Ohm's Law Calculator

Ohm's Law states that the voltage across a resistor is proportional to the current through it: V = I × R. Formulated by Georg Ohm in 1827, it is the foundational relationship for analysing DC resistive circuits. The same relationship gives power dissipation: P = V × I = I² × R = V² / R.

In a DC circuit, resistance (R) is the only opposition to current. In an AC circuit, inductors and capacitors also oppose current: this is called reactance (X). The combined opposition is impedance (Z), and Ohm's Law still holds in the form V = I × Z, but Z is a complex number. This calculator handles DC resistive circuits only.

Not directly. LEDs are non-linear: their effective resistance changes with current. The correct approach is to account for the LED's forward voltage drop (typically 1.8–3.5 V depending on colour) and calculate the series resistor from the remaining voltage. Use the LED Resistor Calculator for that.

Every resistor converts electrical energy to heat. If the dissipated power exceeds the component's rating, it will overheat and fail. Standard through-hole resistors are rated ¼ W or ½ W. Always choose a resistor rated at least 2× the calculated power (standard derating practice) to ensure reliability and thermal safety.

Most conductive materials have a positive temperature coefficient: resistance rises as the material heats up, because increased atomic vibration impedes electron flow. This is quantified in ppm/°C and matters in precision voltage references, oscillators, and sensor circuits. Metal-film resistors (50–100 ppm/°C) are significantly more stable than carbon-film (200–500 ppm/°C).

Ohm's Law applies to linear, time-invariant resistive elements. It breaks down for non-linear devices (diodes, transistors, varistors), and at high frequencies where parasitic inductance and capacitance in component leads and PCB traces dominate. It also assumes steady-state conditions. During transients, capacitors and inductors draw current in ways a simple R cannot model.