Calculate Voltage, Current, Resistance & Power

A free browser-based Ohm's Law calculator that computes voltage, current, resistance, and power from any two known values. Features an interactive formula wheel, animated circuit diagram, auto unit conversion, and practical presets for common scenarios.

How to Use the Ohm's Law Calculator

1
Enter two known values

Type any two values — voltage, current, resistance, or power. Select the appropriate unit from the dropdown.

2
Get instant results

The remaining two values are calculated automatically using Ohm's Law and the power equation.

3
Use presets for common scenarios

Click a preset button to load typical values for USB chargers, LED strips, car circuits, and more.

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Solve any electrical circuit value with the interactive Ohm's Law wheel

Enter any two values to calculate the rest
⚠️ High power — check component ratings!
+ − V = ? I = ? R = ? P = ?
Quick Presets
All Formulas
Find V
V = I × R
Find V
V = P / I
Find V
V = √(P×R)
Find I
I = V / R
Find I
I = P / V
Find I
I = √(P/R)
Find R
R = V / I
Find R
R = V² / P
Find R
R = P / I²
Find P
P = V × I
Find P
P = I² × R
Find P
P = V² / R
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Features

Enter any 2 of 4 values (V, I, R, P) — the rest are calculated instantly Interactive Ohm's Law wheel showing all 12 formulas at a glance Auto unit conversion: mV/V/kV, µA/mA/A, Ω/kΩ/MΩ, mW/W/kW Practical presets: USB charger, LED strip, car battery, and more

FAQ

What is Ohm's Law?

Ohm's Law states that voltage (V) equals current (I) multiplied by resistance (R): V = I × R. Combined with the power equation (P = V × I), you can calculate any electrical quantity from two known values.

What formulas does this calculator use?

It uses all 12 combinations of Ohm's Law and the power equation: V=IR, I=V/R, R=V/I, P=VI, P=I²R, P=V²/R, and their rearrangements.

What units are supported?

Voltage: mV, V, kV. Current: µA, mA, A. Resistance: Ω, kΩ, MΩ. Power: mW, W, kW. The calculator auto-converts between them.

Why does my wire heat up?

Power (P = I²R) is dissipated as heat. Higher current or resistance means more heat. The calculator shows a visual warning when power exceeds safe levels for typical components.

Can I use this for AC circuits?

This calculator is for DC circuits. For AC, you need to account for impedance (not just resistance), power factor, and reactive power. A dedicated AC calculator would be needed.

💡 Want us to improve this tool just for you?

We can — and it's free! Just send us a quick message with your idea. If you'd like to discuss it in detail, leave your email and we'll get back to you. You can stay anonymous.

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Who is this tool for?
🔌 Electronics Enthusiasts
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