Free Online Ohm's Law Calculator
Calculate voltage, current, resistance, and power.
Enter any two values and leave the others empty. The calculator solves the remaining quantities using Ohm's law and the power equations. Values must be positive; use the clear button to start again.
Formulas and working
- Enter two values to see the formulas with numbers.
About the Ohm's Law Calculator
Ohm's law links voltage, current and resistance in a simple electrical circuit: V = I × R. Power adds a fourth relationship, P = V × I, and together they let you solve for any missing quantity when two are known. This calculator accepts any two of voltage, current, resistance and power, computes the other two, and formats the results with sensible prefixes such as milliamps, kilo-ohms and watts. It also reports the energy used if the power is sustained for one hour.
The four relationships
Voltage is the electrical pressure, current is the flow of charge, and resistance opposes that flow. The three combine as V = I × R, with rearrangements I = V / R and R = V / I. Power is the rate of energy transfer and can be written as P = V × I, P = I²R or P = V²/R. The calculator chooses the pair you entered, applies the appropriate rearrangement, and then verifies all four values against each other. Every formula used is shown with the substituted numbers.
Choosing which values to enter
Enter exactly two positive values and leave the other fields blank. Common pairs are voltage and resistance for a simple resistor, voltage and current for a device's operating point, or power and resistance for a heating element. If you enter more than two, the calculator checks whether they are mutually consistent; a mismatch usually means a decimal point, a unit prefix or a transcription error. Zero is rejected because it leads to undefined divisions or trivial circuits that do not exercise the relationships.
Unit prefixes and practical checks
Results are scaled automatically: 0.02 A appears as 20 mA, 4,700 Ω appears as 4.7 kΩ, and 0.5 W appears as 500 mW. This matches how components and multimeters are labeled. Always check the order of magnitude against expectations. A 12 V supply across a 4 Ω resistor draws 3 A and dissipates 36 W, which is a lot of heat for a small component. Real resistors have a power rating, and exceeding it causes overheating even when the resistance value is correct.
Limits of the model
Ohm's law applies to linear resistive elements at a steady state. Diodes, LEDs, transistors and motors are nonlinear, and reactive components such as capacitors and inductors behave differently with AC signals, where impedance includes phase. Temperature changes resistance, and real power supplies have internal resistance and current limits. Use this calculator for DC resistor circuits and as a first estimate elsewhere, then verify with the component datasheet and a proper measurement.
Frequently asked questions
Why does the calculator reject zero values?
A zero voltage, current or resistance can make one of the rearrangements divide by zero, and a zero power with nonzero voltage or current is contradictory. Requiring positive values keeps the solution well defined.
What does the energy per hour card mean?
One watt is one joule per second, so a 36 W load uses 36 watt-hours in one hour, or 0.036 kWh. The card converts the power result into watt-hours and kilowatt-hours for a one-hour period.
Can I use this for AC circuits?
Only as a rough estimate for purely resistive AC loads. Reactive loads involve impedance and phase, and the simple DC relationships do not capture them. Use an impedance calculator for capacitors and inductors.