Power CalculatorP = W ÷ t · P = Fv
Power is the rate of doing work: P = W ÷ t. For something moving at a steady speed against a force, P = Fv. One watt is one joule per second. Calculate power from work and time, from force and speed, or from how fast you climb a flight of stairs, in watts, kilowatts and horsepower, compared with everyday machines.
Formula verified against OpenStax — University Physics, Volume 1 · HyperPhysics, Georgia State University
By Manzoor Ahmad · Pharmacist & calculator developerReviewed & updated
Power inputs
Choose the pair of values you know.
Step-by-step working
What is power?
Power is the rate at which work is done or energy is transferred: how many joules per second. The SI unit is the watt.
Two people can do the same work lifting a box onto a shelf, but the one who does it faster delivers more power. Engines, motors, light bulbs and athletes are all rated by power.
Quick answers
How do you calculate power in physics?
Divide work by time: P = W ÷ t. Lifting 50 kg through 2 m (981 J) in 3 s takes 327 W.
How do I calculate power from force and velocity?
Multiply them: P = Fv. A cyclist pushing against 25 N of drag at 40 km/h (11.1 m/s) needs 278 W. The drag force calculator gives you the force.
How do I measure my own power output?
Time yourself climbing stairs of known height and use P = mgh ÷ t. Most people produce 300–700 W in a short sprint up a flight.
What is the difference between watts and watt-hours?
Watts measure power, the rate of energy use. Watt-hours measure energy. A 100 W bulb left on for 10 hours uses 1,000 Wh = 1 kWh.
Power formulas
1 W = 1 J/s. 1 mechanical horsepower = 745.7 W.
| Symbol | Meaning | SI unit |
|---|---|---|
| P | Power | W |
| W | Work or energy transferred | J |
| t | Time | s |
| F | Force | N |
| v | Speed along the force | m/s |
How to calculate power
- Work out the energy transferred or the work done, in joules.
- Measure the time it took, in seconds.
- Divide: P = W ÷ t gives watts.
- For steady motion, multiply the force by the speed instead.
Worked example: running up the stairs
A 70 kg person runs up a 3 m flight of stairs in 4 s.
P = W ÷ t = 2,059.4 ÷ 4 = 514.8 W ≈ 0.69 hp
That’s a short burst. A fit cyclist can hold about 250–300 W for an hour, and elite riders around 400 W.
Power of everyday things
Typical power ratings and outputs.
| Device or activity | Power |
|---|---|
| LED bulb | 8–12 W |
| Laptop | 45–90 W |
| Human, sustained effort | 75–150 W |
| Microwave | 800–1,200 W |
| Electric kettle | 2,000–3,000 W |
| Family car, cruising at 100 km/h | ≈ 15–25 kW |
| Wind turbine (onshore) | 2–5 MW |
Common mistakes to avoid
- Mixing up power and energy. kW is a rate; kWh is an amount of energy.
- Leaving time in minutes. Watts need seconds.
- Using P = Fv when the force isn’t along the motion. Use the component of force in the direction of travel.
Manzoor Ahmad
Pharmacist turned Web Developer & Calculator Engineer · 13+ years in web developmentI’m a pharmacist by training who turned into a full-time tech geek, and for 13+ years I’ve designed and coded web tools. My pharmacy degree included physical pharmacy and pharmaceutics — states of matter, heat, flow and reaction kinetics — so the physics behind these tools is familiar ground, not a new hobby.
I’m not a physicist, so I don’t ask you to take my word for it: every formula on this page is traced to an open university textbook or NIST reference data, and the calculator is tested against published worked examples before it goes live. Spot an error? Tell me and it will be fixed and dated on the next update.
Power Calculator: frequently asked questions
P = W ÷ t (work divided by time). For constant speed against a force, P = Fv.
The watt (W), equal to one joule per second. 1 kW = 1,000 W; 1 hp ≈ 745.7 W.
No, power is a scalar. It’s the dot product of force and velocity (P = F·v).
Energy is an amount (joules). Power is how fast that energy is used or delivered (joules per second).
One mechanical (imperial) horsepower is 745.7 W. One metric horsepower (PS) is 735.5 W.
References & sources
- OpenStax — University Physics, Volume 1 — Ch. 7.4, Power
- HyperPhysics, Georgia State University — power
- NIST Special Publication 811 — Guide for the Use of the SI — watt and horsepower definitions
Disclaimer: This calculator is an educational tool built on idealised textbook models. It is not a substitute for professional engineering or safety calculations.