Power Calculator
Power as work over time, or as force times velocity at constant speed.
What is Power?
Power is the rate of doing work, measured in watts: one watt is one joule per second. The same amount of work done twice as fast needs twice the power, which is why a bigger engine does not produce more energy — it produces energy more quickly.
How it works
Two relationships are offered because they answer different questions. P = W ÷ t is the average power over an interval, whatever happened in between. P = F × v only holds at constant velocity, where the driving force exactly balances the resistance — it gives the power needed to keep a car at a steady speed on a hill, for instance. Pick the arrangement that matches the problem and leave one value blank.
- Enter what you know. Every field recalculates as you type — there is no submit button. Leave the quantity you want blank and the page solves for it.
- Read the answer and the working. The result appears above the fold; underneath it is the formula with your numbers substituted, so the arithmetic can be checked rather than trusted.
- Check the units and the size. Units are shown on every value and selectable per field, so a result in kJ, kPa or km/h is converted rather than assumed. Sanity-check the magnitude against a known landmark such as 1 g/cm³ for water.
Examples
A textbook problem with a known answer
Every tool here is checked against standard example problems — the molar mass of C6H12O6 is 180.16 g/mol, and free fall from 20 m takes about 2.02 s at g = 9.8 m/s².
A result you want to reason backwards from
Leave the quantity you already know blank instead: given the final answer, most tools recover the starting value — the mass needed for a target concentration, or the height a fall must have had.
Common mistakes
Forgetting that a formula is only as good as its subscripts
H2O and H2O2 are different compounds. The formula field shows its parse — element by element — so a mis-typed subscript is visible before it reaches the arithmetic.
Mixing units in the middle of a calculation
The most common source of a wrong answer is a value in grams next to a formula expecting kilograms, or Celsius next to a gas constant expecting kelvin. This category converts on the way in and labels what it used.
Frequently asked questions
What is the difference between power and energy?
Energy is the total amount, measured in joules or kilowatt-hours. Power is how fast it is used or produced, measured in watts. A 100 W bulb uses 100 J every second, so a kilowatt-hour is one kilowatt sustained for one hour.
When do I use P = Fv instead of P = W/t?
When the force and a steady speed are what you know — a vehicle pulling a load at constant velocity, for instance. The two agree: Fv is just W/t with the distance per second folded in.
How many watts is one horsepower?
About 745.7 W for mechanical horsepower and 735.5 W for the metric variant. Both are selectable here, and the Power converter in Units & Numbers converts between them and kilowatts.