What this calculator does
Convert power from one unit to another, such as horsepower to kilowatts or BTU per hour to watts. Power is how fast energy is used or delivered. Its standard metric (SI) unit is the watt, which is one joule of energy per second. Different trades have kept their own units. Car engines are rated in horsepower, and many European spec sheets use metric horsepower (PS). In North America, air conditioners and furnaces are sized in BTU per hour. Nutrition and exercise science sometimes use kilocalories per hour.
This converter stores each unit as its size in watts, so it can convert between any two units directly. Enter a value and choose the two units. You'll get the result, the working through watts, and a table showing the same power in all eight units.
Where it's used
Why this math matters outside the classroom:
- Car engines. Car buyers comparing specs from different countries read a 200 hp engine as about 149 kW.
- Air conditioners. A 12,000 BTU/h air conditioner moves about 3.5 kW of heat.
- Pump motors. A 1/2 hp pool pump motor is rated at about 373 W.
- Metric horsepower. European car specs in metric horsepower list 150 PS, which is about 148 mechanical hp.
- Cycling power. A cyclist holding 250 W for an hour does about 215 kcal of mechanical work.
Grade level and standards: Grades 6–7; Common Core State Standards for Mathematics 6.RP.A.3.d. © Copyright 2010. National Governors Association Center for Best Practices and Council of Chief State School Officers. All rights reserved.
How to use it
- Enter the power. Zero and decimals are allowed.
- Choose the unit you're converting from and the unit you want.
- Read the result, or scan the table for every unit. The working shows each unit's size in watts.
What the inputs mean
- Power
- The power to convert, in the From unit.
- From
- Mechanical horsepower (hp) and metric horsepower (PS) are different units. Check which one a spec sheet uses.
- To
- The unit you want the answer in. BTU/h is a heating and cooling rate, not an amount of energy.
Formula and method
result = value × (from-unit in W) ÷ (to-unit in W) 1 hp (mechanical) = 550 ft·lbf/s ≈ 745.69987 W 1 PS (metric hp) = 75 kgf·m/s = 735.49875 W 1 BTU/h = 1,055.05585262 J ÷ 3,600 s ≈ 0.29307107 W 1 kcal/h = 4,184 J ÷ 3,600 s ≈ 1.1622222 W; 1 ft·lbf/s ≈ 1.3558179 W
In the formulas, W is watts, J is joules and s is seconds. ft·lbf/s is foot-pounds per second: lifting one pound one foot each second. kgf·m/s is the metric version, lifting one kilogram one meter each second.
Mechanical horsepower was defined as 550 foot-pounds of work per second. The foot and the pound-force both have exact metric values, so it comes to about 745.7 watts. Metric horsepower lifts 75 kilograms-force through one meter each second. That's exactly 735.49875 watts, about 1.4% less.
BTU per hour uses the International Table British thermal unit. Kilocalories per hour use the thermochemical kilocalorie, which is 4,184 joules. The converter multiplies by the from-unit's size in watts, then divides by the to-unit's size.
Worked example
An engine produces 100 mechanical horsepower. What is that in kilowatts?
- 100 hp × 745.6998715823 W = 74,569.987158227 W.
- 74,569.987158227 W ÷ 1,000 = 74.5699871582 kW.
Result: 100 hp is about 74.57 kW.
Understanding the result
Quick rules to remember: 1 kW is about 1.34 hp, 1 hp is about 0.746 kW, and 1 kW is about 3,412 BTU/h. A 12,000 BTU/h air conditioner (a "one-ton" unit) moves heat at about 3.5 kW.
Cooling capacity in BTU/h is how fast heat is removed, not the electricity the unit uses. A unit's electrical input is usually much smaller than its rated capacity.
Assumptions and limitations
- Horsepower means mechanical (imperial) horsepower, unless you choose metric (PS).
- BTU is the International Table BTU. The kilocalorie is the thermochemical one (4,184 J).
- Electrical horsepower (exactly 746 W) and boiler horsepower aren't included.
- It converts rates of power only. For amounts of energy, such as kWh, use an energy converter.
- Engine ratings depend on the test standard used, which this tool can't account for.
Frequently asked questions
What is the difference between hp and PS?
Both are called horsepower. Mechanical horsepower (hp) is about 745.7 W and comes from the imperial system. Metric horsepower (PS, CV or ch) is 735.5 W. So a car rated at 150 PS has about 148 hp.
Why is BTU per hour used for air conditioners?
In North America, heating and cooling equipment is rated by how much heat it moves each hour, measured in British thermal units. One BTU per hour is only about 0.29 W, so typical ratings run into the thousands.
Are watts and watt-hours the same?
No. A watt is a rate: one joule per second. A watt-hour is an amount of energy: one watt kept up for an hour, or 3,600 joules. Your electricity bill counts kilowatt-hours of energy, not kilowatts.
How this calculator is checked
Method: Definitions of mechanical horsepower (550 ft·lbf/s) and metric horsepower (75 kgf·m/s); International Table BTU (1,055.05585262 J); thermochemical calorie (4.184 J). The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.
Embed this calculator
Put this Power Converter on your own website or blog for free. Paste this code where you want it to appear: