Ohm's Law Calculator

The Triangle

Ohm's Law describes the fundamental relationship between voltage (V, in volts), current (I, in amps), and resistance (R, in ohms) in an electrical circuit. The core equation is V = I × R: voltage equals current multiplied by resistance. Rearranging gives the other two forms: I = V ÷ R (current equals voltage divided by resistance) and R = V ÷ I (resistance equals voltage divided by current). The 'Ohm's Law triangle' is a memory aid — write V at the top with I and R below; cover the quantity you want, and the position of the other two tells you whether to multiply or divide. Power (P, in watts) extends the law: P = V × I, and by substitution P = I² × R = V² ÷ R. These give you four interlinked quantities (voltage, current, resistance, power), and knowing any two lets you find the others. A worked example: a 12V supply across a 4-ohm resistor drives a current of I = 12 ÷ 4 = 3 amps, dissipating P = 12 × 3 = 36 watts. These relationships describe the behaviour of every simple resistive circuit and underpin practically all electrical and electronic design, from sizing a current-limiting resistor for an LED to understanding why thick cables are needed for high-current loads.

Everyday Application

Ohm's Law isn't just theory — it governs everyday electrical situations and helps explain device ratings and safety limits. A UK household circuit at 230V protected by a 13A fuse (as in a standard plug) can safely supply up to P = 230 × 13 ≈ 2,990 watts, which is why a 3kW kettle is about the most powerful single appliance on a standard plug. A USB charger outputting 5V at 2A delivers P = 5 × 2 = 10 watts. When choosing a resistor to protect an LED, you use Ohm's Law: if your supply is 5V, the LED drops about 2V and needs around 20mA (0.02A), the resistor must drop the remaining 3V, so R = 3 ÷ 0.02 = 150 ohms. Cable sizing depends on it too — current causes heating proportional to I²R, so high-current circuits need thicker (lower-resistance) cable to avoid overheating, and this is why long cable runs suffer voltage drop. Battery life, speaker impedance matching, fuse selection, and power-supply design all rest on these relationships. Understanding Ohm's Law lets you predict how much current a device will draw, whether a circuit is within safe limits, and how components will behave together.

Limitations

Ohm's Law, in its simple form, applies to 'ohmic' (linear) components — chiefly resistors — where resistance stays constant regardless of the voltage or current. For these, a graph of voltage against current is a straight line through the origin, and the relationships V = IR hold exactly. But many real components are 'non-ohmic', and the simple law doesn't fully describe them. A filament light bulb's resistance rises sharply as it heats up, so its current doesn't increase proportionally with voltage. Diodes conduct in only one direction and have a non-linear voltage-current curve with a 'turn-on' threshold. Transistors, thermistors (resistance varies with temperature), and LDRs (resistance varies with light) all change resistance with conditions. Semiconductors generally are non-linear. Ohm's Law also assumes direct current (DC) or treats AC simply; in AC circuits with capacitors and inductors, you need the broader concept of impedance (which includes frequency-dependent reactance) rather than plain resistance. And at extremes — very high voltages, very low temperatures (superconductivity), or very high frequencies — additional physics applies. For everyday resistive circuits, Ohm's Law is accurate and indispensable; for non-ohmic components and AC analysis, it's a starting point that needs extending. Knowing where it applies — and where it doesn't — is part of using it correctly.

Units and Significant Figures

Science calculations require consistent units throughout. Mixed units (some in metres, some in centimetres) are the most common source of errors. Always check: are all values in SI units (metres, kilograms, seconds, kelvin) before substituting into formulas? The number of significant figures in an answer should not exceed the least precise measurement used in the calculation. When quoting a result, scientific notation (e.g. 3.7 x 10 to the power 4) avoids ambiguity about significant figures and