Calculate voltage, current, or resistance using Ohm's Law
Ohm's Law describes the fundamental relationship between three electrical quantities: voltage (V), current (I), and resistance (R). Published by Georg Simon Ohm in 1827, it states that the current through a conductor is directly proportional to the voltage and inversely proportional to the resistance:
V = I × R I = V ÷ R R = V ÷ I
Select what you want to calculate from the dropdown above, enter the two known values, and the result appears instantly.
A simple memory aid: draw a triangle divided into three sections. V at the top, I and R at the bottom. Cover the quantity you want to find — what remains shows the formula:
| Find | Cover | Formula | Example |
|---|---|---|---|
| Voltage (V) | V | I × R | 2A × 12Ω = 24V |
| Current (I) | I | V ÷ R | 24V ÷ 12Ω = 2A |
| Resistance (R) | R | V ÷ I | 24V ÷ 2A = 12Ω |
Combining Ohm's Law with the power formula P = V × I gives us a family of equations for calculating power in any configuration:
| Known values | Power formula |
|---|---|
| Voltage + Current | P = V × I |
| Current + Resistance | P = I² × R |
| Voltage + Resistance | P = V² ÷ R |
Ohm's Law holds for ohmic conductors — materials where resistance stays constant regardless of voltage or current. Most metals at constant temperature are ohmic. Copper wire, resistors, and most passive components obey Ohm's Law reliably.
Some components are non-ohmic: their resistance changes with voltage or current. LEDs have a forward voltage threshold and then drop very little additional voltage as current increases. Diodes conduct in one direction only. Thermistors change resistance with temperature. Ohm's Law cannot be directly applied to these components in the same way.
LED current limiting: To run an LED from a 5V supply, you need a series resistor. If the LED has a 2V forward voltage and should draw 20mA: available voltage = 5V − 2V = 3V. R = 3V ÷ 0.02A = 150Ω. Choose the nearest standard value, 150Ω or 180Ω.
Fuse selection: A 12V circuit powering a 60W device draws I = P ÷ V = 60 ÷ 12 = 5A. A 7.5A fuse would protect this circuit with adequate margin.
Wire sizing: A circuit drawing 8A continuous should use wire rated for at least 10A (125% of continuous load) — which corresponds to 1.5mm² copper in most wiring standards.
Ohm's Law states that voltage equals current multiplied by resistance (V = I × R). It defines the linear relationship between these three quantities for ohmic conductors at constant temperature.
Divide voltage by resistance. A 9V battery connected to a 180Ω resistor produces 9 ÷ 180 = 0.05A (50mA) of current.
Yes, for purely resistive AC loads (heaters, incandescent bulbs), Ohm's Law applies directly using RMS values of voltage and current. For circuits with capacitors or inductors, resistance is replaced by impedance (Z), which is frequency-dependent.
Georg Simon Ohm, a German physicist, published the law in 1827. His work was initially met with scepticism but was later recognised as a fundamental contribution to electrical science. The unit of resistance, the ohm (Ω), is named in his honour.
Not directly. Semiconductors like diodes and transistors are non-ohmic — their resistance changes with the applied voltage and current. Specialised device models (diode equations, transistor models) are used instead of simple Ohm's Law calculations.