Quick answer
Electrical power P = V × I, and the calculator solves for any one quantity when the other two are given. 120 V × 10 A is 1,200 W.
Enter any two of voltage, current and power, leaving one blank. The calculator returns the missing value. Use this wattage calculator to check your own figures.
Related tools: Ohm's Law Calculator: Solve voltage, current or resistance.
How to use the Wattage Calculator
- Optionally, enter power (P) (W).
- Optionally, enter voltage (V) (V).
- Optionally, enter current (I) (A).
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| Power (P) (optional) | A number, in W. Leave blank to solve for this value | — |
| Voltage (V) (optional) | A number, in V. Leave blank to solve for this value | — |
| Current (I) (optional) | A number, in A. Leave blank to solve for this value | — |
Formula and method
Formula
P = V × I
IEEE-754 double precision; shown to at most 6 decimal places.
Fill in any two values and leave the third blank; the blank one is calculated. Use consistent units.
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: Voltage (V) = 120; Current (I) = 10.
| Result | Value |
|---|---|
| Power | 1,200 W |
Example 2
Inputs: Power (P) = 60; Voltage (V) = 12.
| Result | Value |
|---|---|
| Current | 5 A |
The first line is the headline result (power). IEEE-754 double precision; shown to at most 6 decimal places.
The three forms
P = V × I, I = P ÷ V and V = P ÷ I. A 1,200 W heater on 120 V draws 10 A, which a 15 A circuit can carry. Combining with Ohm's law gives P = I²R and P = V²/R.
Circuit limits
Keep continuous loads under 80% of a breaker's rating: a 15 A circuit is limited to 12 A (1,440 W at 120 V). Exactly one field must be blank, and dividing by zero is rejected.
How the result changes with voltage
Holding every other input at the first worked example, this table changes only voltage. It shows how sensitive the answer is to that one value; each row was produced by running this calculator.
| Voltage | Power |
|---|---|
| 60 | 600 W |
| 120 | 1,200 W |
| 240 | 2,400 W |
| 480 | 4,800 W |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- Power (P): entering "1" shows "Leave exactly one field blank and fill in the other two." beside that field and no result.
Choosing between the Wattage Calculator and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [Ohm's Law Calculator](/engineering/ohm-s-law-calculator/) | Current | |
| [Voltage Drop Calculator](/engineering/voltage-drop-calculator/) | current, one-way length and wire gauge | Voltage drop |
| [Wire Size Calculator](/engineering/wire-size/) | current, one-way length and supply voltage | Smallest AWG for the voltage drop |
| [Three-Phase Power Calculator](/engineering/three-phase-power-calculator/) | line-to-line voltage, line current and power factor | Real power |
| [Microwave Wattage Calculator](/engineering/microwave-wattage/) | minutes, wattage on the package and your microwave wattage | Adjusted time |
Background: electrical circuits
Ohm's law links voltage, current and resistance: V = I × R, so I = V ÷ R and R = V ÷ I. Power is P = V × I, which also equals I² × R and V² ÷ R.
These relations hold for ohmic components, where resistance is constant. Diodes and transistors are non-linear, and in AC circuits with reactive parts impedance (Z) takes the place of resistance. Check component ratings and local electrical code before relying on any calculation.
Assumptions and limitations
- Exactly one field must be left blank
- Division by zero is rejected
Use results as engineering estimates and check them against the governing standards, safety factors and a qualified engineer's judgement before relying on them.
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for power (P) to confirm this.
Frequently asked questions
How many watts is a 15 A circuit?
1,800 W at 120 V, or 1,440 W continuous.
What is a watt?
One joule per second.
Does it work for AC?
Yes for resistive loads; reactive loads need power factor.
What does the Wattage Calculator calculate?
Solve power, voltage or current for a DC or resistive load. It returns Power.
What formula does the Wattage Calculator use?
P = V × I. Fill in any two values and leave the third blank; the blank one is calculated.
What do I need to enter in the wattage Calculator?
You enter . Optional: Power (P) (W), Voltage (V) (V) and Current (I) (A).
How is the result rounded or limited?
IEEE-754 double precision; shown to at most 6 decimal places. Exactly one field must be left blank.
What are the limits of this calculator?
Exactly one field must be left blank. Division by zero is rejected.
What is an example of a wattage result?
With voltage (V) 120 and current (I) 10, the power is 1,200 W.
Can I rely on this calculator for a decision?
Use results as engineering estimates and check them against the governing standards, safety factors and a qualified engineer's judgement before relying on them. For this calculator in particular: Exactly one field must be left blank.
Related calculators
- Ohm's Law Calculator — Solve voltage, current or resistance.
- Three-Phase Power Calculator — Real, apparent and reactive power of a balanced three-phase load.
- Parallel Resistor Calculator — Equivalent resistance of resistors in parallel and in series.
- Watt-Hours Calculator — Energy in watt-hours from power and time, or from amp-hours and voltage.
- LC Resonant Frequency Calculator — Resonant frequency of an LC circuit.
Sources
- OpenStax, University Physics Volume 2 (Rice University): electricity, circuits and thermodynamics
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.