Ohm's Law Calculator
Calculate voltage, current or resistance using Ohm's law (V = I × R).
Tip: Remember the Ohm's triangle: V on top, I×R below. Cover one and see the formula: V=IR, I=V/R, R=V/I.
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About this digital tool
The Ohm's law calculator finds voltage, current or resistance when you know two of the three quantities. Choose what you are looking for, enter the two known values with their units (mV, V, kV; mA, A; Ω, kΩ, MΩ) and get the result plus the power in watts.
Ohm's law states V = I × R, so I = V ÷ R and R = V ÷ I. Power is calculated as P = V × I. The tool converts units to base before calculating and stops you dividing by zero. It is useful for physics and electronics students, for technicians and for sizing resistors in simple circuits.
How to use
- Choose what you want to find (V, I, or R)
- Enter the two known values
- Choose the units (mV/V/kV, mA/A, Ω/kΩ/MΩ)
- See the result plus the power (W)
- See the circuit diagram
Frequently asked questions
What does Ohm's law say?
The current through a conductor is directly proportional to the voltage and inversely proportional to the resistance: I = V/R. It holds for circuits with pure (ohmic) resistance.
When doesn't Ohm's law apply?
It doesn't apply to: diodes, transistors, incandescent lamps (resistance changes with temperature), and any non-linear element where resistance depends on voltage/current.
How is power related to Ohm's law?
P = V × I = I²R = V²/R. Power (watts) shows how much energy the circuit consumes. E.g., a 100Ω resistor at 0.1A: P = 0.1² × 100 = 1W.
How do I use the units mA, kΩ or MΩ correctly?
Choose the unit next to each value and the tool converts to base units (A, V, Ω) itself before calculating. So with 2 kΩ and 5 mA you don't need to convert by hand.
What does Ohm's law say in simple terms?
Voltage equals current multiplied by resistance (V = I × R). If you know two of them, you find the third: I = V ÷ R and R = V ÷ I.