calc.ing

Ohm's Law Calculator

Solve voltage, current, resistance and power.

V
3.3 V5 V12 V120 V230 V
Ω
101001K10K100K
A
0.01 A0.1 A0.5 A1 A5 A
Power
1.4400 W

What does Ohm's Law mean?

Ohm's Law is a fundamental principle in electrical engineering that describes the relationship between voltage (V), current (I), and resistance (R) in an electrical circuit. It states that the current flowing through a conductor is directly proportional to the voltage across it and inversely proportional to the resistance. Pair it with the power equation P = V × I and the same three numbers also tell you how many watts a component has to dissipate — which is why this calculator shows power alongside voltage, current, and resistance. Together they are essential for designing circuits, sizing resistors, and troubleshooting electrical systems.

How to calculate Ohm's Law

Ohm's Law is expressed as V = I × R. Rearranged, it gives current I = V / R and resistance R = V / I. Enter any two of the three values above and the calculator solves for the third as you type, then reports the power in watts.

For example, a 12 V battery connected to a 100 Ω resistor draws 12 / 100 = 0.12 A (120 mA), and dissipates 12 × 0.12 = 1.44 W.

How to calculate power

Power is the rate at which a circuit converts electrical energy to heat or work, measured in watts (W). It has three interchangeable forms, because Ohm's Law lets you substitute any one of V, I, or R:

  • P = V × I — when you know voltage and current (the form this calculator uses).
  • P = I² × R — when you know current and resistance, e.g. sizing a series resistor.
  • P = V² / R — when you know voltage and resistance, e.g. a resistor across a supply rail.

All three give the same answer for the same circuit. For the 12 V / 100 Ω example: 12 × 0.12 = 1.44 W, 0.12² × 100 = 1.44 W, and 12² / 100 = 1.44 W. Pick a resistor rated comfortably above that figure — a common rule of thumb is at least twice the calculated power.

FAQ

Ohm's Law states that the current through a conductor between two points is directly proportional to the voltage across the two points and inversely proportional to the resistance. The formula is V = I × R, where V is voltage in volts, I is current in amperes, and R is resistance in ohms.

Multiply them: P = V × I, where P is power in watts, V is voltage in volts, and I is current in amperes. For example, 12 V at 0.12 A dissipates 12 × 0.12 = 1.44 W. This calculator applies P = V × I automatically, so once voltage, current, and resistance are consistent it shows the power in watts under the inputs.

Voltage is measured in volts (V), current in amperes or amps (A), and resistance in ohms (Ω). Power, which can be derived from Ohm's Law, is measured in watts (W). For small values, milliamps (mA) and kilohms (kΩ) are commonly used.

Use P = V² / R when you know voltage and resistance, or P = I² × R when you know current and resistance. Both are derived from P = V × I by substituting Ohm’s Law. For instance, 12 V across 100 Ω gives 12² / 100 = 1.44 W, and the same circuit at 0.12 A gives 0.12² × 100 = 1.44 W.

Ohm's Law applies to ohmic (linear) conductors where resistance remains constant regardless of voltage. Many real-world components like resistors follow this law. However, non-ohmic devices such as diodes, transistors, and thermistors have variable resistance and do not strictly follow Ohm's Law.

If resistance approaches zero (a short circuit), the current becomes extremely high, which can damage components or cause fires. Circuit protection devices like fuses and circuit breakers are used to prevent damage from excessive current caused by very low resistance paths.

Use Ohm's Law: V = I × R. To find voltage, multiply current (amps) by resistance (ohms). To find current, divide voltage by resistance: I = V / R. To find resistance, divide voltage by current: R = V / I. For example, with 12 V across a 4 Ω resistor, the current is 12 / 4 = 3 A, and the power is 12 × 3 = 36 W. Enter any two known values above and the calculator will solve for the third.

Related calculators

  • Force— Solve F = ma for force, mass or acceleration.
  • Kinetic Energy— Energy of motion.
  • Density— Mass per volume.
  • Pressure— Solve P = F/A for pressure, force or area.