Ohms Law Calculator – V = I × R
Calculate voltage, current, resistance, or power using Ohms Law (V=IR).
Ohm's Law: The Foundation of Electronics
Ohm's Law states that the current through a conductor between two points is directly proportional to the voltage across those points: V = I × R (Voltage = Current × Resistance).
From this single equation, you can derive any of the three values if you know the other two: I = V/R (current); R = V/I (resistance). This makes Ohm's Law the starting point for almost every electronics calculation.
Power: The Fourth Variable
Adding Power (P, measured in Watts) extends Ohm's Law into the Power Triangle: P = V × I = I² × R = V²/R. These relationships allow you to calculate any electrical quantity from any two known values.
For example, a 12V LED drawing 0.5A consumes 6W of power. A 100W light bulb on 120V draws 0.83A and has a resistance of 144 Ω when operating.
Practical Applications
LED resistor sizing: To limit current to 20mA on a 5V supply with a 2V LED forward voltage: R = (5−2)/0.02 = 150 Ω. Fuse selection: Calculate maximum expected current to choose the right fuse rating. Power supply sizing: Sum all device power draws to size your supply. Wire gauge selection: Higher current requires lower resistance wire (larger gauge) to minimize heat and voltage drop.
Frequently Asked Questions
Does Ohm's Law apply to all components?
Ohm's Law applies to ohmic (linear) conductors where resistance is constant regardless of voltage. It does not apply to non-ohmic components like diodes, LEDs, and transistors, which have non-linear voltage-current relationships.
What is the unit of electrical resistance?
Ohms (Ω), named after Georg Simon Ohm who formulated the law in 1827. 1 Ohm = 1 Volt per Ampere. Practical resistances range from milliohms (wire) to megaohms (insulation).
What happens when resistance is zero?
Zero resistance with any voltage produces infinite current — a short circuit. In practice this causes overheating, damage, or fire. Fuses and circuit breakers are designed to interrupt the circuit before this causes catastrophic failure.