Ohm's Law: Resistance & Power

What the Ohm's Law Calculator Does

This Ohm's Law calculator finds the missing value in a simple electrical circuit when you know any two of voltage (V), current (I), resistance (R), or power (P). Enter the values you have, and it returns the others — for example, resistance and power from a known voltage and current.

It is built for electronics hobbyists, students learning circuit basics, electricians, and anyone sizing a resistor, checking a power supply, or verifying that a component stays within its ratings. Voltage is measured in volts (V), current in amperes (A), resistance in ohms (Ω), and power in watts (W).

How It Works: The Ohm's Law Formula

Ohm's law states that the current through a conductor is proportional to the voltage across it and inversely proportional to its resistance. The core relationship and the power equation are:

Rearranging these covers every case, so any two known quantities give you the rest.

• V = I × R (voltage = current × resistance)
• I = V / R (current = voltage / resistance)
• R = V / I (resistance = voltage / current)
• P = V × I (power = voltage × current)
• P = I² × R and P = V² / R (power from current or voltage alone)

Worked Example With Real Numbers

Suppose an LED circuit runs on a 12 V supply and you measure a current of 0.02 A (20 mA) flowing through a series resistor. To find the resistance, divide voltage by current: R = V / I = 12 / 0.02 = 600 Ω.

To find how much power that resistor dissipates, multiply voltage by current: P = V × I = 12 × 0.02 = 0.24 W. Because a common 1/4-watt (0.25 W) resistor sits right at that limit, you would choose a 1/2-watt part to run it cool and safe. This is exactly the kind of check the calculator automates.

Tips and Common Mistakes

Most errors come from mixing units rather than from the math itself. Keep everything in base units before you calculate, or let the calculator handle the conversion.

• Convert prefixes first: 1 kΩ = 1,000 Ω, 1 mA = 0.001 A, 1 mV = 0.001 V.
• Don't confuse current (I, amps) with power (P, watts) — they answer different questions.
• Always check the power result against a component's wattage rating, not just its resistance.
• Ohm's law assumes a steady DC circuit with a constant (ohmic) resistance.

Factors That Affect the Result

Ohm's law is exact for ideal resistors, but real components shift the numbers. Resistors carry a tolerance (often ±1% or ±5%), so a labeled 600 Ω part may actually measure anywhere within that band, changing the current you get.

Resistance also rises with temperature in most metals, and semiconductor devices like diodes and transistors are non-ohmic — their resistance changes with voltage, so a single V/I value only describes one operating point. For AC circuits with capacitors or inductors, you must use impedance instead of plain resistance. Treat the calculator's output as accurate for DC and resistive loads, and as a close estimate elsewhere.