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Ohm's Law Calculator

Free Ohm's law calculator — enter any two of voltage, current, resistance and power to work out the other two, with mA, kΩ and other unit prefixes. V = I × R, P = V × I.

Fill in exactly two boxes.

Voltage
12 V
Current
54.545 mA
Resistance
220 Ω
Power
654.55 mW

V = I × R

I = V ÷ R

R = V ÷ I

P = V × I = I²R = V²/R

Pick a resistor rated at least double the power: here 1.3091 W or more (common ratings: ¼ W, ½ W, 1 W, 2 W, 5 W).

What is the Ohm's Law Calculator?

Ohm's law says voltage = current × resistance (V = IR). Combined with power = voltage × current, any two of the four values give you the other two.

How to use it

  1. 1

    Fill in two boxes

    e.g. 12 V and 220 Ω.

  2. 2

    Pick units

    From the dropdowns.

  3. 3

    Read the rest

    Calculated values are shown in bold.

Features

  • Any two values

    Voltage, current, resistance or power.

  • Unit prefixes

    mV, mA, µA, kΩ, MΩ, kW and more.

  • Engineering format

    Results like 54.5 mA, not 0.0545 A.

  • Resistor rating tip

    Suggests a safe wattage.

Popular uses

  • LED resistors

    Work out the current-limiting resistor.

  • Electronics

    Check current draw and heat.

  • Physics homework

    V = IR problems.

Frequently asked questions

Does it work for AC?

For resistive loads, yes, using RMS values. Motors and capacitors need impedance.

How do I pick a resistor's wattage?

Use at least double the calculated power.

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