Ohm's Law Calculator
Enter any two of the three values — voltage, current, or resistance — and the calculator finds the third using Ohm's law, then reports the power dissipated as a bonus.
Ohm's law says voltage, current, and resistance in a simple circuit are related by \(V = IR\). Knowing any two, you can solve for the third. The power dissipated in a resistor is \(P = VI\) (which can also be written as \(I^2 R\) or \(V^2 / R\) by substitution).
The Formulas
| To find | Use |
|---|---|
| Voltage | \(V = IR\) |
| Current | \(I = V / R\) |
| Resistance | \(R = V / I\) |
| Power | \(P = VI = I^2 R = V^2 / R\) |
The three power forms are all equivalent — substitute one of \(V = IR\) or \(I = V/R\) into \(P = VI\) to derive the others.
Worked Example
A \(12\) V battery drives a current of \(2\) A through a resistor. What's the resistance, and how much power is dissipated?
\[R = \dfrac{V}{I} = \dfrac{12}{2} = 6 \text{ Ω}\]
\[P = V I = (12)(2) = 24 \text{ W}\]
Tips for Using the Calculator
- Keep units consistent. Standard SI is volts (V), amperes (A), and ohms (Ω); everything else (milliamps, kilohms) should be converted first.
- Resistance can't be negative for an ordinary resistor — if your math produces \(R < 0\), recheck the signs on \(V\) and \(I\).
- For AC circuits, Ohm's law still applies in instantaneous form, but RMS values are what most calculators expect when "voltage" and "current" are quoted without further qualification.
Frequently Asked Questions
What units should I use?
The calculator assumes SI base units throughout: volts for voltage, amperes for current, and ohms for resistance. Power comes out in watts. If your problem gives you milliamps (mA), millivolts (mV), or kilohms (kΩ), convert first: \(1\) mA = \(0.001\) A, \(1\) kΩ = \(1000\) Ω, and so on.
Does Ohm's law work for non-resistive elements?
Strictly, no. Ohm's law applies to ohmic materials, where the current is proportional to the voltage. Most metal resistors at moderate temperatures are ohmic, but diodes, transistors, light bulbs (because their resistance changes with temperature), and electrolytes are not. For those, the V-I relationship is nonlinear and requires a different model.
What about series and parallel resistor combinations?
This calculator handles a single resistor. To combine resistors first, use the standard rules:
- Series: \(R_{\text{total}} = R_1 + R_2 + R_3 + \cdots\)
- Parallel: \(\dfrac{1}{R_{\text{total}}} = \dfrac{1}{R_1} + \dfrac{1}{R_2} + \cdots\)
Find the equivalent resistance, then plug that into this calculator with the voltage or current.
Why are there three formulas for power?
They're algebraically identical, just rearranged. Start with \(P = VI\). Substitute \(V = IR\) and you get \(P = I^2 R\). Substitute \(I = V/R\) instead and you get \(P = V^2 / R\). Use whichever form uses the variables you already know — no extra calculations needed.
What happens if resistance is zero?
A perfect zero-resistance path is a short circuit. By \(I = V/R\), the current would be infinite, which in practice means the current is limited only by the source's own internal resistance — usually large enough to overheat the source or trip a breaker. This calculator returns an error for \(R = 0\) when solving for current.