Ohm's Law Calculator

What is Ohm's Law?

Ohm's Law describes the relationship between voltage, current, and resistance in an electrical circuit. It helps you find any one of these values when the other two are known.

Voltage is measured in volts (V), current in amperes (A), and resistance in ohms (Ω).

Ohm's Law formulas

Rearrange the main equation to solve for each quantity:

Voltage, current, and resistance

Voltage is the electrical potential difference that pushes charge through a circuit. Current is the rate at which charge flows. Resistance opposes that flow. With resistance held constant, increasing voltage increases current; with voltage held constant, increasing resistance decreases current.

Electrical power formulas

Power measures how quickly electrical energy is transferred. Power is measured in watts (W), and it can be calculated using voltage, current, and resistance:

Worked examples

To find current from 12 V and 4 Ω:

To find power from 2 A and 10 Ω:

To find resistance from 120 V and 60 W:

Unit conversions

The calculator converts entered values to base SI units before calculating. For example, 4 kΩ is 4,000 Ω, 12 V ÷ 4,000 Ω is 0.003 A, and that is 3 mA. It also converts mV, kV, µA, MΩ, mW, and kW automatically.

Frequently asked questions

How the calculator works

Choose the quantity you want to find from the "Solve for" menu, then enter any two of the remaining three. The calculator converts each input to its base unit (volts, amperes, ohms, or watts), picks the first formula that fits the values you provided, and fills in the unknown. Because any two quantities fix the other two, it also derives the fourth value and lists all four in the results panel.

Each field has its own unit menu: V, mV, or kV for voltage; A, mA, or µA for current; Ω, kΩ, or MΩ for resistance; and W, mW, or kW for power. The answer is displayed in whichever unit is selected for the calculated field, so you can enter 5 V and 250 Ω and read the current directly in mA. Results are rounded to four decimal places, and very small or very large values (below 0.0001 or above 1 billion) switch to scientific notation.

Resistance must be greater than zero and power cannot be negative; values that break those rules are ignored rather than producing nonsense. Voltage and current may be negative, which is useful when you are tracking polarity in a circuit.

Ohm's law and the power formula

Ohm's law says the current through a conductor is proportional to the voltage across it and inversely proportional to its resistance. The constant of proportionality is the resistance itself. Electrical power is the rate at which energy is delivered and equals voltage times current. Combining the two gives the alternative power forms P = I² × R and P = V² ÷ R.

Worked example: a 12 V battery connected across a 4 Ω resistor. Current is I = 12 ÷ 4 = 3 A. Power dissipated in the resistor is P = 12 × 3 = 36 W. Checking with the other forms gives the same answer: I² × R = 9 × 4 = 36 W, and V² ÷ R = 144 ÷ 4 = 36 W.

All twelve rearrangements

The classic Ohm's law wheel lists three ways to find each quantity, one for each pair of known values. These are exactly the formulas the calculator tries, in this order:

• Voltage: V = I × R, V = P ÷ I, V = √(P × R)
• Current: I = V ÷ R, I = P ÷ V, I = √(P ÷ R)
• Resistance: R = V ÷ I, R = P ÷ I², R = V² ÷ P
• Power: P = V × I, P = I² × R, P = V² ÷ R

Units, prefixes, and the most common slip

The SI units are the volt (V) for potential difference, the ampere (A) for current, the ohm (Ω) for resistance, and the watt (W) for power. Real circuits rarely use round numbers of those units, so metric prefixes are everywhere: µ (micro) is one millionth, m (milli) is one thousandth, k (kilo) is one thousand, and M (mega) is one million. A 4.7 kΩ resistor is 4,700 Ω; a 20 mA LED current is 0.02 A.

The most frequent mistake is entering a value in the wrong prefix, typically typing 20 for current while the menu still says A instead of mA. That error makes every downstream number a thousand times too large. As a sanity check, 5 V across 250 Ω gives 0.02 A, which is 20 mA, and dissipates 5 × 0.02 = 0.1 W, or 100 mW.

When Ohm's law applies

Ohm's law holds for ohmic materials, which means most metals, wire, and ordinary resistors at a steady temperature. It does not hold for components whose resistance changes with voltage or temperature: diodes and LEDs, incandescent filaments, thermistors, and semiconductors. For those you use the manufacturer's curve rather than a single resistance value, although the power formula P = V × I is still valid.

The formulas here are for direct current or for purely resistive AC circuits. Once inductors or capacitors are involved in an AC circuit, resistance is replaced by impedance and the phase between voltage and current matters. Finally, use the power result to pick a component rating: a resistor expected to dissipate 100 mW should be rated at 250 mW (1/4 W) or more to leave a safety margin.

Frequently asked questions

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