Enter two known values
Positive magnitudes only
Base relationship: V = I × R. Power adds P = V × I.
Free circuit calculator
Ohm's Law Calculator
An Ohm's law calculator finds voltage, current, resistance, and power from any two known circuit values. Choose your units, enter two measurements, and see the remaining values instantly.
How to use this calculator
- 1
Choose two quantities
Select two different values you already know, such as voltage and current.
- 2
Enter values and units
Use the unit menus for volts, milliamps, kilohms, watts, and other common scales.
- 3
Check the complete result
Read all four circuit values and the exact formulas used to derive the unknowns.
Ohm's law formula guide
V = I × R
I = V ÷ R
R = V ÷ I
P = V × I
P = I² × R
P = V² ÷ R
Results model ideal resistive circuits. Confirm component ratings, tolerances, and safe working limits before building a real circuit.
Frequently asked questions
What is Ohm's law?
Ohm's law states that voltage equals current multiplied by resistance: V = I × R. Its related power formulas connect those values with watts.
Which two values do I need?
Enter any two different known values from voltage, current, resistance, and power. The calculator derives the other two.
Can I use milliamps and kilohms?
Yes. Select the unit beside each input. The calculator converts micro, milli, kilo, and mega units to base SI units before solving.
Does Ohm's law work for every circuit?
It works for ideal resistive DC calculations. Real components can change with temperature, frequency, tolerance, or non-linear behavior.
How is electrical power calculated?
Power is P = V × I. Equivalent forms are P = I² × R and P = V² ÷ R.
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