How to Calculate Resistance with Ohm's Law

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

Explanation

Measure the voltage across a component and the current through it, and Ohm's law gives its resistance. This is how you turn readings from an experiment into a resistance.

R = \dfrac{V}{I}

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

Resistance measures how strongly a component opposes the flow of charge. The less current a given voltage can push through, the higher the resistance.

Example

The defaults are a voltage of 100 V and a current of 2 A.

R = \dfrac{100}{2} = 50\,\Omega

The resistance is 50 Ω.

Notes

The current cannot be zero, since that would mean dividing by zero.

$V$ and $I$ have to belong to the same component: the voltage across that resistor and the current through that resistor. Mixing the supply voltage with the current in one branch of a circuit gives a meaningless answer.

Plot current against voltage, and a straight line through the origin means the component is ohmic. The resistance is the reciprocal of the slope.

Resistance changes with temperature. A lamp filament resists far more when it is glowing than when it is cold, so note the conditions of the measurement.

Watch the prefixes. Divide milliamps by 1000 to get amps; volts divided by milliamps comes out in kilohms.