Tools

Ohm's Law Calculator: Voltage, Current, Resistance, and Power

By David Brown · January 2026 · 3 min read

Ohm's Law describes the relationship between voltage, current, and resistance in an electrical circuit. It's the most fundamental equation in electronics and is used constantly in electrical work, electronics design, and troubleshooting.

The Equations

Ohm's Law: V = I × R

Where:

V = Voltage (volts)
I = Current (amperes, amps)
R = Resistance (ohms, Ω)

Power equation: P = V × I

And derived:

P = I² × R
P = V² ÷ R

Given any two values, the other two can be calculated.

Practical Examples

What resistor limits an LED to safe current?

LED supply: 5V. LED forward voltage: 2V. Desired current: 20mA.

Voltage across resistor: 5V - 2V = 3V

R = V ÷ I = 3V ÷ 0.02A = 150Ω

How much current does a 60W bulb draw at 120V?

I = P ÷ V = 60W ÷ 120V = 0.5 amps

What's the power dissipated by a 100Ω resistor carrying 50mA?

P = I² × R = (0.05)² × 100 = 0.25 watts

Why This Matters for Practical Work

Fuse sizing: Know the current draw of a circuit, use Ohm's law to verify the fuse is appropriate.

Wire gauge: Current-carrying capacity of wire gauges is based on the resistance of copper at different cross-sections.

LED resistors: Every LED needs a current-limiting resistor; Ohm's law calculates the correct value.

Battery life estimation: Current draw × battery capacity (amp-hours) = operating time.

[Use the Ohm's Law calculator →](https://doesitaddup.com)

Try it yourself: All free calculators at doesitaddup.com →

This article is for informational purposes only. See our disclaimer.