Ohm's Law Calculator

Electronics Tools

Enter any two values — voltage, current, resistance, or power — and the other two are solved for you.

Voltage (V)

Current (I)

Resistance (R)

Power (P)

Enter exactly two values — clear the others and I'll solve for them.

Solved circuit values

—

Voltage— V

Current— A

Resistance— Ω

Power— W

Ohm's law: V = I × R. Power: P = V × I. Any two known values fully determine the circuit.

Disclaimer: This calculator is provided for general informational and educational purposes only and assumes an ideal, purely resistive DC circuit. It does not account for reactance, temperature effects, component tolerances, or AC circuits. Always verify calculations before working with live electrical circuits, and follow proper safety precautions.

Written by Magesh | Financial Tools & Calculation Specialist · Last updated July 31, 2026

In two sentences: An Ohm’s Law calculator solves for any one of four electrical values — voltage (V), current (I), resistance (R), or power (P) — as long as you know any two of the other three, using the foundational relationship V = I × R plus the power formula P = V × I. This guide breaks down every formula variation, worked examples for common circuits, and how to know which two values to start with.

What Is an Ohm’s Law Calculator?

An Ohm’s Law calculator applies the relationship first described by physicist Georg Ohm — that voltage across a conductor equals the current flowing through it multiplied by its resistance — to solve circuit problems instantly. Because voltage, current, resistance, and power are all mathematically linked, knowing any two lets you calculate the other two without needing to measure everything directly.

The Ohm’s Law Formula and Its Variations

V = I × R          (Voltage = Current × Resistance)
I = V ÷ R          (Current = Voltage ÷ Resistance)
R = V ÷ I          (Resistance = Voltage ÷ Current)

P = V × I          (Power = Voltage × Current)
P = I² × R         (Power = Current² × Resistance)
P = V² ÷ R         (Power = Voltage² ÷ Resistance)

An Ohm’s Law calculator uses this full set of relationships, not just the basic V = I × R formula, so it can solve for power even when you only know two of voltage, current, or resistance rather than needing power measured directly.

The Ohm’s Law Wheel: Solving for Any Value

Because there are four variables (V, I, R, P) and multiple formulas connecting them, it helps to think of Ohm’s Law as a “wheel” of 12 possible formula combinations, grouped by what you’re solving for:

Solving For	If you know V and I	If you know V and R	If you know I and R
Voltage (V)	—	—	V = I × R
Current (I)	—	I = V ÷ R	—
Resistance (R)	—	—	R = V ÷ I
Power (P)	P = V × I	P = V² ÷ R	P = I² × R

Worked Examples

Example 1: Solving for current

A circuit has a 12V battery connected to a 4-ohm resistor.

I = V ÷ R = 12 ÷ 4 = 3 amps

Example 2: Solving for voltage

A component draws 0.5 amps of current through a 100-ohm resistor.

V = I × R = 0.5 × 100 = 50 volts

Example 3: Solving for resistance

A 9V battery pushes 0.3 amps through an unknown resistor.

R = V ÷ I = 9 ÷ 0.3 = 30 ohms

Example 4: Solving for power using two different formula paths

A 120V circuit draws 2 amps of current.

P = V × I = 120 × 2 = 240 watts

If instead you only knew the current (2A) and resistance (60 ohms), you’d reach the identical answer a different way:

P = I² × R = (2)² × 60 = 4 × 60 = 240 watts

This is the core value of an Ohm’s Law calculator — it doesn’t matter which two values you start with, since all valid formula paths converge on the same correct answer.

Step-by-Step: How to Use an Ohm’s Law Calculator

Identify which two of the four values (V, I, R, P) you already know.
Identify which value you need to solve for.
Select the formula that connects your two known values to your unknown — an Ohm’s Law calculator does this selection automatically.
Enter your known values and solve.
Double-check your units — voltage in volts, current in amps, resistance in ohms, and power in watts — since mixing units (like milliamps with amps) is the most common source of calculation errors.

Common Applications of Ohm’s Law

Sizing a resistor for an LED circuit, where you know the supply voltage, the LED’s forward voltage, and the desired current, and need to solve for the correct resistance to avoid burning out the LED.
Calculating power draw of a household appliance from its voltage and current rating, useful for sizing circuit breakers or estimating energy costs.
Troubleshooting a circuit where a multimeter gives you voltage and current readings, and you need to verify the expected resistance matches a component’s rated value.
Battery and charging calculations, where knowing voltage and current lets you calculate power delivered or consumed over time.

Using an Ohm’s Law Calculator for Real Projects

An Ohm’s Law calculator is most useful the moment you’re actually building something — sizing a resistor, checking a component rating, or troubleshooting a circuit that isn’t behaving as expected. Rather than memorizing all twelve formula variations, most people simply keep an Ohm’s Law calculator on hand and let it select the right relationship based on whichever two values they know.

Common Ohm’s Law calculator scenarios for hobbyists

LED resistor sizing — enter your supply voltage, the LED’s forward voltage drop, and your target current, and an Ohm’s Law calculator gives you the exact resistor value needed to protect the LED from excess current.
Verifying a component’s rated power — if a resistor is rated for a maximum wattage, an Ohm’s Law calculator using P = I²R lets you confirm your circuit won’t exceed that rating before you power it on.
Diagnosing an unexpected reading — if a multimeter shows a voltage or current that doesn’t match your expectations, running the numbers through an Ohm’s Law calculator can reveal whether a component’s actual resistance differs from its labeled value.
Battery life estimation — combining an Ohm’s Law calculator’s power result with a battery’s rated capacity gives a rough estimate of how long a circuit will run before the battery depletes.

Frequently Asked Questions

What’s the difference between Ohm’s Law and the power formulas?

Ohm’s Law (V = I × R) relates voltage, current, and resistance directly. The power formulas (P = V × I, P = I²R, P = V²/R) extend this relationship to include power, letting you calculate wattage from any two of the three base variables.

Does Ohm’s Law apply to AC circuits the same way as DC?

For purely resistive AC circuits, yes — the same relationships hold using RMS (root mean square) values. For circuits containing capacitors or inductors, impedance replaces simple resistance, and the calculations become more complex than basic Ohm’s Law.

Why do I get different power results using different formulas?

You shouldn’t, if your inputs are accurate — all three power formulas (P=VI, P=I²R, P=V²/R) are mathematically equivalent and will produce the same answer when working from consistent, correctly measured values. A discrepancy usually points to a measurement or unit error.

Can an Ohm’s Law calculator handle circuits with multiple resistors?

Basic Ohm’s Law calculators handle single-resistor calculations. For circuits with resistors in series or parallel, you first need to calculate the total equivalent resistance using series/parallel resistance formulas, then apply Ohm’s Law to that combined value.

Related Calculators

In summary, an Ohm’s Law calculator turns one of the most fundamental relationships in electronics into an instant solver for voltage, current, resistance, or power, and understanding the full set of formula variations — not just V = I × R — lets you work confidently from whichever two values you happen to measure first.

About the author: Magesh is a Financial Tools & Calculation Specialist focused on building and fact-checking online calculators across finance, electronics, and everyday measurement topics.

Note: This calculator and article are provided for general educational and informational purposes only and do not constitute professional electrical engineering advice. Always follow proper safety procedures and consult a licensed electrician when working with electrical circuits.