Ohm's law describes how voltage, current and resistance relate in an electrical circuit: the voltage across a resistor equals the current through it multiplied by its resistance. Enter any two to find the third, along with the power dissipated.
Formula
V = I × R
Where V is the voltage (V), I is the current (A), R is the resistance (Ω).
Choose any value to calculate — the calculator rearranges the formula for you.
Worked example
A 12 V supply across a 24 Ω resistor drives a current of 12 ÷ 24 = 0.5 A. The resistor dissipates 12 × 0.5 = 6 W of power.
Power formulas
Combining Ohm’s law with P = V × I gives two more useful forms: P = I² × R and P = V² ÷ R.
Frequently asked questions
What is Ohm's law?
V = I × R: voltage (volts) equals current (amperes) times resistance (ohms). It holds for ohmic conductors such as ordinary resistors.
How do I calculate current from voltage and resistance?
Divide the voltage by the resistance: I = V ÷ R. 9 V across 450 Ω gives 0.02 A (20 mA).
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