Ohm's law calculator
Short answer
Enter any two values (volts, amps, ohms or watts) and the calculator works out the other two. It uses Ohm's law, E = I × R, and the power formula, P = E × I.
Ohm's law formulas
| To find | Formulas |
|---|---|
| Volts (E) | E = I × R E = P ÷ I E = √(P × R) |
| Amps (I) | I = E ÷ R I = P ÷ E I = √(P ÷ R) |
| Ohms (R) | R = E ÷ I R = E² ÷ P R = P ÷ I² |
| Watts (P) | P = E × I P = I² × R P = E² ÷ R |
Worked examples
- A 120-volt circuit feeds a 24-ohm load: I = 120 ÷ 24 = 5 A, and P = 120 × 5 = 600 W.
- A 12 A current flows through a 10-ohm resistor: E = 12 × 10 = 120 V.
- A 1,500 W heater on 120 volts: I = 1,500 ÷ 120 = 12.5 A, and R = 120 ÷ 12.5 = 9.6 ohms.
These are the same kinds of questions that open the electrical theory section of most journeyman exams. Try more in our free practice test.
Frequently asked questions
Does Ohm's law work for AC circuits?
For purely resistive AC loads such as heaters and incandescent lamps, yes. For motors and other inductive loads, current also depends on reactance and power factor, so you use impedance (Z) in place of resistance.
What is the difference between E and V?
Both mean voltage. Electrical textbooks and exam questions often use E (electromotive force) in formulas, while meters and nameplates use V.