Ohm's Law Calculator
Voltage, current, resistance and power.
Show detailsHide details
Ohm's Law links the three fundamental quantities in any electrical circuit: voltage, current, and resistance. Enter any two and this calculator finds the third — plus the power dissipated — showing the exact formula it used.
Loading calculator...
Ohm's Law — The Quick Answer
Voltage equals current times resistance. From V = I × R, you can solve for any one quantity if you know the other two. Power equals voltage times current.
Shortcut
V = I × R | I = V ÷ R | R = V ÷ I | P = V × I
The three Ohm's Law relationships
| Quantity | Symbol | Unit | Formula |
|---|---|---|---|
| Voltage | V | volts (V) | V = I × R |
| Current | I | amps (A) | I = V ÷ R |
| Resistance | R | ohms (Ω) | R = V ÷ I |
| Power | P | watts (W) | P = V × I |
A helpful memory aid is the Ohm's Law triangle: cover the quantity you want, and the remaining two show the operation — V on top over I and R means V = I × R.
How It Works
The Formulas
One relationship, three rearrangements
Formula
V = I × R I = V ÷ R R = V ÷ I P = V × I
Variables
Voltage (volts)
The electrical 'pressure' driving current around a circuit. One volt drives one amp through one ohm of resistance.
Current (amps)
The rate of flow of electric charge. It rises with more voltage and falls with more resistance.
Resistance (ohms)
How much a component opposes the flow of current. Higher resistance means less current for the same voltage.
Note: Ohm's Law holds for ohmic (linear) components like resistors. Non-ohmic devices — diodes, filament lamps at varying temperature — don't keep a constant R across their range, so the relationship is only approximate for them.
Last updated: October 6, 2026
Creators
Computer Scientist & Developer Tools Author
Reviewers
Technical Reviewer
Table of Contents
Quick Facts
Related Tools
Privacy Guaranteed
Your data never leaves your browser. All calculations are 100% private.
Frequently Asked Questions
What is Ohm's Law?
Ohm's Law states that the current through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance — written as V = I × R. It lets you work out any one of voltage, current, or resistance when you know the other two.
How do I calculate resistance from voltage and current?
Divide the voltage by the current: R = V ÷ I. For example, 12 volts across a component carrying 3 amps means a resistance of 12 ÷ 3 = 4 ohms.
How do I find the power in a circuit?
Power equals voltage times current: P = V × I, in watts. For 12 volts and 3 amps that's 36 watts. Combined with Ohm's Law you can also write P = I²R or P = V²/R, depending on which quantities you know.
Does Ohm's Law always apply?
It applies to ohmic components — those whose resistance stays constant, like fixed resistors. Non-ohmic devices such as diodes, thermistors, and filament bulbs don't obey a simple constant-resistance relationship across all conditions, so results there are only approximations.