Ohms Law Calculator
Enter any two of voltage, current, resistance and power, in volts, millivolts, amps, milliamps, ohms, kilohms, watts or kilowatts, and the Ohm's law calculator solves the other two and shows the formula it used.
Ohm's law calculator
Enter any two values and leave the other two empty.
Ohm's law formulas
V = I × R: voltage (volts) equals current (amps) times resistance (ohms). Combined with power, P = V × I, any two values give the other two:
| Known | Formulas |
|---|---|
| V and I | R = V ÷ I · P = V × I |
| V and R | I = V ÷ R · P = V² ÷ R |
| I and R | V = I × R · P = I² × R |
| P and V | I = P ÷ V · R = V² ÷ P |
| P and I | V = P ÷ I · R = P ÷ I² |
| P and R | V = √(P × R) · I = √(P ÷ R) |
Example: a 470 Ω resistor on 12 V draws 12 ÷ 470 = 25.5 mA and dissipates 12² ÷ 470 = 0.306 W, so a ½-watt resistor is the right rating. For household circuits, remember breakers are rated in amps: a 1,500 W heater on 120 V draws 12.5 A.
Common questions
What is Ohm's law?
Voltage equals current times resistance: V = I × R.
How do I calculate current from voltage and resistance?
Divide voltage by resistance: I = V ÷ R. 12 V across 470 Ω is about 25.5 mA.
How do I calculate watts from volts and amps?
Multiply them: P = V × I. 120 V × 10 A = 1,200 W.
How many amps does a 1,500-watt heater use?
1,500 W ÷ 120 V = 12.5 A, which is why it shouldn't share a 15 A circuit with other big loads.
What resistor do I need for an LED?
R = (supply voltage − LED forward voltage) ÷ LED current. 5 V, a 2 V LED at 20 mA: 3 ÷ 0.02 = 150 Ω.
Sources
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General information, not financial, tax or legal advice. Check important numbers with the official source before acting on them.