Ohm's Law Calculator
Solve voltage, current, resistance and power.
Enter any two of the three.
Results update as you type.
How it works
Ohm's law is the single most important relationship in electronics, tying together voltage, current, and resistance in one clean equation. This ohms law calculator lets you enter any two of the three quantities and instantly returns the missing one along with the power dissipated. Voltage is the electrical pressure pushing charge through a circuit, current is the rate of charge flow, and resistance is how strongly the material opposes that flow. Understanding this trio matters because almost every practical circuit decision depends on it: sizing a resistor to protect an LED, choosing wire gauge so it does not overheat, or confirming a power supply can deliver enough current. When you know two values, the third is fixed, so a good ohms law calculator removes the mental arithmetic and the risk of a slipped decimal that could fry a component. The formula works because resistance behaves as a constant ratio between voltage and current in ohmic materials. Hobbyists, electricians, and engineers all reach for an ohms law calculator daily, whether they are debugging a breadboard, planning household wiring, or teaching the fundamentals of electricity to students for the first time.
V = I × R · P = V × I
Ohm's law says the current through a conductor is proportional to the voltage across it and inversely proportional to its resistance, because resistance is the constant of proportionality that limits electron flow. Rearranging V = I × R lets you recover whichever quantity is unknown. Power follows as P = V × I, since power is the rate at which each coulomb of charge delivers its voltage-worth of energy.
Source:Standard physics (Ohm's law)
We review all our tools from time to time to ensure accuracy.
Worked example
How to use this calculator
- 1
Enter your values
Enter your values in the fields above.
- 2
Adjust the options
Adjust voltage, current and resistance if they apply — sensible defaults are already set.
- 3
Read your result
Power updates instantly as you type — no button to press.
Frequently asked questions
How many values do I need to enter?
You need to enter exactly two of the three quantities — voltage, current, or resistance. From those two the calculator solves the remaining unknown using Ohm's law and then computes power. If you enter only one value the system cannot determine the circuit, and if you enter all three inconsistently the power figure will simply reflect the numbers you typed rather than a corrected result.
What units does this calculator use?
Voltage is in volts (V), current in amperes (A), resistance in ohms (Ω), and power in watts (W). These are the standard SI units for electrical circuits. If your values are in milliamps or kilohms, convert them first: 1,000 mA equals 1 A and 1,000 Ω equals 1 kΩ. Keeping everything in base units ensures the power result comes out correctly in watts.
How is power calculated from voltage and current?
Power equals voltage multiplied by current, written P = V × I. This tells you how fast electrical energy is being converted to heat, light, or motion. You can also express power as I² × R or V² ÷ R by substituting Ohm's law. Power matters when sizing components, because a resistor must be rated to dissipate at least the wattage it will actually handle without overheating.
Does Ohm's law apply to every component?
Ohm's law applies cleanly to ohmic materials like most resistors and metal conductors, where resistance stays constant regardless of voltage. It does not hold for non-ohmic components such as diodes, transistors, and light bulbs, whose resistance changes with temperature or applied voltage. For those parts you use their specific characteristic curves, but Ohm's law still governs the resistors and wiring around them in the circuit.
What happens if resistance is zero?
A resistance of zero represents a short circuit, and Ohm's law predicts that current would become infinite for any non-zero voltage since I = V ÷ R. In reality wires have small resistance, so current spikes to a very high value that can blow fuses, damage components, or start a fire. That is exactly why circuits include fuses and current-limiting resistors to cap the flow.