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LED Resistor Calculator

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An LED does not limit its own current. Once the voltage across it passes its forward threshold it will conduct as hard as the supply allows, which for most supplies means until something fails. A resistor in series fixes that by soaking up the difference between the supply and the LED's forward voltage. This works out how big that resistor has to be, which stocked value to buy instead of the exact figure, what current you actually end up with, and how much heat the resistor will be making.

Pick the nearest, or type your own below.

V
V

From the datasheet

mA

20 mA is typical

On this one resistor

Typical forward voltage:

How to use the led resistor calculator

  1. 1Enter the supply voltage, or pick one of the common rails.
  2. 2Enter the LED's forward voltage from its datasheet, or use one of the typical figures for its colour.
  3. 3Enter the forward current you want — 20 mA is the usual figure for a standard indicator, and less is often plenty.
  4. 4If several LEDs share one resistor in a series chain, set the count.
  5. 5Read the resistor to fit, and check the wattage line before choosing a part.

Examples

A red indicator on 5 V

Input
5 V supply, 2 V forward, 20 mA
Result
150 Ω exactly; fit 150 Ω, giving 20 mA and 60 mW

One of the rare cases where the arithmetic lands directly on a stocked value.

A white LED on 12 V

Input
12 V supply, 3.2 V forward, 20 mA
Result
440 Ω exactly; nearest E24 is 430 Ω, giving 20.5 mA

The resistor burns 180 mW against the LED's 64 mW — nearly three quarters of the energy becomes heat, which is why 12 V strips use drivers rather than resistors.

Three LEDs in series

Input
12 V supply, 3.2 V forward, 20 mA, 3 LEDs
Result
120 Ω, and only 48 mW wasted in the resistor

Stacking the LEDs uses up the supply voltage productively instead of dropping it across a resistor.

About the led resistor calculator

Why a diode needs help and a resistor does not

A resistor obeys a straight line: double the voltage across it and you double the current through it. That self-limiting behaviour is what makes it safe to connect one directly across a supply, as long as the wattage is right. A diode obeys an exponential instead. Below its forward voltage almost nothing flows; above it, the current rises so steeply that the device effectively pins the voltage across itself and lets the rest of the circuit decide the current.

That is the whole problem in one sentence. If the LED sets the voltage and nothing sets the current, the current is determined by whatever accidental resistance exists in the wires and the supply — which is neither predictable nor small enough to save the part. Putting a resistor in series makes the current predictable again, because now the resistor takes the leftover voltage and the leftover voltage divided by its resistance is the current.

This also explains why the calculation subtracts the forward voltage before dividing. The resistor does not see the whole supply; it sees what is left after the LED has taken its share. On a 5 V rail with a 3.2 V white LED, the resistor is only working with 1.8 V, which is why the required resistance is so much lower than a first guess suggests.

When a resistor is the wrong answer

The efficiency figure this page reports is the thing to watch. Every watt the resistor drops is heat, contributing nothing to light. On a 5 V supply with a red LED that is a reasonable trade. On a 12 V supply with a single LED, three quarters of the energy is wasted, and on a 24 V rail it is closer to seven eighths. For one indicator lamp nobody cares. For a hundred of them, or for anything battery powered, it is the dominant cost.

Two fixes exist and both appear in this calculator's output. The first is to stack LEDs in series so the supply voltage is used by the diodes rather than the resistor — the count field does exactly this, and the wasted fraction falls sharply with each one added. The second is a constant-current driver, an active circuit that regulates the current directly instead of relying on a fixed resistance. Every commercial LED lamp, torch and strip controller uses one.

There is one more reason drivers win outside the hobby bench: a resistor sets the current only for the supply voltage you designed around. A vehicle's nominal 12 V is anywhere from 11 V to 14.5 V depending on whether the engine is running, and a resistor sized for the middle of that range gives noticeably different brightness at the ends of it. A current source does not care.

Frequently asked questions

What happens if I leave the resistor out?
The LED draws whatever the supply can deliver and destroys itself, usually in well under a second. The reason is that the current through a diode rises exponentially with the voltage across it: a tenth of a volt above the forward threshold can be the difference between 20 mA and several hundred. Since nothing else in the circuit sets the current, tiny variations in supply voltage or in the LED itself become enormous variations in current, and the junction overheats.
Where do I find the forward voltage?
On the datasheet for the specific part, listed as Vf at a stated test current. It varies mainly with the chemistry, which is why it tracks colour: red and yellow sit near 2 V, while blue, white and pure green need around 3.2 V because they emit higher-energy photons. The presets here are typical figures rather than promises — an individual part can sit a couple of tenths either side, and Vf also drifts down as the die warms up.
Can I wire several LEDs to one resistor?
In series, yes, and it is the efficient arrangement — the same current passes through each one, so they match automatically, and each LED uses up supply voltage that would otherwise be wasted as heat. In parallel it is a bad idea: the LEDs will not have identical forward voltages, so the one with the lowest takes more than its share of current, warms up, drops its Vf further and takes more still. Give each parallel branch its own resistor.
Why does the calculator suggest a different resistor from the exact answer?
Because the exact answer is usually a value nobody manufactures. Resistors are sold from the E series, so a computed 440 Ω has to become 430 Ω or 470 Ω in practice. This page names the nearest E24 value, which covers ordinary 5% parts, and the nearest E96 for 1% parts, then recalculates what current actually flows with the E24 one fitted — because that recalculated figure, not the ideal, is what your circuit will do.
Is 20 mA always the right target?
It is a convention rather than a requirement, and it is often higher than necessary. Modern indicator LEDs are bright enough at 2–5 mA for anything read indoors, which saves power and heat and extends the life of both the LED and whatever is driving it. Run at the maximum only when the thing genuinely needs to be seen in daylight, and never above the absolute maximum on the datasheet — that figure has no headroom in it at all.

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