Ohm’s Law Calculator
Enter any two of voltage, current, and resistance to solve for the third, plus the power the circuit dissipates.
Circuit values
Reading the Ohm’s law triangle
Ohm’s law, V = I × R, is the single most useful equation in beginner electronics. Voltage (V, in volts) is the electrical “push” from a battery or supply, current (I, in amps) is how much charge flows per second, and resistance (R, in ohms) is how much the circuit opposes that flow. Knowing any two lets you find the third with simple algebra: I = V / R, or R = V / I.
Power (P, in watts) tells you how fast that circuit turns electrical energy into heat or light, and it follows straight from the same three numbers: P = V × I. That’s why a resistor’s wattage rating matters as much as its ohm value — too small a resistor for the power it has to dissipate will overheat.
Frequently Asked Questions
What is Ohm's law?
Ohm's law states that voltage equals current times resistance: V = I × R. It describes how the three quantities relate in a simple resistive DC circuit — raise the voltage and current rises with it; raise the resistance and current falls.
How do I find power once I know V, I, and R?
Power in watts is P = V × I. Because V = I × R, that same power can also be written as P = I² × R or P = V² / R — all three forms give the same answer, so use whichever two values you already have.
Why does the calculator only need two values?
Voltage, current, and resistance are locked together by V = I × R, so any two of them fix the third. Entering all three would be over-specified (and might not agree with each other), which is why this tool asks for exactly two.
Does Ohm's law apply to LEDs and batteries the same way?
Ohm's law describes ideal resistors precisely. Real components like LEDs and batteries are more complex — an LED's voltage barely changes with current once it's conducting, and a battery has its own small internal resistance — but Ohm's law is still the starting point for sizing a series resistor or estimating current draw.
Educational tool for low-voltage DC circuits only. Verify with a multimeter before relying on a result, and never apply this to household mains wiring or high-voltage equipment.