Ohm's Law states that the voltage (V) across a conductor equals the current (I) through it multiplied by its resistance (R): V = I × R. It is the foundational relationship in electrical circuit analysis.

Watt's Law defines electrical power (P) as the product of voltage and current: P = V × I. Combined with Ohm's Law, this allows you to derive any one of the four quantities V, I, R, or P from any two others.

Why do I need to enter exactly two values?

The four quantities V, I, R, and P are related by two independent equations. Two knowns uniquely determine the remaining two unknowns. Fewer than two gives an underdetermined system; more than two is over-constrained and may be inconsistent.

What do the unit prefixes mean?

milli (m) = × 10⁻³, base (1) = no scaling, kilo (k) = × 10³, mega (M) = × 10⁶. For example, 4.7 kΩ means 4700 Ω.

Does this tool work for AC circuits?

Ohm's Law in its basic form applies to purely resistive DC circuits. For AC circuits with capacitors or inductors, impedance (Z) replaces resistance and the relationships become frequency-dependent. This calculator assumes purely resistive loads.

Ohm's Law Calculator

Name: Ohm's Law Calculator
Availability: InStock
Author: Nham Vu

Enter any two of voltage (V), current (A), resistance (Ω), or power (W) to instantly compute the remaining two using Ohm’s Law and Watt’s Law.

Enter Any Two Values

Fill in exactly two fields. Leave the other two blank.

Voltage — V

Current — I

Resistance — R

Power — P

Results

Enter any two values and click Calculate.

V = I × R • P = V × I

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Summary

Enter any two of voltage (V), current (A), resistance (Ω), or power (W) to instantly compute the remaining two using Ohm’s Law and Watt’s Law.

How it works

Select the unit prefix (m / 1 / k / M) for each quantity you want to enter.
Type a numeric value into exactly two of the four fields: Voltage, Current, Resistance, or Power.
Click Calculate (or press Enter) to compute the remaining two values.
Read the result panel to see all four values, which two were given, and the formulas used.
Click Reset to clear all fields and start over.

Use cases

Students verifying circuit calculations for electronics coursework.
Engineers quickly checking power dissipation in a resistor.
Hobbyists sizing resistors and calculating LED current-limiting values.
Technicians troubleshooting electrical circuits in the field.

Frequently Asked Questions

Last updated: 2026-06-09 · Reviewed by Nham Vu