Ohm's Law Calculator

This Ohm's Law calculator solves for voltage, current, resistance, and power in a DC electrical circuit. Enter any two of the four values and it instantly derives the remaining two, along with the power dissipated. It is built for students revising physics, electronics hobbyists sizing a series resistor for an LED, electricians checking load on a circuit, and engineers who need a quick sanity-check without reaching for a notebook. Because the relationships are fixed by physics, the same tool works whether you are wiring a 5 V breadboard project or analysing a 230 V Indian mains appliance.

Ohm's Law, published by German physicist Georg Simon Ohm in 1827, states that the current through a conductor between two points is directly proportional to the voltage across those points and inversely proportional to the resistance. In symbols that is V = I × R. The constant of proportionality, resistance, is what makes a material oppose the flow of charge. The law is empirical and applies to ohmicmaterials — metals and ordinary resistors whose resistance stays effectively constant across the operating range. It does not hold for non-ohmic components such as diodes, transistors, or filament lamps once they heat up, where resistance changes with voltage.

Core Formulas

V = I × R — Voltage (Volts) = Current (Amps) × Resistance (Ohms)

P = V × I — Power (Watts) = Voltage × Current

Derived: P = I²R = V²/R, I = V/R = P/V, R = V/I = V²/P

The two base equations combine into the "Ohm's Law wheel" — twelve rearrangements that let you reach any one quantity from any two others. The power forms P = I²R and P = V²/R are especially useful: they let you calculate heat dissipation without first solving for the missing voltage or current, which is exactly what you need when choosing a resistor's wattage rating.

The power triangle

A second memory aid, the power triangle, places P on top with V and I below it: cover P to get V × I, cover V to get P ÷ I, and cover I to get P ÷ V. Used together with the voltage triangle above, these two shapes recover every formula on the wheel without rote memorisation. Keep your units consistent — volts, amperes, and ohms — and the answers come out in watts. If a current is given in milliamps (mA), convert to amps first (divide by 1000); a stray factor of a thousand is the single most common arithmetic slip in these problems.

Worked example

Suppose a resistor has 12 V across it and 2 Aflowing through it. Resistance is R = V ÷ I = 12 ÷ 2 = 6 Ω. Power dissipated is P = V × I = 12 × 2 = 24 W, which you could equally get from P = I²R = 2² × 6 = 24 W. So this component must be rated for at least 24 W, and in practice you would choose a part with comfortable headroom (say 50 W) so it runs cool. Enter 12 in Voltage and 2 in Current above to see these same numbers produced step by step.

Tips and common mistakes

Tip: when sizing a resistor, the power rating matters as much as the resistance value — an under-rated resistor overheats, drifts, and eventually burns out. Tip: for AC mains work, use impedance (Z) in place of resistance and remember that quoted mains voltages are RMS values. Common mistake: mixing units, such as plugging in kilo-ohms while treating the result as ohms. Common mistake: applying Ohm's Law to a diode or LED directly — an LED is non-ohmic, so you size the series resistor using the supply voltage minus the LED's forward voltage, not the supply voltage alone. Common mistake: forgetting that resistance itself rises with temperature in real conductors, so a cold-circuit calculation slightly under-estimates resistance once the part warms up.