Enter any two of voltage, current, resistance and power to find the other two.
Voltage = current × resistance (V = I × R). Power = voltage × current (P = V × I). With 12 V across a 6 Ω resistor, the current is 2 A and the power is 24 W.
Ohm's law links four electrical quantities — voltage (V), current (I), resistance (R) and power (P). Know any two and the other two follow. This calculator accepts any pair, with convenient units from millivolts and microamps to kilovolts and megohms, and shows all four results in sensible SI prefixes.
It is useful for electronics projects, choosing a resistor for an LED, checking a heater or appliance, and school physics. For resistor colour bands use the Resistor Color Code Calculator; for losses along a cable use the Voltage Drop Calculator.
The calculator converts your two inputs to base SI units, then applies the formulas below.
| Known values | Voltage | Current | Resistance | Power |
|---|---|---|---|---|
| V = 12 V, I = 2 A | 12 V | 2 A | 6 Ω | 24 W |
| V = 230 V, R = 46 Ω | 230 V | 5 A | 46 Ω | 1.15 kW |
| V = 10 V, P = 100 W | 10 V | 10 A | 1 Ω | 100 W |
| I = 2 A, R = 5 Ω | 10 V | 2 A | 5 Ω | 20 W |
| I = 3 A, P = 27 W | 9 V | 3 A | 3 Ω | 27 W |
| R = 4 Ω, P = 100 W | 20 V | 5 A | 4 Ω | 100 W |
| To find | Using V and I | Using V and R | Using I and R | Using P and V / I / R |
|---|---|---|---|---|
| Voltage V | — | — | V = I × R | V = P ÷ I, V = √(P × R) |
| Current I | — | I = V ÷ R | — | I = P ÷ V, I = √(P ÷ R) |
| Resistance R | R = V ÷ I | — | — | R = V² ÷ P, R = P ÷ I² |
| Power P | P = V × I | P = V² ÷ R | P = I² × R | — |
Ohm's law holds for resistors and most conductors at constant temperature. Diodes, LEDs, transistors and lamp filaments are non-linear, so their resistance changes with voltage and current. For AC circuits with reactive loads, power also depends on the power factor; the formulas here give the real power only for resistive loads.
Ohm's law states that the current through a conductor is proportional to the voltage across it: V = I × R, where R is the resistance.
Divide voltage by resistance. A 230 V supply across a 46 Ω heater drives 5 A.
Multiply voltage by current (P = V × I). You can also use P = I² × R or P = V² ÷ R.
R = P ÷ I². With 27 W at 3 A, R = 27 ÷ 9 = 3 Ω.
Millivolts to kilovolts, microamps to kiloamps, milliohms to megohms and milliwatts to kilowatts.
Two values fully define the circuit. Entering three could be inconsistent, so leave two boxes blank.
Use R = (supply voltage − LED voltage) ÷ LED current. For a 5 V supply, a 2 V LED and 20 mA, R = 150 Ω.
For purely resistive AC loads, yes, using RMS values. For motors and other reactive loads, impedance and power factor must be considered.
One watt is one joule per second, or one volt times one amp.
No. Everything runs in your browser.
