Ohm's Law Calculator
Enter any two electrical values to instantly calculate the other two. Select the unit for each input.
The two most recently edited values are used as the calculation basis.
Recommendations include a 2× power margin and a 25% current margin. Confirm protection coordination and inrush current for your application.
Fundamentals guide
Apply Ohm’s law to a real circuit
Ohm’s law relates voltage, current, and resistance. Together with P = V·I, it helps size a resistor, estimate dissipation, and catch inconsistencies before powering a circuit.
Method and assumptions
The calculator isolates the unknown in V = R·I, I = V/R, and R = V/I, then derives power. Input units are normalised to avoid confusion between mA, A, kΩ, and Ω.
Inputs to verify
Two known quantities
Use consistent values measured at the same point and operating state.
Allowable power
The electrical result must be compared with component rating, derating, and ambient temperature.
Recommended workflow
- 1Identify the component or branch under study and fix the units.
- 2Calculate the missing quantity and dissipated power.
- 3Check worst-case voltage, tolerance, and temperature before final selection.
Checkable example
At 12 V, a 120 Ω resistor passes 0.1 A and dissipates 1.2 W. A 1 W part is insufficient continuously; a thermally derated choice is needed.
Limits to keep in mind
Ohm’s law describes an ideal local relationship. Diodes, transistors, inductors, capacitors, and constant-current loads require an additional model.
Frequently asked questions
Why add power margin?
Power rating depends on temperature, mounting, and cooling. Margin reduces the risk of thermal drift or failure.
Does it apply to AC?
Yes for a pure resistor when using the right RMS values. For reactive components, replace resistance with complex impedance.
References to consult
- Method reference: Ohm’s law, dissipated power, and the component manufacturer’s derating curves.
Original educational content, reviewed for technical clarity on 20 August 2026. Always verify datasheets, applicable standards, and your design before power-up or manufacture.