Ohm's Law Calculator
Computes electrical resistance from voltage and current using Ohm's Law. Ideal for electronics hobbyists, students, and engineers sizing resistors or diagnosing circuits.
About this calculator
Ohm's Law states that the resistance of a conductor equals the voltage across it divided by the current flowing through it: R = V / I. Rearranged, you can also find voltage (V = I × R) or current (I = V / R). The law holds for linear (ohmic) components at constant temperature — resistors, wire runs, and many passive devices. Resistance is measured in ohms (Ω), voltage in volts (V), and current in amperes (A). In practice, Ohm's Law is used to select the correct resistor value to limit LED current, calculate voltage drops across wiring, and verify circuit behaviour before powering up. Non-linear devices such as diodes and transistors do not strictly obey Ohm's Law.
How to use
Suppose a 12 V battery powers a circuit that draws 0.5 A. Enter Voltage = 12 V and Current = 0.5 A. The calculator computes R = V / I = 12 / 0.5 = 24 Ω. This tells you the total circuit resistance is 24 ohms. If you then want to add a resistor to limit current to 0.3 A from the same 12 V source, rearrange to find R = 12 / 0.3 = 40 Ω, so select the nearest standard value of 39 Ω or 47 Ω.
Frequently asked questions
What is Ohm's Law and how is it used in everyday electronics?
Ohm's Law (R = V / I) describes the relationship between voltage, current, and resistance in a circuit. It is one of the most fundamental tools in electronics, used to size current-limiting resistors for LEDs, calculate the voltage drop across a cable run, and troubleshoot malfunctioning circuits. Any time you know two of the three quantities, Ohm's Law instantly gives you the third. It applies reliably to resistors, heating elements, and conductive traces at normal operating temperatures.
Why does Ohm's Law not apply to all electronic components?
Ohm's Law assumes a linear relationship between voltage and current — resistance stays constant regardless of how much voltage is applied. Components like diodes, transistors, and thermistors have resistance that changes with voltage, current, or temperature, making them non-ohmic. For example, a diode allows current to flow freely in one direction but blocks it in the other, and its I-V curve is exponential, not linear. For these components, more advanced models such as the Shockley diode equation are required.
How do I use Ohm's Law to choose the right resistor for an LED?
LEDs require a specific forward current — typically 10–20 mA — and have a forward voltage drop of roughly 1.8–3.3 V depending on colour. Subtract the LED's forward voltage from the supply voltage to get the voltage across the resistor, then divide by the desired current: R = (V_supply − V_LED) / I_LED. For a 5 V supply, a red LED (2 V drop), and 20 mA current, R = (5 − 2) / 0.02 = 150 Ω. Always round up to the nearest standard resistor value to avoid exceeding the LED's current rating.