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.

Typing values in follows one convention: every field reads base units. Voltage goes in as volts, current as amperes, resistance as ohms, and power as watts, and no prefix is parsed along the way, so you convert before typing. A 20 mA LED current is 0.02 in the Current field, a 4.7 kΩ pull-up is 4700 in the Resistance field, and a 250 mW budget is 0.25 in the Power field. Reading a result back out is the same shift in reverse: a calculated 0.0152 A is 15.2 mA once the decimal moves three places. Keeping that shift straight is the whole difference between an answer that matches the datasheet and one that lands a factor of a thousand away.

The VIR triangle and the formula wheel

The VIR triangle is the classic shortcut for remembering which arrangement to use. Picture voltage at the top with current and resistance side by side along the bottom, then cover the letter you want with a finger. Whatever remains spells out the operation: covering V leaves I next to R, so voltage equals current times resistance; covering I leaves V over R; covering R leaves V over I. The triangle adds no new physics, because it is the single relationship V = I × R drawn as a picture. What it buys you is speed: the finger does the algebra that a rearrangement would otherwise demand.

The formula wheel scales the same idea to all four quantities. Every arrangement on it, all twelve, comes from just two laws: V = I × R and P = V × I. Knowing any two values pins down the other two, which makes exactly six know-any-two pairs: V with I, V with R, V with P, I with R, I with P, and R with P. The last pair is the only one that resolves through square roots, because resistance and power together hide both voltage and current behind them. This calculator runs all six pairs the same way, so picking the formula is never your job; entering two knowns is the whole task.

Getting real values and checking your math

Where does the resistance you enter actually come from? A through-hole part states it in colored bands: the first two carry the digits, the third is the multiplier, and the fourth states the tolerance.² That last band deserves more attention than it usually gets, because it defines a window around the printed value rather than a guarantee. A part marked 220 ohms with a ten-percent band is in spec anywhere from 198 to 242 ohms, and no calculator can know where inside that window your particular part sits. Entering the marked value is the right default; measuring the actual part is the upgrade path.

Checking the math takes three measurements, each with its own placement rule. Voltage is read with the probes across the component while the circuit runs, because a voltmeter measures in parallel.³ Current is the invasive one: the meter goes in series, so the path is broken and the flow runs through it.³ Resistance belongs to the powered-off bench, ideally with the part lifted out of the circuit, since anything in parallel with it skews the reading and even the test leads contribute a few tenths of an ohm of their own.⁴ With two measured values in hand, enter them and the other two fields fill themselves in.

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).⁵ 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.⁶ 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.⁵

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.⁷

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.⁷

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.⁸ Surface-mount resistors in 0402 or 0603 packages are typically rated at 0.063 W or 0.1 W.⁹¹⁰ 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.⁸

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.¹¹

That exactness carries one qualifier: resistance itself moves with temperature. The drift follows R = R₀(1 + αΔT), and the direction depends on the material, because metal resistivity climbs as the vibrating lattice impedes the electron flow while carbon and most semiconductors drift the other way.¹² Across the few tens of degrees a bench circuit sees, the shift stays at percent scale. A tungsten filament shows what a large swing does: the textbook example starts at 0.35 ohms cold and climbs past 4.8 ohms at operating glow, a more-than-tenfold rise, which is why a lamp's switch-on current briefly exceeds its steady current.¹² Any component that heats itself under load is quietly editing its own resistance while you measure.

When a measurement disagrees with the calculation

Tolerance is the first reason a bench reading disagrees with the math. The calculator is exact for the numbers it is fed, but those numbers are nominal, and a real part sits somewhere inside its window rather than on the center line.² A measurement a few percent away from the prediction is usually confirming the calculation, not contradicting it. Small series resistances come next: every socket, joint, and pair of test leads adds a fraction of an ohm, invisible next to a kilo-ohm pull-up but decisive when the resistance you are measuring is itself under an ohm.⁴ When the gap runs wide instead of percent-scale, suspect the entered values before you suspect the law.

The power source is the remaining reason. Inside every real battery and supply sits an internal resistance, so the terminal voltage reads below the nameplate figure and falls further as the current draw rises.¹³ Under a heavy load, the 12 V you typed no longer reaches the resistor intact, because the internal drop claims part of it first. The tool cannot know any of this, because it only knows the values you enter. Measure the voltage across the component while the circuit runs, then re-solve with the measured numbers, so the calculation describes the circuit on the bench rather than the one on paper.