Watts to Horsepower Converter

1 horsepower is 745.7 W (hp) or 735.5 W (PS)

Horsepower measures power, the amount of work done each second, and the watt (W) is the SI unit for the same thing. The trouble is that "one horsepower" is not a single number. The mechanical horsepower used in the United States and the United Kingdom (hp, defined as 550 ft·lbf/s) is about 745.7 W. The metric horsepower (PS) is 735.49875 W, and the electric horsepower used for motors is exactly 746 W. That is why 100 kW comes out as 134.10 hp but 135.96 PS.

This converter fills eight fields at once: kilowatts, watts, three kinds of horsepower, kilocalories per hour, Btu per hour and foot-pounds-force per second. Type a number into any field and the rest follow. It also shows how much weight that power can raise by 1 ft each second, and which motor nameplate rating sits closest to your number.

When you need it

• Reading a motor or pump label: catalogs list pairs such as 5 HP and 3.7 kW, or 7.5 kW and 10 HP. Converting 7.5 kW directly gives 10.06 hp, which the catalog rounds to a 10 HP rating.
• Comparing cars from different markets: spec sheets may give engine output in PS or kW, while US and UK sources use hp. The same 100 kW is 135.96 PS and 134.10 hp, so the PS figure looks about 1.4% larger.
• Checking an e-bike motor: the UK limit is a continuous rated power of 250 W, which is only 0.34 hp.
• Turning a household wattage into hp: a 746 W appliance is 1.00 hp, a 750 W one is 1.01 hp and a 1,500 W one is 2.01 hp.
• Heating and cooling figures: equipment data often uses Btu/h. 1 hp is 2,544.43 Btu/h and 1 kW is 3,412.14 Btu/h.

How to use it

1. Put the value you know in the matching field. The two large fields at the top are kilowatts and horsepower (hp). The six smaller ones below are watts, metric PS, electric hp(E), kcal/h, Btu/h and ft·lbf/s. You can type 1,234.5 with a thousands comma, or a fraction such as 1/2 or 1 1/2.
2. The field you typed in becomes the reference (thick border) and the other seven change. When a shown value is rounded, the "=" between the two large fields turns into "≈".
3. Under "Decimals", pick 0, 1, 2, 4 or 6 places. The sentence under the fields shows six decimal places, and the formula line below it shows which factor was used.
4. "See it as a lift" stacks 550 lb weights and raises them 1 ft in one second. The number of weights is the hp value.
5. The "Compare with motor nameplate ratings" strip points to the nearest North American HP rating and the nearest IEC kW rating. Click a rating, or focus the strip and press ← → (HP ratings) or ↑ ↓ (IEC ratings), to load that rating into the fields.
6. "Common values" puts a typical number in the fields. "Copy results" copies all eight values, and "Copy link" stores your input in the address.

How the conversion works

Every unit is first turned into watts, then divided by the number of watts in the target unit. The value of each unit in watts is in this table.

1. kW to hp: kW × 1,000 ÷ 745.69987 (100 kW is 134.10 hp)
2. kW to PS: kW × 1,000 ÷ 735.49875 (100 kW is 135.96 PS)
3. hp to kW: hp × 0.74569987 (300 hp is 223.71 kW)
4. PS to kW: PS × 0.73549875 (150 PS is 110.32 kW)
5. hp and PS: 1 hp is 1.01387 PS, and 1 PS is 0.9863 hp. A hp is about 1.39% bigger, so the same power written in hp gives a slightly smaller number.

The factors are kept as exact fractions and rounded only once, at the end. That is why 1 PS is 735.49875 W here, while NIST rounds it to 735.4988 W.

kW, hp and PS conversion table

There is more than one horsepower

The NIST conversion tables list six different horsepower values. These four are the ones you meet most often.

NIST also lists a U.K. horsepower (745.70 W) and a water horsepower (746.043 W). Boiler horsepower only shares the name: at 9,809.50 W it is about 13.2 times a mechanical horsepower and describes a steam boiler, so do not mix it with engine or motor ratings.

Britannica puts the metric horsepower at 4,500 kilogram-metres per minute, which equals 0.9863 of a mechanical horsepower. Its figure for the electrical equivalent of one horsepower is 746 W, and for heat it gives 2,545 Btu per hour. This tool computes 2,544.43 Btu/h for exactly 745.69987 W, which is consistent with that rounded figure.

Motor nameplates: HP and kW

Motor catalogs pair an HP rating with a kW rating. The WEG W22 catalog lists a 5 HP motor as 3.7 kW, but the standard IEC output list in the ABB Motor Guide (Table 3.1, based on IEC 60072-1) has no 3.7 kW step. It has 4 kW. A nameplate horsepower is a rounded rating name, so it differs slightly from a direct conversion of the hp number.

The kW or horsepower on a nameplate is the mechanical output at the shaft, at the rated voltage, current and frequency. The motor draws more than that from the supply, by the amount of its losses, so a motor rating converted to hp is not its electrical consumption.

550 lb raised 1 ft each second: how big is one horsepower?

A mechanical horsepower is the power needed to raise 550 lb by 1 ft every second. Dividing a power in ft·lbf/s by 550 gives hp, so the number of weights in the picture above is the hp value. 1 kW can raise about 737.56 lb by 1 ft each second, and 100 kW about 73,756 lb. The picture ignores friction and losses and assumes all of the power goes into lifting.

Britannica says James Watt arrived at the unit in the late 18th century after experiments with strong dray horses, and that one horsepower is 33,000 foot-pounds of work per minute. That is about 50 percent more than an average horse can sustain over a working day. For a few seconds, though, a horse can do far more. A 1993 Nature article by Stevenson and Wassersug used records from a 1925 horse pulling contest at the Iowa State Fair and reported peak mechanical power of 12 to 14.9 hp for a horse. Horsepower is the name of a defined rate of work, not a measurement of an actual horse.

Where the units show up in rules

• UK electric bikes (gov.uk): the motor "must have a 'continuous rated power' output of no more than 250 watts". 250 W is 0.34 hp, or 0.34 PS.
• hp versus PS: NIST gives 745.6999 W for the 550 ft·lbf/s horsepower and 735.4988 W for the metric one. In the US and UK, hp means the first. The two numbers are close but not equal, so check which unit a source uses before comparing.

Common mistakes

• Mixing hp and PS: 200 PS is 147.10 kW but 200 hp is 149.14 kW, a gap of more than 2 kW. Convert both to the same unit before comparing.
• Reading input power as horsepower: the watts on a vacuum cleaner or heater are what it draws from the wall. The kW or HP on a motor nameplate is the power at the shaft. A 1,000 W appliance equals 1.34 hp on paper, but it does not deliver that much mechanical work.
• Treating kVA as kW: generator and UPS ratings in kVA are apparent power. Multiply by the power factor to get kW before converting to horsepower.
• Re-converting a rounded nameplate: converting the 3.7 kW on a 5 HP motor gives 4.96 hp or 5.03 PS. HP and kW on a nameplate are paired rating names, not exact equivalents.
• Confusing heat output with power: Btu/h is a rate of heat. 1 hp is 2,544.43 Btu/h, but a heater or cooler rated in Btu/h is not an engine rating.

Limits

• The tool only changes units using their definitions. The real output of an engine or motor depends on how it is measured (speed, temperature, accessories) and on efficiency, and none of that is reflected here.
• "See it as a lift" assumes an ideal lift where all of the power raises the load. A real hoist or crane loses power to friction and lifts less.
• The nameplate strip shows only the HP and kW pairs from the WEG catalog (1 to 600 HP) and the IEC output list from the ABB guide (0.06 to 132 kW). Ratings and markings differ by maker and market, so check the real nameplate.
• Each field accepts a power from 0 up to 1 GW (1,000,000 kW).
• Your input is calculated in this browser and is not sent anywhere. "Copy link" puts the input into the address.

Basis: the factors are definitions from NIST SP 811 Appendix B.8, so they do not change from year to year. The UK e-bike rule, the catalogs and the other sources were checked against the originals on 21 October 2026.

Sources

• 01NIST SP 811 Appendix B.8: Factors for units listed alphabeticallyhorsepower: 550 ft·lbf/s 745.6999 W, metric 735.4988 W, electric 746 W, boiler 9,809.50 W, U.K. 745.70 W, water 746.043 W · Btu_IT/h 0.2930711 W
• 02Britannica: horsepower1 hp = 33,000 ft·lb/min, set by James Watt in the late 18th century, about 50% more than a horse sustains over a working day · metric horsepower = 4,500 kg·m/min = 0.9863 hp · 746 W, 2,545 Btu/h
• 03GOV.UK: Electric bike rulesMotor must have a "continuous rated power" output of no more than 250 watts
• 04WEG: W22 Three-Phase Electric Motor catalogHP–kW pairs: 1–0.75, 5–3.7, 7.5–5.5, 10–7.5 … 600–440
• 05ABB: Motor Guide, Table 3.1 (EN 50347 and IEC 60072-1)Standard outputs 0.06, 0.09 … 3, 4, 5.5 … 110, 132 kW
• 06CED Engineering: Understanding Motor Nameplate Information, NEMA vs. IEC StandardsNameplate kW and horsepower are the shaft output at rated voltage, current and frequency
• 07Stevenson & Wassersug, Horsepower from a horse, Nature 364:195 (1993)1925 Iowa State Fair horse pulling contest: peak mechanical power of a horse 12 to 14.9 hp

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