How to Calculate Current with Ohm's Law

Calculates current with Ohm's law I = voltage ÷ resistance. Voltage is in volts, resistance in ohms, and current in amperes.

Explanation

Given the voltage across a component and its resistance, Ohm's law gives the current flowing through it.

I = \dfrac{V}{R}

$I$ is the current, in amperes (A)
$V$ is the voltage, in volts (V)
$R$ is the resistance, in ohms (Ω)

At a fixed voltage the current is inversely proportional to the resistance: the more the component resists, the less flows.

Example

The defaults are a voltage of 100 V and a resistance of 50 Ω.

I = \dfrac{100}{50} = 2\,\mathrm{A}

The current is 2 A.

Notes

The resistance cannot be zero, since that would mean dividing by zero. Real circuits punish this too: give the current a path with almost no resistance and an enormous current flows. That is a short circuit, and it is dangerous.

Ohm's law is for ohmic conductors, where current stays proportional to voltage. Diodes and lamp filaments, whose resistance shifts with direction or with temperature, do not obey it directly.

Mind the prefixes. Multiply kilohms by 1000 to get ohms; volts divided by kilohms comes out in milliamps.

Mains voltage is 100 V in Japan and about 230 V across much of Europe. A 1000 W kettle on a 100 V supply draws about 10 A.