Ohm's Law Calculator
Enter any two of voltage, current, and resistance — leave the third field blank to calculate it.
Inputs
Result
Step-by-step working
Formula
- V = IR
- I = V/R
- R = V/I
V is voltage in volts, I is current in amps, R is resistance in ohms.
About this calculation
Ohm's law links the voltage across a resistive component to the current through it: the two are in proportion, and the constant of that proportion is the resistance. Enter any two of voltage, current and resistance and the calculator returns the third, converting the prefixes (µA, mA, A; Ω, kΩ, MΩ) before it works.
Assumptions and limits
- It assumes one ideal resistive element carrying a steady (DC) current, or AC values given as RMS.
- Resistance is treated as constant. A real resistor drifts with temperature, and some components (filament lamps, diodes, thermistors) are deliberately not ohmic.
- The resistance of the wires, the source's internal resistance and the load of any meter are ignored.
A worked example
A 12 V supply across a 4 Ω resistor carries I = V / R = 12 / 4 = 3 A, and that current is what a meter in series with the resistor would read.
See also Power and Voltage Divider.