Ohm's law calculator

Ohm’s law ties the three quantities you can actually measure on a bench into one relationship. Give it any two and the third is fixed.

Voltage is in volts, current in amperes and resistance in ohms. Mixing units is where most errors come from: a 4.7 kΩ resistor is 4700 Ω, and 20 mA is 0.02 A. If you enter 20 instead of 0.02 you get an answer that is off by a factor of a thousand and still looks plausible.

Working through an example

Suppose you have a 9 V battery and a 330 Ω resistor and you want the current. Rearranged, I = V / R = 9 / 330 = 0.0273 A, or 27.3 mA. The power the resistor turns into heat is P = V × I = 9 × 0.0273 = 0.246 W. A common quarter-watt resistor is rated for 0.25 W, so this part is running right at its limit and will get hot. Use a half-watt part instead, or raise the resistance.

Power, and why it matters more than people expect

Power has three equivalent forms, and which one you reach for depends on what you already know:

The squared terms are the reason small changes in current matter so much. Double the current through a fixed resistor and you get four times the heat. This is why a resistor that is fine at 5 V can fail at 12 V in the same circuit.

Where the law does not apply

Ohm’s law describes ohmic components — resistors, and wire at ordinary temperatures. It does not describe:

• Diodes and LEDs. Current rises exponentially with voltage past the forward drop. You size a series resistor instead of solving for the LED itself.
• Transistors. The channel or junction resistance depends on the control terminal, not just the voltage across it.
• Filaments and thermistors. Resistance moves with temperature, so the cold value and the hot value differ, sometimes by an order of magnitude.
• Anything at AC with reactance. Capacitors and inductors need impedance, which has a phase term Ohm’s law does not carry.

Quick reference