Ohm's law calculator: voltage, current or resistance

Example: a 9 V battery drives a current of 300 mA through a resistor. Convert first: 300 mA = 0.3 A. Then $R = 9 \div 0.3 = 30$ Ω.

For a resistor at a constant temperature, current is directly proportional to potential difference, so its I–V graph is a straight line through the origin. Other components behave differently:

• Filament lamp: resistance increases as it gets hotter, so the graph curves.
• Diode: current flows in one direction only; resistance is very high in the reverse direction.
• Thermistor: resistance falls as temperature rises.
• LDR (light-dependent resistor): resistance falls as light intensity increases.

$V = IR$ still gives the resistance at any one point.

• Series: the total resistance is the sum, $R_{\text{total}} = R_1 + R_2$. The current is the same everywhere.
• Parallel: the total resistance is less than the smallest single resistance. For two resistors, $\dfrac{1}{R_{\text{total}}} = \dfrac{1}{R_1} + \dfrac{1}{R_2}$ (used in Cambridge IGCSE Extended and higher-level courses).

See series and parallel circuits for worked examples.

• Forgetting to convert milliamps (divide by 1,000) or kilohms (multiply by 1,000).
• Rearranging incorrectly, for example $R = I \div V$.
• Using the battery's voltage for one component in a series circuit, when the potential difference is shared between components.

$V = IR$ is one of the most used equations in GCSE and IGCSE electricity, alongside power ($P = IV$) and charge ($Q = It$). Practical questions often ask you to find resistance from a table of voltmeter and ammeter readings. Read our Ohm's law guide for more.