Ohm's Law Calculator

About the Ohm's Law Calculator

Ohm's law is the foundation of electrical circuits. It links three quantities: voltage, the electrical "pressure" that pushes charge around a circuit; current, the rate at which charge flows; and resistance, how strongly a component opposes that flow. Add the power formula, which tells you how much energy per second a component uses or turns into heat, and you can work out almost anything about a simple circuit.

This Ohm's law calculator needs just two of the four values — voltage, current, resistance and power — and calculates the other two. It accepts current in amps or milliamps, resistance in ohms, kilohms or megohms, and power in watts, milliwatts or kilowatts, and shows results with sensible prefixes such as mA and kΩ.

How to Use the Ohm's Law Calculator

Enter any two of the four values: voltage, current, resistance or power.

Leave the other two blank or at 0.

Choose the units for current, resistance and power.

The Formulas

  Ohm's law      V = I × R       I = V ÷ R       R = V ÷ I

  power          P = V × I = I² × R = V² ÷ R

  V = voltage (volts, V)        I = current (amps, A)
  R = resistance (ohms, Ω)      P = power (watts, W)

Step-by-Step Examples

12 V across 4 Ω.

  I = 12 ÷ 4 = 3 A
  P = 12 × 3 = 36 W

A 2,000 W kettle on 230 V.

  I = 2,000 ÷ 230 = 8.6957 A
  R = 230² ÷ 2,000 = 26.45 Ω

Limiting 9 V to 20 mA.

  R = 9 ÷ 0.02 = 450 Ω
  P = 9 × 0.02 = 0.18 W = 180 mW

A 60 W, 240 Ω lamp.

  V = √(60 × 240) = 120 V
  I = √(60 ÷ 240) = 0.5 A

Understanding Voltage, Current and Resistance

A common analogy compares electricity to water in pipes. Voltage is like water pressure, pushing the flow along. Current is like the rate of flow, measured in litres per second for water and amperes — coulombs of charge per second — for electricity. Resistance is like a narrowing in the pipe, restricting the flow. Raise the pressure and more water flows; narrow the pipe and less flows. Ohm's law says the current is directly proportional to the voltage and inversely proportional to the resistance.

Power and Heat

Whenever current flows through a resistance, energy is converted to heat at a rate given by P = I²R. That is useful in kettles, toasters and electric heaters, but wasteful in wires and electronics, where it must be kept low. It is also why components have power ratings: a resistor rated at a quarter of a watt will overheat if made to dissipate much more. The same formula explains why electricity is transmitted at very high voltages: for the same power, a higher voltage means a lower current, and the I²R losses in the cables fall dramatically.

Where Ohm's Law Applies

Ohm's law describes ohmic materials: metals and ordinary resistors whose resistance stays constant as long as the temperature does. Many components are non-ohmic. A filament bulb's resistance rises as it heats up, so its current does not rise in proportion to voltage. Diodes and LEDs barely conduct until a threshold voltage is reached, then conduct very easily, which is why an LED always needs a series resistor to limit the current. Semiconductors, batteries and electric motors all behave in more complicated ways. Even so, the ratio V ÷ I still gives a useful "resistance" at a particular operating point.

Georg Ohm

The law is named after the German physicist Georg Simon Ohm, who published it in 1827 after careful experiments with wires of different lengths and thicknesses. His work was not well received at first, but it was eventually recognised as fundamental, and the unit of resistance, the ohm, symbol Ω, was named in his honour. One ohm is the resistance that lets one amp flow when one volt is applied.

Electrical Safety

Mains electricity at 230 V or 120 V can be lethal, because even a small current through the body — a few tens of milliamps — can disturb the heart. The body's resistance falls sharply when skin is wet, which is why water and electricity are so dangerous together. Fuses and circuit breakers protect wiring by cutting the supply when the current exceeds the rating, and residual current devices protect people by detecting leakage currents of around 30 mA. Always use properly rated plugs, cables and fuses, and leave mains work to qualified people.

Series and Parallel Resistors

Ohm's law becomes even more useful combined with the rules for connecting resistors. In series, one after another, resistances add: 100 Ω and 220 Ω make 320 Ω, and the same current flows through both. In parallel, side by side, the reciprocals add, so the total is always less than the smallest resistor: two 100 Ω resistors in parallel make 50 Ω, and the voltage across each is the same. With these rules and Ohm's law, the current and voltage anywhere in a simple circuit can be found step by step.

Understanding Your Result

The headline shows all four values together.

The voltage, current, resistance and power lines give each one separately, with the power also expressed as energy used in an hour.

When Should You Use This Calculator?

Use it for physics and electronics homework.

Use it to choose a resistor for an LED or a sensor circuit.

Use it to work out the current an appliance draws.

Common Mistakes

Entering more or fewer than two values. Enter exactly two.

Forgetting to convert milliamps. 20 mA is 0.02 A; the calculator converts for you.

Ignoring power ratings. Resistors overheat if they dissipate too much power.

Applying Ohm's law to non-ohmic parts. LEDs and bulbs do not have constant resistance.