About this calculator
Ohm's Law is the relationship every other piece of electronics is built on: the current through a resistor is proportional to the voltage across it. Written out, that is V = I × R. Pair it with the power equation P = V × I and any two of voltage, current, resistance and power are enough to pin down the other two.
This calculator does exactly that. Type in the two values you know, leave the rest blank, and it solves for everything else — so you never have to rearrange the formula in your head or remember which version of the power equation you need.
How it works
There are four quantities and two independent equations linking them, which means twelve valid rearrangements in total. That is what the classic "Ohm's Law wheel" on workshop posters is showing. The calculator picks whichever rearrangement matches the pair you entered.
If you give it voltage and resistance, it divides to get current, then squares the voltage over resistance to get power. Give it power and current instead, and it works backwards to voltage and resistance. Any of the six possible pairings works.
Everything is in base SI units: volts, amps, ohms and watts. A 20 mA LED current is entered as 0.02, and a 10 kΩ resistor as 10000. If you would rather work in milliamps and kilohms, run the numbers through the unit converter first.
Worked example
You have a 220 Ω resistor across a 5 V supply. What current flows, and is a ¼ W resistor big enough?
- Known: V = 5 V, R = 220 Ω
- I = V / R = 5 / 220 = 0.0227 A = 22.7 mA
- P = V² / R = 25 / 220 = 0.114 W = 114 mW
The resistor passes 22.7 mA and dissipates 114 mW. A ¼ W (250 mW) part handles that with roughly a 2× margin, which is about right — see the notes on derating below.
Practical notes
- Enter base units.
20 mAis0.02,4.7 kΩis4700,1 MΩis1000000. - Ohm's Law describes resistors. Diodes, LEDs and transistors are non-linear — their resistance changes with the voltage across them, so you cannot pick a single R and expect it to hold.
- Size resistors for roughly twice the calculated dissipation. A part running at its full rating gets hot enough to scorch a board and drift out of tolerance.
- The law works for AC as long as you use RMS values and the load is purely resistive. Once capacitors or inductors are involved you need impedance, which varies with frequency.
- Resistance drifts with temperature. For precision work, check the temperature coefficient (ppm/°C) on the datasheet.
Frequently asked questions
What is Ohm's Law in simple terms?
Push harder and more current flows; add resistance and less current flows. Voltage is the push, resistance is the restriction, and current is what actually moves. Doubling the voltage across a fixed resistor doubles the current through it.
How do I calculate the power a resistor dissipates?
Use P = V² / R if you know the voltage across the resistor, or P = I² × R if you know the current through it. Both give watts. Compare that to the resistor's power rating and pick a part rated at least twice as high.
Does Ohm's Law apply to LEDs?
Not to the LED itself — an LED's forward voltage stays roughly constant while its current climbs steeply, so it has no fixed resistance. It does apply to the series resistor you put with the LED, which is exactly how the LED resistor calculator sizes that part.
Why does the calculator need two values?
One value leaves the system underdetermined. Knowing only that a circuit runs at 5 V tells you nothing about its current until you also know the resistance or the power. Any two of the four quantities is enough.
Does Ohm's Law work with AC?
For resistive loads, yes — use RMS voltage and RMS current. For anything reactive, replace resistance with impedance (Z), which depends on frequency. At that point the RC and LC calculators are the better tools.