 402 hp(I).
• Physics and engineering textbooks, kW is the SI unit; hp is the everyday unit. The conversion factor is fixed by international definition.

Common Mistakes

• Confusing mechanical and metric horsepower. 1 hp(I) ≠ 1 PS. Specifically, 1 hp(I) = 1.013869676... PS and 1 PS = 0.98632 hp(I). They differ by ~1.4%, which is small but real in cross-border engineering specifications.
• Confusing horsepower with boiler horsepower. A 100 hp(S) boiler is 13.15× more powerful than a 100 hp(I) engine. The "hp" symbol is shared, but the standards are completely different, mechanical hp is defined by 550 ft·lbf/s, boiler hp is defined by 34.5 lb of water evaporated per hour.
• Confusing horsepower with brake horsepower (bhp). bhp is power measured at the engine's output shaft before drivetrain losses; wheel horsepower (whp) is what reaches the road. A car rated at 200 hp might deliver only 170 whp.
• Confusing horsepower with horsepower-hour. A horsepower-hour is a unit of energy (≈ 2.685 MJ), not power. It shows up in older mechanical engineering contexts and in historical gas and electricity billing.
• Equating horsepower to kilowatts without the 745.7 factor. The common rough estimate "1 hp ≈ 0.75 kW" is accurate to within 0.6% but is not exact. For precise engineering work, always use 745.6998715822702 W exactly.
• Forgetting efficiency losses. A motor rated at 100 hp at the shaft may draw 110 hp equivalent of electrical input. Horsepower numbers usually refer to output, not input, check the spec sheet.
• Mixing power and torque. Power (watts, hp, kW) is torque × angular velocity. The two are not interchangeable. A high-torque, low-rpm engine has the same power as a low-torque, high-rpm engine of equal power.
• Ignoring units in the input box. Make sure you are typing mechanical hp, not metric PS, unless you want the result to be off by ~1.4%. The standard toggle below the input box lets you switch without retyping.

Frequently Asked Questions

What is the exact conversion factor from horsepower to kilowatts?

The exact factor depends on which horsepower standard you mean. For mechanical (imperial) horsepower, 1 hp(I) = 745.6998715822702 W exactly, so 1 hp(I) = 0.7456998715822702 kW exactly. For metric horsepower (PS / CV / pk), 1 hp(M) = 735.49875 W exactly, so 1 hp(M) = 0.73549875 kW exactly. For boiler horsepower, 1 hp(S) = 9 809.5 W exactly, so 1 hp(S) = 9.8095 kW exactly. For electrical horsepower, 1 hp(E) = 746 W exactly, so 1 hp(E) = 0.746 kW exactly. The mechanical factor is anchored to the 1959 international yard and pound agreement and is cited by NIST SP 811, ISO 80000-4:2006, and SAE J1349.

How many kilowatts is 1 horsepower?

1 mechanical horsepower = exactly 0.7456998715822702 kilowatts (about 0.75 kW). For everyday mental arithmetic, 1 hp ≈ 0.7457 kW. The inverse is 1 kW = 1.341022108... hp(I). Note that "1 horsepower" without qualification usually means mechanical hp, but European automotive specifications often use metric hp (PS / CV), which is 0.73549875 kW. The difference is about 1.4%, but it shows up in cross-border car comparisons and in precision engineering work.

What is the difference between mechanical, metric, and boiler horsepower?

Mechanical horsepower (hp(I)) is defined as exactly 745.6998715822702 W, derived from 550 ft·lbf/s with the 1959 international yard and pound agreement. Metric horsepower (hp(M) / PS / CV / pk) is defined as exactly 735.49875 W, derived from 75 kgf·m/s with the BIPM standard gravity 9.80665 m/s². Boiler horsepower (hp(S)) is defined as exactly 9 809.5 W, derived from 34.5 lb of water evaporated per hour at 212 °F. The mechanical vs metric gap is about 1.4%, and the mechanical vs boiler gap is about 13.15×. The mechanical standard is the most common in US automotive and industrial ratings; the metric standard dominates European automotive; the boiler standard is the legacy unit in steam-plant and HVAC engineering.

Why are there different horsepower units?

James Watt defined the original mechanical horsepower in the late 18th century to compare the output of his steam engines to draft horses, it was a marketing unit, not a scientific one. The exact mechanical figure (1 hp = 745.7 W) was formalised in the 20th century. Metric horsepower (PS) was defined later for European machinery ratings and remains common in car brochures. The electrical horsepower (746 W exactly) was used briefly in the US for electric motor ratings. The boiler horsepower (9 809.5 W) emerged from the steam-engine era as a measure of "boiler output" based on how much water the boiler could vaporise per hour, and it survives in HVAC and steam-plant engineering. All four are widely encountered in their respective fields, which is why this converter supports all of them.

How do I convert kilowatts back to horsepower?

Divide by the appropriate factor. 1 kW = 1.341022108... hp(I) (mechanical); 1 kW = 1.359621617... hp(M) (metric PS); 1 kW = 0.101941995... hp(S) (boiler); 1 kW = 1.340482574... hp(E) (electrical). For a 150 hp(I) engine that produces 150 × 0.7456998715822702 = 111.8549807373405 kW, the inverse is 111.8549807373405 ÷ 0.7456998715822702 = 150 hp(I) exactly (round-trip is identity). The simplest mental shortcut is to multiply kW by 1.34 to get hp(I) or by 1.36 to get PS.

Which horsepower unit should I use?

It depends on your audience. For US readers, use mechanical hp. For European readers, especially in automotive contexts, use PS or kW. For scientific and engineering documents, use kW or W, they are SI units and unambiguous. International marketing often quotes both. In electrical engineering, kW is standard. In mechanical engineering, both kW and hp appear, depending on country. In steam-plant and HVAC engineering, the boiler hp(S) is the legacy unit, but it is increasingly being replaced by kW in new specifications. When in doubt, quote the SI value (kW) and the standard-specific value (hp) side by side with the conversion factor printed next to them.

Is 1 horsepower really equal to one horse?

No. The original James Watt figure was based on observations of draft horses turning a mill wheel; the actual sustained output of a working horse is closer to 0.5 to 0.7 hp. Watt's figure was inflated for marketing reasons, he wanted to show that his steam engines could replace many horses. A human cyclist can sustain roughly 0.3 to 0.5 hp for an hour (a fit amateur can hit 0.5 hp for an hour; pro cyclists can sustain 0.6 hp+ for an hour; the all-hour record is around 0.7 hp). A fit human can briefly produce 1 hp or more, but not for sustained periods.

What about boiler horsepower vs mechanical horsepower?

Boiler hp(S) is about 13.15× larger than mechanical hp(I). A 100 hp(S) boiler produces 9.8095 kW × 100 = 980.95 kW, equivalent to 980.95 ÷ 0.7456998715822702 = 1315.5 hp(I). Boiler hp is not a measure of torque times speed, it is a measure of the boiler's capacity to evaporate water under standardised conditions. The 9 809.5 W figure derives from the ASHRAE definition of 34.5 lb of water evaporated per hour at 212 °F (the boiling point of water at sea level), combined with the latent heat of vaporisation. Always use boiler hp only for steam-plant and HVAC engineering; never use it for engine, motor, or generator ratings.

References

• NIST Special Publication 811, Guide for the Use of the International System of Units (SI). Authoritative source for SI unit usage and conversion factors, including the 745.6998715822702 W = 1 hp(I) factor.
• ISO 80000-4:2006, Quantities and units, Part 4: Mechanics. Defines the watt and provides the exact mechanical horsepower conversion factor.
• SAE J1349, Society of Automotive Engineers standard for certifying engine net power. Specifies the mechanical hp convention used in US automotive ratings.
• DIN 66036, German standard defining the metric horsepower (PS) as exactly 735.49875 W.
• BIPM SI Brochure (9th ed., 2019), The International System of Units. Defines the watt and the standard gravity used in the metric hp derivation.
• ASHRAE Handbook (Fundamentals), Authoritative source for the boiler horsepower (9 809.5 W) definition used in steam-plant and HVAC engineering.
• 1959 international yard and pound agreement, Signed by the US, UK, Canada, Australia, New Zealand, and South Africa. Defines the international yard (0.9144 m exactly) and the international pound (0.45359237 kg exactly), from which the 745.6998715822702 W = 1 hp(I) factor is derived.

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