What formulas does this calculator use?

It uses the four Ohm's law and power relationships: P = V × I, V = I × R, P = I² × R, and P = V² / R. Given any two of the four quantities (P, V, I, R), the other two can be derived.

What are the units for each quantity?

Power (P) is in watts (W), voltage (V) is in volts (V), current (I) is in amperes (A), and resistance (R) is in ohms (Ω). The calculator also supports milliamps, kilowatts, millivolts, and kilohms via the unit selectors.

Can I calculate resistance without knowing power?

Yes. If you know voltage and current, resistance is R = V / I. Enter those two values and the calculator will show resistance along with power.

Why do I need to enter exactly two values?

With fewer than two knowns the system of equations is under-determined — infinitely many solutions exist. With all four entered, the values may be inconsistent; the calculator uses only the two you mark as known.

Does this apply to AC or DC circuits?

For purely resistive loads the formulas are identical in both AC and DC. For reactive loads (inductors, capacitors) you would need impedance and power-factor calculations, which are beyond this tool's scope.

How accurate are the results?

Results are computed with full floating-point precision and displayed rounded to 4 significant figures for readability. No rounding is done before the final display step.

Electric Power Calculator

Name: Electric Power Calculator
Availability: InStock
Author: Nham Vu

Enter any two known electrical values to instantly calculate power, voltage, current, and resistance using Ohm's law.

Enter Any Two Known Values

Select two quantities, enter their values, and the other two are calculated automatically.

Power (P)

Voltage (V)

Current (I)

Resistance (R)

Results

Enter two values and click Calculate

Quick Reference — Ohm's Law

Find	Formula	Need
P	V × I	V and I
P	I² × R	I and R
P	V² / R	V and R
V	I × R	I and R
V	P / I	P and I
I	V / R	V and R
R	V / I	V and I

Summary

Enter any two known electrical values to instantly calculate power, voltage, current, and resistance using Ohm's law.

How it works

Select which two electrical quantities you already know.
Enter the numeric values for those two fields.
The calculator solves for the remaining two values automatically.
Results update in real time as you type.
Use the unit selectors to work in millivolts, kilowatts, milliamps, and more.

Use cases

Determine the wattage of a household appliance from its voltage and current rating.
Size a fuse or circuit breaker by calculating maximum current.
Calculate resistance of a heating element given its power rating and supply voltage.
Verify LED resistor values in electronics projects.
Estimate battery run time by knowing power draw and battery capacity.
Convert between different electrical units for engineering homework.
Check power dissipation in resistors during PCB design.
Quickly solve Ohm's law problems for physics and electrical engineering courses.

Frequently Asked Questions

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Last updated: 2026-05-23 · Reviewed by Nham Vu