Fill in any two of the four boxes (leave the others empty) and the calculator solves for the rest.
Ohm’s law says the voltage across a resistor equals the current through it times its resistance: V = I x R. Electrical power is P = V x I. Combining the two gives every pair below, so any two known values are enough to find the other two.
| To find | If you know V and I | V and R | I and R | P and V | P and I | P and R |
|---|---|---|---|---|---|---|
| Voltage V | - | - | I x R | - | P / I | √(P x R) |
| Current I | - | V / R | - | P / V | - | √(P / R) |
| Resistance R | V / I | - | - | V² / P | P / I² | - |
| Power P | V x I | V² / R | I² x R | - | - | - |
| Voltage | Resistance | Current | Power |
|---|---|---|---|
| 12 V | 100 Ω | 0.12 A | 1.44 W |
| 5 V | 100 Ω | 0.05 A | 0.25 W |
| 24 V | 100 Ω | 0.24 A | 5.76 W |
| 120 V | 100 Ω | 1.2 A | 144 W |
| 230 V | 100 Ω | 2.3 A | 529 W |
Ohm’s law states that the current through a conductor is proportional to the voltage across it: V = I x R, where V is in volts, I in amps and R in ohms.
Divide the voltage by the resistance: I = V / R. A 12 V supply across a 100 ohm resistor drives 0.12 A (120 mA).
Multiply voltage by current: P = V x I. You can also use P = I² x R or P = V² / R when you do not know all three values.
It holds for ordinary resistors and wires at a steady temperature. Components such as diodes, LEDs and light bulb filaments are non-ohmic: their resistance changes with voltage or temperature, so V = IR gives only the value at one operating point.
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