Ohm's Law Calculator

Ohm's law states that the voltage across a resistor equals the current through it multiplied by its resistance: V = IR. Rearranged, it also gives current (I = V/R) and resistance (R = V/I), which is what this calculator switches between.

It works exactly for an ideal resistor, which is why R stays constant however V or I changes. Many real components — diodes, LEDs, transistors — are non-linear, so their resistance isn't fixed and Ohm's law only gives an approximate, instantaneous relationship for them, not a fixed R you can look up.

Voltage in volts (V), current in your choice of µA, mA or A, and resistance in your choice of Ω, kΩ or MΩ. Pick the unit that matches the value on your component or datasheet — the calculator converts internally before computing the answer.

Check the units on your current and voltage first — a decimal point or unit prefix error (typing 20 instead of 0.02 for 20 mA, for instance) is the most common cause of an answer that's out by a factor of 1000.

Yes, in the same form, but resistance is replaced by impedance (Z), which accounts for the phase shift capacitors and inductors introduce. This calculator only handles pure resistance (V = IR); for reactive AC components you need V = IZ with Z as a complex number.

Ohm's law describes the relationship between V, I and R for a single component. Kirchhoff's current and voltage laws describe how currents and voltages combine across a whole circuit. You typically need both together to solve anything beyond a single resistor.