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Ohm’s Law Calculator

How do I use Ohm’s law?

Enter any two

Result

Enter exactly two values
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How it is calculated

Ohm's law links voltage V (volts), current I (amperes) and resistance R (ohms): V = I × R. Fill in exactly two of the three fields and the tool solves the missing one: I = V / R, R = V / I or V = I × R. It then gives the electrical power in watts, P = V × I.

If you enter fewer or more than two values, or non-numeric text, it asks for exactly two. A zero resistance or zero current that would require dividing by zero gives no result. Values are shown to two decimals. The law applies to simple resistive circuits with steady (DC or effective AC) values; it does not describe lamps, diodes or coils whose resistance changes with conditions.

Worked example

A 12 V supply drives a 6 Ω resistor. Enter V = 12 and R = 6, and leave current empty. The current is I = V / R = 12 / 6 = 2.00 A. The power is P = V × I = 12 × 2 = 24.00 W. The result shows 12.00 V, 2.00 A, 6.00 Ω and 24.00 W.

Try this example →

Common mistakes

  • Filling in all three fields or only one. Exactly two must be filled, otherwise the tool asks for two values.
  • Mixing units, such as milliamps or kilo-ohms typed as amps and ohms. Convert first: 250 mA is 0.25 A.
  • Using Ohm's law for components whose resistance varies, such as LEDs or incandescent bulbs, or for AC circuits with capacitors and coils.
  • Reading the power as the supply's capability. It is what this load draws at that voltage.

About this calculator

This tool applies Ohm's law (V = I × R) and the related power formula (P = V × I) to a simple circuit: enter any two of voltage, current and resistance, and it solves for the missing one plus the power dissipated. Because the three quantities are locked together by the same equation, knowing any two always determines the rest — you never need to solve the algebra by hand.

Electronics hobbyists and students use it constantly when building circuits — picking a resistor value to limit current to an LED, checking whether a component will exceed its power rating, or verifying a circuit design before soldering it. Electricians and engineers use the same relationship to size wiring and troubleshoot circuits where a measured voltage and current don't match the expected resistance.

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Comments (1)

  • Editorial team

    A classic use is sizing a resistor for an LED. Enter the voltage across the resistor, which is the supply minus the LED's forward drop, and the current in amperes, not milliamps. For 5 V, a 2 V LED and 20 mA, enter 3 V and 0.02 A to get 150 Ω and 0.06 W. Then choose a resistor rated well above the computed power. What circuit are you trying to size?

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