🔌Ohm's Law Calculator

Calculate voltage, current, or resistance using Ohm's law from any two known values

How to Use the Ohm's Law Calculator

Ohm's Law (V = I × R) describes the relationship between voltage (volts), current (amperes), and resistance (ohms) in an electrical circuit. Select which value you want to calculate, enter the other two, and the calculator instantly finds the unknown value. It also calculates power (watts) automatically, since P = V × I.

Common uses include choosing the right resistor for an LED circuit, calculating the current draw of a device, or finding the voltage drop across a resistor. For example, a 12V source through a 6Ω resistor drives 2A of current and dissipates 24W of power.

Frequently Asked Questions

How do I use kilohms (kΩ) or milliamps (mA)?

Convert to base units: 1 kΩ = 1,000 Ω and 1 mA = 0.001 A. Enter 4700 for 4.7 kΩ, or 0.02 for 20 mA.

Does this calculator work for AC circuits?

For pure resistive loads, yes. For circuits with capacitors or inductors, you'd need to use impedance (Z) instead of resistance (R), which this calculator doesn't cover.

How do I calculate the resistor value for an LED?

Use R = (Supply voltage − LED forward voltage) ÷ LED current. For example: (5V − 2V) ÷ 0.02A = 150Ω. Use the nearest standard resistor value above that.