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Ohm’s Law Calculator

Find voltage, current or resistance from the other two, with the power in watts.

By Nitin Raturi · Last reviewed

Your circuit

How to use the Ohm's Law Calculator

Find voltage, current or resistance from the other two, plus the power.

  1. Choose what to find.
  2. Enter the two values you know.
  3. Read the answer and the power in watts.

Good to know

  • Convert milliamps to amps (÷ 1,000) and kilohms to ohms (× 1,000).

V = I × R. Use volts, amps and ohms.

Find

Find

Choose the quantity you want; enter the other two.

How to fill it in

  • Resistance from voltage and current.
  • Current from voltage and resistance.
  • Voltage from current and resistance.

Voltage (V)

The electrical push across the component, in volts.

How to fill it in

  • Enter a number above 0, such as 12 or 230.

Example

230 V across 1,000 Ω drives 0.23 A.

Current (A)

How much charge flows each second, in amperes. 20 mA is 0.02 A.

How to fill it in

  • Enter a number above 0.

Example

2.5 A at 12 V means 4.8 Ω.

Resistance

4.8 Ω

12 V = 2.5 A × 4.8 Ω. Power: 30 W.

Voltage
12 V
Current
2.5 A
Resistance
4.8 Ω
Power
30 W

Ohm’s law V = I × R for a resistor at steady current; power P = V × I. Rounded to six decimals for display.

ChangelogHow we test this calculatorEmbed this calculator

How this calculator works

  1. Step 1ResistanceR = V ÷ IFrom voltage and current.
  2. Step 2CurrentI = V ÷ RFrom voltage and resistance.
  3. Step 3VoltageV = I × RFrom current and resistance.
  4. Step 4PowerP = V × IWatts used by the component.

Ohm’s law links voltage, current and resistance: the current through a resistor is the voltage across it divided by its resistance. Knowing any two gives the third, and their product gives the power.

Worked example: a car headlight

Resistance of a headlight drawing 2.50 A at 12.0 V
Ohm's law
R = V ÷ I
  1. Resistance12.0 V ÷ 2.50 A4.8 Ω
  2. Power12.0 V × 2.50 A30 W

This is OpenStax College Physics Example 20.4, which gives 4.80 Ω.

Small and large currents

  • LED resistor9.4 V20 mA through 470 Ω, using 0.188 W.
  • 1 kΩ on 230 V0.23 ADissipating 52.9 W, so it needs a suitably rated resistor.

Read the result

The headline is the quantity you chose to find; the table shows all four values.

Assumptions and limits

  • Ohmic componentsResistance stays the same as current changes.
  • DC or resistive ACFor AC, use RMS values with purely resistive loads.
  • Not safety adviceMains wiring should be done by a qualified electrician.

LEDs, motors and lamps that heat up do not follow Ohm’s law exactly.

Ohm's law links voltage, current and resistance. Circuits and cables build on it.

To combine resistors, use the series and parallel resistor calculator. For loss along a cable, try the voltage drop calculator, and for capacitor timing, the RC time constant calculator.

Common mistakes

  • Mistake: Entering 20 mA as 20.

    Instead: Use amperes: 20 mA is 0.02 A.

  • Mistake: Ignoring the power rating.

    Instead: Check that a resistor can handle the watts shown.

Checking the method

Every golden case is rechecked with exact fractions using P = I²R and R = V²/P.

An independent script uses other forms of the same laws, and reproduces OpenStax’s headlight example.

Read how we test our calculators, or browse more everyday tools.

Frequently asked questions

What is Ohm's law?

Voltage equals current times resistance: V = I × R. Knowing any two gives the third.

How do I find resistance?

Divide voltage by current. 12 V at 2.5 A is 4.8 Ω.

How do I find current?

Divide voltage by resistance. 230 V across 1,000 Ω is 0.23 A.

How is power worked out?

Multiply voltage by current. The headlight uses 30 W.

How do I use milliamps?

Divide by 1,000 first. 20 mA through 470 Ω drops 9.4 V.

Sources and methodology

The formulas are standard time-value-of-money results. We check every figure against golden values worked out independently, by closed form and by month-by-month simulation; see the methodology.

Written and maintained by Nitin Raturi, a software engineer (about the author). Last reviewed 3 Nov 2026; next review due by . Found an error? Tell us.

Changelog

  • : Initial release.