Watts to Horsepower Converter

Convert between watts, mechanical horsepower (hp), and metric horsepower (PS) using exact ISO 80000-4 conversion factors. This free online calculator is fast, accurate, and works in any modern browser, no signup, no ads, no tracking.

Introduction

Power is the rate at which work is done or energy is transferred, measured in watts (W) in the SI system. But horsepower predates the SI system by more than a century and is still widely used in automotive, marine, and industrial contexts. The catch: there is more than one kind of horsepower, and they differ by about 1.4%. Mixing them up can lead to errors of a few percent when comparing engines, motors, or generators across regions.

This converter accepts an input in watts and instantly reports the equivalent in mechanical horsepower (hp), metric horsepower (PS), and kilowatts (kW). It uses the exact conversion factors from ISO 80000-4, the international standard for mechanical quantities and units. Whether you are sizing an electric motor, comparing a German car brochure to a US spec sheet, or just curious how many horses your power supply really is, this tool gives you the answer in a single click.

The default value of 1000 W (= 1 kW) is loaded on page load so you can see the conversions immediately, 1.3410 mechanical hp or 1.3596 metric PS. Change the input to any positive number and click Calculate to refresh.

How to Use This Tool

1. Enter the power in watts in the input box. Any non-negative number works; decimals are accepted.
2. Click Calculate.
3. Read the mechanical horsepower, metric horsepower (PS), and kilowatt values displayed below the button.
4. To convert in the reverse direction, multiply hp by 745.6998 (or PS by 735.49875), or just punch the result back into the input box divided by 1000 to get kW first, then by 1000 again to get W.

Negative inputs are rejected (you cannot have negative power in the steady-state sense, though power can flow in either direction, we treat magnitude here). Non-numeric input also produces no result.

The Formulas

Mechanical horsepower (hp): 1 hp = 745.6998715822702 W exactly, per ISO 80000-4. To convert watts to mechanical hp:

hp = W ÷ 745.6998715822702

Metric horsepower (PS): 1 PS = 735.49875 W exactly. The unit is also known as "Pferdestärke" (German), "CV" (French for "cheval-vapeur"), or "pk" (Dutch). To convert watts to metric hp:

PS = W ÷ 735.49875

Kilowatts: 1 kW = 1000 W by definition. To convert:

kW = W ÷ 1000

Reverse direction (hp/PS to watts): just multiply. 1 hp × 745.6998 = 745.6998 W. 1 PS × 735.49875 = 735.49875 W.

Cross-conversion between hp and PS:

PS = hp × (745.6998 ÷ 735.49875) = hp × 1.01387
hp = PS × (735.49875 ÷ 745.6998) = PS × 0.98632

So a 200 PS engine is about 197.31 mechanical hp, and a 150 hp engine is about 152.08 PS.

Worked Examples

Example 1, 1 kW motor. A common small electric motor is rated at 1000 W. In mechanical hp: 1000 ÷ 745.6998 = 1.3410 hp. In metric PS: 1000 ÷ 735.49875 = 1.3596 PS. In kW: 1.000 kW (no surprise). This is a useful sanity check: any time you see a 1 kW rating, the equivalent horsepower is roughly 1.34 (US) or 1.36 (European).

Example 2, 150 hp engine. A US-spec 150 hp engine produces 150 × 745.6998 = 111,854.98 W ≈ 111.85 kW. In metric PS: 111,854.98 ÷ 735.49875 = 152.08 PS. This is the kind of conversion that shows up in cross-border car comparisons, the same engine is rated higher in PS than in hp.

Example 3, 200 PS car. A European-spec 200 PS car produces 200 × 735.49875 = 147,099.75 W ≈ 147.10 kW. In mechanical hp: 147,099.75 ÷ 745.6998 = 197.31 hp. Notice the ~1.4% gap, never assume 1 PS = 1 hp.

Example 4, Round-trip check. Starting with 745.6998 W (exactly one mechanical hp, truncated to 4 decimals), the tool reports 1.0000 hp (to 4 decimal places). This confirms the inverse is consistent: hp × 745.6998 = W.

Example 5, Large industrial motor. A 500 kW industrial pump motor: 500,000 W ÷ 745.6998 = 670.54 hp (mechanical) or 500,000 ÷ 735.49875 = 679.81 PS. Industrial spec sheets often quote kW for the electrical input and hp or PS for the mechanical output, so a converter like this is exactly the right tool.

Example 6, Small appliance. A 60 W incandescent light bulb: 60 ÷ 745.6998 = 0.0805 hp. Yes, even a light bulb has a measurable horsepower equivalent, though you would not use that bulb to pull a plow.

Where It Shows Up

• Car engines, German and other European manufacturers usually rate in PS or kW; US-market brochures use hp. When comparing, you must convert.
• Electric motors, typically rated in kW or hp depending on the regional market. Industrial motors often list both.
• Boats and small engines, outboard motors are usually rated in hp, even in Europe. A 9.9 hp outboard is a common class.
• 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.
• Industrial machinery, older European equipment is often quoted in PS. Newer equipment uses kW per EU efficiency regulations.
• Air conditioners and HVAC, rated in BTU/h, tons of refrigeration, or sometimes kW. The watts-to-hp conversion is less common here but useful for comparing compressor power.
• Power tools, workshop tools are rated in watts (EU) or hp (US). A "1.5 hp" table saw might be 1100 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 e-bikes and EVs).

Common Mistakes

• Confusing mechanical and metric horsepower. 1 PS ≠ 1 hp. Specifically, 1 PS = 0.98632 hp. They differ by ~1.4%, which is small but real in technical specifications.
• Confusing horsepower with brake horsepower (bhp). bhp is power measured at the crank 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.
• Equating horsepower to watts without the 745.7 factor. The common rough estimate "1 hp ≈ 750 W" is accurate to within 0.6% but is not exact. For precise engineering work, always use 745.6998715822702.
• Confusing electrical and mechanical horsepower. The electrical horsepower (746 W exactly) is sometimes used in the US for electric motor ratings, but is essentially identical to mechanical hp at 4 significant figures. Most modern sources use the 745.6998 figure.
• 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) is torque × angular velocity. The two are not interchangeable. A high-torque, low-rpm engine has the same hp as a low-torque, high-rpm engine of equal power.
• Ignoring units in the input box. Make sure you are typing watts, not kilowatts, unless you want the result to be off by a factor of 1000.

Frequently Asked Questions

Q? What is the difference between hp and PS?

Mechanical horsepower (hp) is defined as exactly 745.6998715822702 watts. Metric horsepower (PS, also written "CV" or "pk") is defined as exactly 735.49875 watts. The two differ by about 1.4%. The same engine rated in both units will show a higher number in PS than in hp, a 200 PS engine is roughly 197 hp. The difference comes from historical definitions: mechanical hp traces back to James Watt's late-18th-century observations of draft horses, while metric horsepower was standardised for European machinery in the 19th and 20th centuries.

Q? Why are there different horsepower units?

James Watt defined the original 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 definition (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 but has largely been superseded by the same 745.6998 figure.

Q? What is brake horsepower (bhp)?

Brake horsepower is the power measured at the engine's output shaft, using a dynamometer (a "brake" that absorbs the power). It excludes drivetrain losses from the gearbox, differential, and accessories. Wheel horsepower (whp) is what actually reaches the road, measured at the hubs. The typical loss between bhp and whp in a modern car is 15 to 25%, depending on drivetrain layout, transmission type, and drivetrain accessories. Always check which figure you are looking at.

Q? How many watts is 1 horsepower?

1 mechanical horsepower = exactly 745.6998715822702 watts. 1 metric horsepower = exactly 735.49875 watts. The "round" figure often quoted is 746 W, which is the electrical horsepower used historically in the US for electric motors.

Q? How many horsepower is 1 kilowatt?

1 kW = 1000 W = 1.34102 mechanical horsepower = 1.35962 metric horsepower (PS). This is the inverse of the above. A quick mental shortcut: every kW is roughly 1.34 hp, and every kW is roughly 1.36 PS.

Q? How do I convert horsepower back to watts?

Multiply: 1 hp × 745.6998 = 745.6998 W; 1 PS × 735.49875 = 735.49875 W. If you have an engine rated at 150 hp and want to know its power in watts: 150 × 745.6998 = 111,854.98 W ≈ 111.85 kW. This conversion is exact up to the precision of the original rating.

Q? 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. 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.

Q? What about imperial horsepower vs metric horsepower?

There is no separate "imperial horsepower" in common use, the term "horsepower" almost always means mechanical horsepower (745.6998 W). The metric equivalent is PS (735.49875 W). The UK switched to metric units in the 20th century but retained horsepower in automotive contexts for cultural reasons.

Q? 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 horse is closer to 0.5 to 0.7 hp. Watt's figure was inflated for marketing reasons. 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 do 0.6 hp+ for an hour).

Q? Why do electric cars sometimes quote horsepower equivalent?

Because automotive tradition is rooted in hp, electric vehicles are often marketed with "equivalent horsepower" figures so that buyers can compare them directly to internal-combustion vehicles. A 300 kW electric motor is roughly 402 hp (mechanical) or 408 PS. Some manufacturers quote the motor's mechanical output directly; others quote an "equivalent" figure after drivetrain losses.

References

• ISO 80000-4, Quantities and units, Part 4: Mechanics. Defines the watt and provides the exact horsepower conversion factor (745.6998715822702 W = 1 hp).
• IEC 60050, International Electrotechnical Vocabulary. Defines power, energy, and related quantities for electrical engineering contexts.
• NIST Special Publication 811, Guide for the Use of the International System of Units (SI). Authoritative source for SI unit usage and conversion factors.
• James Watt (1782), on the steam engine, historical reference for the original horsepower definition, based on observations of draft horses.
• DIN 66036, German standard defining the metric horsepower (PS) as exactly 735.49875 W.

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