Ohm's Law Calculator
Calculate voltage, current, resistance, or power using Ohm's Law. Enter any two values to find the other two.
Ohm's Law is the foundational relationship in electrical circuits. It comes up every time you are sizing a resistor, calculating power consumption, checking a fuse rating, or troubleshooting a circuit. The law is simple, but having all four derived relationships in one calculator saves the mental juggling of which formula to use when you are starting from different knowns.
You only need two values to find the other two. V and R gives you I and P. V and I gives you R and P. I and R gives you V and P. P and V gives you I and R.
The four Ohm's Law relationships
The core law: V = I x R (voltage equals current times resistance). From this, all four variables are derivable from any other two: I = V / R, R = V / I. Power adds a second relationship: P = V x I = I^2 x R = V^2 / R. With any two knowns from the set {V, I, R, P}, the other two are determined. This calculator handles all combinations.
Practical applications
Resistor selection: you have a 9V supply and an LED that needs 20mA (0.02A) with a 2V forward voltage. Voltage across resistor = 9 - 2 = 7V. R = 7 / 0.02 = 350 ohms, so you choose a 330 ohm or 360 ohm resistor from standard values. Power: P = 7 x 0.02 = 0.14W, so a 1/4W resistor is fine. Fuse sizing: if a circuit draws 3A at 12V, a 5A fuse provides reasonable protection. Wire sizing: higher current requires lower resistance wire to avoid voltage drop and heat.
AC vs DC
Ohm's Law applies directly to DC (direct current) circuits and to resistive AC circuits. For reactive AC circuits (with capacitors or inductors), impedance replaces resistance and the relationships become more complex (involving phase angles and complex numbers). For audio electronics, RF circuits, and power factor calculations, impedance (Z) is used in place of R. For basic circuit analysis and most hobbyist electronics work, Ohm's Law with resistance is sufficient.
Frequently asked questions
What is a kilohm and a megaohm?
1 kilohm (kOhm) = 1,000 ohms. 1 megaohm (MOhm) = 1,000,000 ohms. Resistors in electronics are commonly rated in ohms, kilohms, and megaohms. Enter values in ohms for this calculator (e.g., 4,700 for a 4.7kOhm resistor).
What is a milliamp?
1 milliamp (mA) = 0.001 amps. LEDs typically operate at 10-30mA. Enter in amps for this calculator (e.g., 0.02 for 20mA).