Ohm's Law & Power Calculator

Ohm's Law basics

Ohm's Law states that the current through a conductor between two points is directly proportional to the voltage across those points, which gives the familiar relationship voltage (V) equals current (I) multiplied by resistance (R): V = I × R.¹ Voltage is the electric potential difference measured in volts. Current is the flow of charge measured in amperes. Resistance is how much a component opposes that flow, measured in ohms.

The core equation in practice

Rearranging the same relationship into I = V/R and R = V/I lets you solve for whichever quantity you're missing. A 10-ohm resistor connected to a 5V supply will pass 0.5A of current. Double the resistance to 20 ohms and the current drops to 0.25A at the same voltage. Halve it to 5 ohms and the current rises to 1A. Because the three quantities are linked by a single equation, knowing any two of them is always enough to pin down the third without ambiguity. These relationships are linear, so doubling the voltage across a fixed resistance doubles the current in lockstep.

Power relationships

Electrical power (P) in watts is the product of voltage and current: P = V × I. Combined with Ohm's Law, this gives two more useful forms: P = I²R (power from current and resistance) and P = V²/R (power from voltage and resistance).² Choosing which form to use depends on the two quantities you already know, letting you avoid an extra step of rearranging the equation first.

Power forms for common cases

A 220-ohm resistor dropping 5V will dissipate about 114 milliwatts. That's fine for a quarter-watt resistor but would overheat a tenth-watt part. A 470-ohm resistor at the same 5V drops roughly 53 milliwatts. Across the standard values used for LED current limiting these losses stay well inside a quarter-watt rating, which is why a 220-ohm or 470-ohm part rarely needs anything bigger. Use this calculator to check dissipation whenever you pick resistor wattage for LED circuits, voltage dividers, or pull-up resistors.

Practical examples

The equations behind Ohm's Law are simple, but applying them well means choosing which quantities to work with and which assumptions to check. The two scenarios below are the ones you will encounter most often when you are picking a resistor for an LED or sizing a wire run in a higher-current circuit, and working through them step by step shows how the algebra turns into a design decision.

Choosing a current-limiting resistor for an LED

For a typical LED running at 20mA with a forward voltage of 2.1V on a 5V supply: the resistor needs to drop 2.9V at 0.02A, so R = 2.9/0.02 = 145 ohms. The nearest standard value is 150 ohms, giving about 19mA, well within the LED's rating.³⁴ Working through the subtraction before you reach for a part keeps your LED current inside the manufacturer's safe range. The same steps apply to any supply voltage or LED colour.

For a 12V automotive circuit drawing 2A through a 0.5-ohm wire run: the voltage drop across the wire is V = I×R = 1V, and the power wasted as heat is P = I²R = 2 watts. That tells you the wire must dissipate 2 watts as heat while carrying that load.²

Resistors in series and parallel

Ohm's Law extends naturally to networks of resistors. In a series circuit, resistances add directly: two 100-ohm resistors in series give 200 ohms total, and the same current flows through both. In a parallel circuit, the combined resistance is lower than any individual resistor: two equal resistors in parallel give half the resistance of either one. The formula for two parallel resistors is R = (R1 × R2) / (R1 + R2). Knowing these relationships lets you use this calculator to find the voltage drop and current at each stage of a multi-component circuit by breaking it into series and parallel groups, solving each group, and working outward.⁵

Voltage dividers, a core building block in analogue electronics, rely on the series relationship. Two resistors R1 and R2 connected from supply to ground produce an output voltage at the midpoint of V × R2 / (R1 + R2). Entering the total series resistance and supply voltage into the calculator gives the total current; multiplying that current by R2 gives the divider's output voltage. Understanding this calculation is essential for setting bias points in transistor circuits and scaling sensor voltages down to ADC input ranges.⁵

Power dissipation and component ratings

Every resistor has a maximum power rating that determines how much heat it can safely dissipate. Standard through-hole resistors are rated at 0.125 W, 0.25 W, 0.5 W, or 1 W.⁶ Surface-mount resistors in 0402 or 0603 packages are typically rated at 0.063 W or 0.1 W.⁷⁸ Use the P = I²R or P = V²/R form of the power equation to check whether a chosen resistor stays within its rating before committing to a design.

A resistor running near its rated power becomes hot, and excessive heat can drift the resistance value upward over time or even cause the part to fail open in extreme cases. For that reason designers typically pick a resistor rated for at least twice the expected dissipation, leaving enough margin to keep the component and surrounding board cool inside an enclosed housing. The same logic applies to pull-up resistors and voltage dividers that sit in a signal path for hours at a time, where a part that runs warm to the touch is a sign you should step up to the next higher power rating.⁶

When Ohm's Law has limits

Ohm's Law assumes resistance stays constant regardless of voltage or current, and most resistors satisfy this assumption over their full operating range even as the ambient temperature rises by several degrees. Semiconductors do not: a diode's forward voltage drop is roughly fixed regardless of current, so V = IR gives misleading results. Transistors and FETs have regions of operation where current depends on voltage in highly non-linear ways. Inductive and capacitive components introduce frequency-dependent behaviour that a resistance-only model cannot capture.

Because the relationship breaks down outside a purely resistive DC circuit, applying this calculator to diodes, transistors, or any AC circuit that includes capacitors or inductors gives only a rough approximation. For those components you need to work with impedance, complex numbers, or device-specific models instead. Within resistive DC circuits, however, Ohm's Law remains exact and reliable, which is why it still underpins the vast majority of hobbyist and breadboard design work.¹⁹