Convert hp to W horsepower ↔ watts Free Online Secure
Type a value in horsepower and read it in watts at once. The factor is exact by definition, the arithmetic happens in your browser, and nothing is sent anywhere.
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Common values — Convert hp to W (horsepower ↔ watts)
| hp | W |
|---|---|
| 1 | 745.69987158227022 |
| 2 | 1491.39974316454044 |
| 3 | 2237.09961474681066 |
| 5 | 3728.4993579113511 |
| 10 | 7456.9987158227022 |
| 20 | 14913.9974316454044 |
| 50 | 37284.993579113511 |
| 100 | 74569.987158227022 |
Where these units come from
Horsepower is a sales figure that outlived its sales pitch. In the 1780s James Watt needed to tell mill owners what his steam engine was worth in the only terms they knew, so he measured what a horse could haul and rounded it to 33,000 foot-pounds a minute. The watt was named after him a century later and is the SI unit, defined with no animals involved: one joule every second. The two have never quite agreed, and Europe added a third — metric horsepower, the PS or CV that European engines are rated in, which is about 1.4% smaller than the mechanical horsepower used on this page.
Where you meet them
Watts and kilowatts measure everything that consumes or produces power: kettles, motors, heat pumps, solar panels, amplifiers, data centres. Horsepower survives almost entirely in engines — cars, boats, tractors and outboards — where it is less a measurement than a way of talking. Electric cars have put the two side by side on the same spec sheet, which is where most people now notice that one number has to be converted into the other.
Questions people ask
Which horsepower does this page use?
Mechanical horsepower, 550 foot-pounds per second, which works out to about 745.7 watts. European engines are usually rated in metric horsepower — written PS, CV, ch or pk — which is 735.49875 watts exactly, roughly 1.4% smaller. A car advertised as 150 PS is about 148 mechanical hp. The difference is small but it is not rounding, and it always flatters the metric figure.
Why are kettles in watts and car engines in horsepower?
Only habit. They are the same quantity, and a 2 kW kettle and a 2.7 hp kettle would be the same appliance. Electricity arrived already metric and was never named after a horse; engines arrived with Watt's own sales figure attached and kept it. The conversion is exact in the sense that the definition is fixed — nothing is measured at the point of conversion.
Is the answer exact?
Where both units are defined in terms of one another by whole numbers, yes — the page says so above the figure, and the only rounding is in how many digits are shown. Where the ratio has no finite decimal, the page shows twelve significant digits and says the relation is approximate rather than pretending otherwise.
Does anything I type leave my device?
No. The arithmetic runs in your browser; nothing is uploaded and there is no server to upload it to. Load the page once and it keeps working with the network switched off.
Horsepower to watts: how a mill horse became 745.7 watts
The horsepower was invented to sell steam engines. James Watt needed to tell brewers and mill owners how many horses a machine would replace, so he turned a horse's work into a number, and that number survives on motors, mowers and car brochures more than two centuries later. To convert hp to W, multiply by 745.7: 1 hp is 745.7 W, 5 hp is 3,728.5 W and 20 hp is 14,914 W. The calculator above multiplies as you type, and the sections that follow look at how the unit was defined, which versions exist, and where fractions of a horsepower still turn up on household motors.
Below come the rule with examples, short notes on each unit, a side-by-side look and a long series of historical and practical cases, ending with the slips people make most. Everything is computed locally in the page. Always check the nameplate of a motor before connecting it.
How to convert hp to W: multiply by 745.7
The rule
A mechanical horsepower equals 550 foot-pounds-force per second, which is 745.6999 watts. For a “convert hp to W” calculation, take the horsepower figure times 745.7: 0.5 hp gives 372.8 W, 3 hp gives 2,237 W and 12 hp gives 8,948 W. To go back, multiply watts by 0.00134102.
Worked examples
The 1903 Wright Flyer engine, around 12 hp, produced about 8,950 W. A Model T Ford of 20 hp had 14,914 W. A garage door opener of 1/2 hp has 373 W, and a furnace blower of 1/3 hp 249 W.
Other horsepowers
The metric horsepower, PS, is 735.5 W. The electrical horsepower on a motor plate is exactly 746 W. The boiler horsepower used for steam plant is 9,809.5 W, nearly thirteen times larger. When someone writes hp, it is the first of these in nearly every American context.
A quick sanity test
Expect the watt result to be over 700 times the hp figure. A smaller number than you began with means 0.00134 was used where 745.7 belonged.
Horsepower
A unit invented as a sales argument
Watt's partner Matthew Boulton sold engines to people who owned working horses, and a number let them compare. Watt is said to have calculated that a horse walking a mill wheel of 12-foot radius, 2.4 times a minute, against a pull of about 180 pounds, did 32,600 foot-pounds a minute, and rounded the figure to 33,000.
Where it appears
American motor and vehicle ratings, engine brochures and appliance labels.
The watt
The unit named after Watt
It is ironic that the SI unit of power bears the name of the man who made the horsepower famous. The watt is one joule per second. The unit was adopted at the end of the nineteenth century and became part of the SI in 1960.
Where it appears
Electrical ratings, bills and the specifications of most of the world's appliances.
Horsepower against watts
Counted in watts, a motor looks more than 700 times bigger than in hp: a third of a horsepower turns into 249 W and four horsepower into 2,983 W. Both describe identical power, and only the size of the unit changes.
Nothing is lost in the change. People most often trip over shaft power versus the power drawn from the supply.
Where the conversion shows up
Household motors
Fractional-horsepower motors run American households: a ceiling fan at 1/10 hp is 75 W, a refrigerator compressor at 1/6 hp is 124 W, a sump pump at 1/3 hp is 249 W, and a garbage disposal at 3/4 hp is 559 W. A “convert hp to W” calculation lets a reader outside America see how those compare with the watts on their own appliances.
Shaft power and supply power
A 1 hp motor delivers 746 W at the shaft, but at 80 per cent efficiency it draws 932 W from the supply. Small motors are less efficient, 60 per cent for 1/3 hp, which then draws 414 W. Bills follow the supply figure.
Operating cost
Running a 1 hp motor for an hour uses about 0.93 kWh. A pool pump of 1.5 hp, 1,119 W at the shaft and around 1.4 kW from the supply, running for 8 hours a day uses about 11 kWh daily, over 330 kWh a month.
The mill-horse story
The popular account has Watt measuring a horse hauling around a mill wheel. With a wheel of 12 feet in radius turned 2.4 times a minute and a pull of 180 pounds, the horse covers 181 feet a minute and does 32,600 foot-pounds in that time. Watt rounded it up to 33,000, and some tellers add that he also allowed a margin so that buyers would not be disappointed.
Hydraulic and drawbar horsepower
Water horsepower is flow times head times density, and drawbar horsepower is the pull at the hitch times speed. A pump lifting 20 litres a second by 30 metres has 5.9 kW, 7.9 hp of water power. A tractor pulling 30 kN at 7 km/h has 58 kW, 78 hp of drawbar power.
Household motor sizes
Typical sizes are 1/20, 1/12, 1/8, 1/6, 1/4, 1/3, 1/2, 3/4 and 1 hp, which are 37, 62, 93, 124, 186, 249, 373, 559 and 746 W. A pool pump of 1.5 hp is 1,119 W and a table saw of 1.75 hp 1,305 W. These sizes come from the days when motors were made in a few frame sizes.
American motors on a European supply
A 1 hp motor built for 60 Hz runs at about 1,725 rpm there and 1,440 rpm at 50 Hz, delivering roughly 620 W instead of 746 W. It also draws about 8 A at 120 V and half of that at 230 V. A “convert hp to W” calculation of the plate figure gives only the nominal power, not what the motor delivers on the wrong supply.
Starting current
An induction motor takes four to seven times its running current for a fraction of a second. A 2 hp compressor, 1,491 W at the shaft, draws about 1,900 W in steady running and several kilowatts at start. A generator feeding such loads needs a margin well above the nominal power.
Air compressors
A shop compressor of 2 hp has 1,491 W at the shaft. Compressors sold with "peak hp" claims of 5 or 6 hp are limited by the socket to about 1,800 W, 2.4 hp. The real figure is the continuous rating, which is often half of the headline.
Fans and blowers
A furnace blower of 1/3 hp is 249 W, one of 3/4 hp is 559 W. At an efficiency of 50 to 60 per cent, the 1/3 hp blower draws 400 to 500 W while running. Variable-speed models cut this by half or more at low speed, because fan power falls with the cube of speed.
Sump and utility pumps
A sump pump of 1/3 hp is 249 W and lifts about 7,000 litres an hour through 4 metres. A utility pump of 1/2 hp is 373 W. They run only when water rises, so the annual energy is small even though the instant power is a few hundred watts.
Lawn and garden
A small electric mower of 1.5 hp equals 1,119 W and a chipper of 2.5 hp 1,864 W. Electric mowers are usually sold by watts or volts, so a horsepower figure like this is mostly seen on imported machines.
A worked example
You import a workshop dust extractor marked 2 hp and 120 V. At the shaft it has 1,491 W. At 75 per cent efficiency it draws 1,988 W, or 16.6 A at 120 V and 8.6 A at 230 V. On a 10 A European circuit it would run, though the start-up surge may trip a fast breaker, so a slow-blow type is better.
The mental shortcut
To convert hp to W in your head, take three quarters of a kilowatt for each horsepower. For 5 hp: 5 × 750 = 3,750 W, against the exact 3,728.5. The error is 0.6 per cent. Take 0.6 per cent off if you need a closer figure.
How strong is a real horse?
A draught horse can sustain about one horsepower for hours, around 750 W. For a few seconds it can reach 10 to 15 hp, 7.5 to 11 kW. A human at 100 W would need seven or eight colleagues to match one mill horse, and even a sprinting cyclist at 2,000 W, 2.7 hp, lasts only seconds.
Spreadsheet formulas
In a spreadsheet, keep the horsepower in column A and use =A2*745.6999 in column B. For a fraction use =(1/3)*745.6999. The reverse is =B2/745.6999. A column that multiplies hp by 0.7457 gives kW and offers a quick cross-check.
Rounding on nameplates
Nameplates give fractions such as 1/3 hp, so 248.57 W is really "about 250 W". Round to the nearest 10 W. 1/3 hp is 249 W, not 248.5667. Extra decimals imply a precision that a motor of that kind does not have.
Landmarks to memorise
1/4 hp is 186 W. 1/2 hp is 373 W. 1 hp is 746 W. 2 hp is 1,491 W. 5 hp is 3,728 W. 10 hp is 7,457 W. 20 hp is 14,914 W. With these landmarks nearly every mental convert hp to W is possible.
Reference ladder
1/20 hp: 37.3 W. 1/5 hp: 149.1 W. 2/3 hp: 497.1 W. 1.25 hp: 932.1 W. 2.5 hp: 1,864 W. 7.5 hp: 5,593 W. 15 hp: 11,185 W. With a list of this kind any result can be checked quickly.
Checking by reversing
Test a “convert hp to W” calculation answer by multiplying it by 0.00134102: 497.1 W gives 0.667 hp, so the 2/3 hp you started from came through intact. A second sign of a good result is that the watt figure is more than 700 times the hp one.
Link to BTU per hour and foot-pounds
In one horsepower there are 745.7 W, 2,544 BTU/h, 642 kcal/h and 550 ft·lbf/s. A 1/2 hp motor therefore equals 1,272 BTU/h of mechanical power. The units differ and the rate does not, so a “convert hp to W” calculation only relabels it.
Two routes to one answer
Go straight to watts with 745.7, or travel via kilowatts: hp × 0.7457 × 1,000. A further path uses the definition, 550 ft·lbf/s × 1.3558. If the answers drift apart, a digit slipped somewhere.
Heating and cooling fans
A window air conditioner carries a compressor motor of about 3/4 to 1 hp, 559 to 746 W, but it removes 2.6 to 3.5 kW of heat because a heat pump moves heat rather than making it. A “convert hp to W” calculation on such a motor shows the electricity, never the cooling capacity.
Early engines
The engine of the Wright brothers, built in 1903, made around 12 hp, 8.9 kW. Benz's first Patent-Motorwagen of 1885 had about three quarters of a horsepower, near 550 W, and the Ford Model T of 1908 had 20 hp, 14.9 kW. A modern small car has 70 to 100 hp, three and a half to five times the Model T.
Ships of the steam age
The Titanic's engines were rated at about 46,000 horsepower in total, which is 34.3 MW. That is the output of roughly 340,000 people pedalling at 100 W, or about 2,300 times the Model T. Ships' ratings in those days were usually indicated horsepower, measured inside the cylinder.
Indicated, shaft and brake
The indicated horsepower, ihp, is the work done by steam on the piston, as measured by an indicator diagram. The brake horsepower, bhp, is what comes off the shaft after friction. The difference was typically 10 to 15 per cent, so 1,000 ihp gave about 870 bhp, which is 649 kW at the shaft.
Nominal horsepower
Before engines could be tested easily, builders used a nominal horsepower worked out from the cylinder size and a standard steam pressure. The figure was a trade classification, not a measurement, and a "10 nhp" engine often gave several times that at the shaft. When you convert hp to W on an old plate, treat the result as a label.
The boiler horsepower
A boiler horsepower is the evaporation of 34.5 pounds of water an hour from and at 212 °F, which carries 33,475 BTU/h or 9,809.5 W. A 50 bhp boiler therefore has a heat output of 490 kW, equal to 658 mechanical horsepower. Never read a boiler's bhp as engine power.
The electrical horsepower
The electrical industry rounded the unit to exactly 746 W, which differs from the mechanical one by 0.04 per cent. It means a motor of 10 hp is 7,460 W by this definition. One electrical horsepower would light 12 incandescent bulbs of 60 W, a comparison that suited the early days of lighting.
Metric horsepower
Continental Europe used a unit of 75 kilogram-force metres per second, which is 735.5 W. Called PS in German, CV in French and CP in Italian and Spanish, it was a tidy definition in the kilogram-force system. A car with 150 PS has 110.3 kW, against 111.9 kW for 150 hp.
More values for convert hp to W
A longer list of conversions
If the value you need is not in the first list, these further conversions may be close enough to read off directly:
- 0.1 hp = 74.57 W
- 0.5 hp = 372.85 W
- 3 hp = 2,237.1 W
- 4 hp = 2,982.8 W
- 7 hp = 5,219.9 W
- 15 hp = 11,185.5 W
- 20 hp = 14,914 W
- 75 hp = 55,927.5 W
- 200 hp = 149,140 W
- 500 hp = 372,850 W
- 2,000 hp = 1,491,400 W
- 10,000 hp = 7,456,999 W
Common slips with a “convert hp to W” calculation
Using the wrong direction
From hp to watts the number grows. If 1/2 hp became 0.0007, you used the factor for the other direction.
Shaft and supply
The 373 W of a 1/2 hp motor is at the shaft, while the supply gives 500 to 600 W. Bills and breakers depend on the supply figure.
Rounding fractions
1/3 hp is not 0.3 hp. Rounding to 0.3 gives 224 W in place of 249 W, an error of 10 per cent. Use the exact fraction, 0.3333.
Short answers
How do I convert hp to W?
Take the horsepower times 745.7.
What is 1 hp in watts?
It is 745.7 W.
What is 1/2 hp in watts?
It is 372.8 W.
What is 1,000 W in hp?
It is 1.341 hp.
Is my number sent anywhere?
No, everything is computed in the browser.
More pages and a wrap-up
Other power conversions are listed in Convert hp to kW, Convert km to mi, Convert mi to km, Convert °C to °F, Convert °F to °C, Convert m to dm, Convert dm to in, Convert Mach to km/h, Convert m/s to Mach. Going the other way is covered by Convert W to hp.
To sum up, a “convert hp to W” calculation needs one multiplication by 745.7 and a unit beside the answer. The figure is worked out on your own device, without registration or upload.