Electrical Power Calculator
Any two of power, voltage, current and resistance, with all four returned.
What is Electrical Power?
Electrical power is the rate at which a circuit transfers energy: P = VI, with the equivalent forms P = I²R and P = V²/R for when one of the other pairs is known. Appliance ratings, fuse sizes and heating calculations all come from it.
How it works
Four quantities are joined by three interchangeable forms of one relationship, so which arrangement applies depends on which *pair* you supplied — there are six possible pairs and every one of them is handled explicitly rather than guessed at. All four values come back labelled, so the page reads as a solved circuit rather than half a table, and the arrangement used is shown.
- 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
A 12 V supply drives 0.5 A. What is the power?
6 W. P = VI = 12 × 0.5, and the resistance comes out as 24 Ω from R = V ÷ I. Both appear when you load the example.
Which form of the power equation should I use?
Whichever matches the values you have. VI when you know voltage and current, I²R when you know current and resistance, V²/R when you know voltage and resistance. This tool picks it for you.
Why does a resistor get hot?
Because it dissipates electrical energy as heat at a rate of I²R. Doubling the current through the same resistor quadruples the heating, which is why fuses have current ratings.