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Ohm’s Law Calculator: Voltage, Current, Resistance, Power

This free Ohm’s law calculator finds the missing values of an electrical circuit. Fill in any two of voltage, current, resistance and power, and it gives the other two with the formula it used.

By Debmedia Technologies LLP · Kolkata, India · Updated

What Is It and When Do You Need It?

Ohm’s law links voltage, current and resistance. With the power formula, any two values let you find the rest. It is the first tool of every electronics hobbyist and electrician.

Enter any two values and read the other two.

How to Use This Tool

  1. Enter two of the four values.
  2. Leave the others empty.
  3. Press Calculate.
  4. Read the formulas used.

How the Calculation Works

The tool uses V = I R and P = V I, and combinations such as I = P ÷ V, R = V² ÷ P, V = √(P R) and I = √(P ÷ R) depending on which two values you give.

Worked Example

With 12 V and 4 Ω, the current is 3 A and the power is 36 W. With 5 V and 0.5 A, the resistance is 10 Ω and the power is 2.5 W.

Common Uses

  • Circuit design.
  • Choosing resistors and fuses.
  • Checking power ratings.
  • Homework and learning.

Tips and Common Mistakes

  • Use consistent units: volts, amps, ohms, watts.
  • Check that parts can handle the power.
  • Add a safety margin for resistors.
  • When you only know milliamps or kilo-ohms, convert to amps and ohms first: 20 mA is 0.02 A and 4.7 kΩ is 4,700 Ω, then enter the numbers in the boxes.
  • The power figure tells you the resistor rating you need, so choose a resistor rated at least twice the calculated watts for a safe margin.

Next, you may need the LED resistor calculator, resistor color code calculator, AWG wire gauge calculator, electricity cost calculator.

Everything runs in your browser. Nothing you type is uploaded, stored or sent to any outside service.

Frequently Asked Questions

Voltage equals current times resistance: V = I × R. A circuit with 12 volts across 4 ohms carries 3 amps.
Power = voltage × current. With the first law, P = I² × R and P = V² ÷ R.
Any two. Leave the other two empty and the tool finds them.
For simple resistive loads, yes, with RMS values. Coils and capacitors need impedance, which is not covered here.

Disclaimer: results are calculated in your browser from the numbers you enter. Check important results before you rely on them for money, health, safety or legal decisions.