An LED is one of the first components anyone wires up — and one of the easiest to destroy. Connect it straight across a battery and it flares for a fraction of a second, then dies. The fix is a single cheap part: a series resistor that limits the current to a safe level. This guide explains why it is needed and how to size it, building directly on Ohm’s law.
Why an LED needs a resistor
Unlike a plain resistor, an LED is a diode: its current does not rise smoothly with voltage. Below its turn-on point almost no current flows; just above it, current rises almost vertically. That means a voltage source alone cannot set a sensible operating point — a few hundredths of a volt is the difference between “off” and “far too much current.”
Worse, LEDs have a slight negative temperature coefficient: as they heat up they conduct more, which heats them further — a runaway that ends in failure. A series resistor breaks this cycle by dropping the leftover voltage and fixing the current, regardless of small changes in the LED or supply.
The formula
The resistor and LED are in series, so the same current flows through both, and their voltages add up to the supply. Rearranging Ohm’s law for the resistor gives:
- Vsupply — your source voltage (e.g. 5 V or 12 V).
- Vforward — the voltage the LED drops when lit (depends on colour, see below).
- I — the target current, often around 20 mA (0.02 A) for a standard indicator LED.
Picking the forward voltage
Forward voltage rises with the energy of the light, so it tracks colour. Use the LED’s datasheet when you have it; otherwise these typical values are close enough:
| LED colour | Typical Vforward |
|---|---|
| Red | 1.8 – 2.0 V |
| Yellow / Amber | 2.0 – 2.2 V |
| Green | 2.0 – 3.0 V |
| Blue / White | 3.0 – 3.4 V |
A worked example
Drive a red LED (Vf = 2.0 V) at 20 mA from a 5 V supply:
The nearest standard value at or above 150 Ω is 150 Ω (or 220 Ω for a dimmer, even safer result). When in doubt, round up — a slightly larger resistor means slightly less current and a longer-lived LED.
Don’t forget the wattage
The resistor turns the excess voltage into heat, so it needs a power rating above what it dissipates: P = (Vsupply − Vforward) × I. In the example that is 3 V × 0.02 A = 0.06 W — a standard 1/4 W resistor is fine. At higher voltages or currents this number grows quickly, and an under-rated resistor will scorch.
Multiple LEDs
For several LEDs, series wiring shares one resistor — add the forward voltages together (you must keep total Vforward comfortably below the supply). Parallel LEDs should each get their own resistor: LEDs are never perfectly matched, so on a shared resistor the one with the lowest forward voltage steals most of the current and runs hot.
Do it instantly
The LED Resistor Calculator does this maths for you — enter supply voltage, LED colour or forward voltage, and target current, and it returns the resistor value, the nearest standard part, and the wattage. For the underlying relationship, see the Ohm’s law guide.
Frequently asked questions
Why does an LED need a resistor?
An LED has an exponential current–voltage curve, so a tiny voltage rise causes a huge current rise. Without a resistor to limit current, the LED draws too much and burns out almost instantly. The resistor sets a safe, predictable current.
What happens if the resistor is too large or too small?
Too large and the LED is dim or barely lights; too small and it runs too much current, overheats, and fails. Sizing it for around the LED’s rated current (often ~20 mA) gives full, safe brightness.
Can I drive several LEDs with one resistor?
LEDs in series can share one resistor (add their forward voltages). LEDs in parallel should each have their own resistor, because small differences between them make one hog the current otherwise.
What forward voltage should I use for my LED?
It depends mostly on colour, because the chemistry differs: red and yellow are around 1.8–2.2 V, green and blue around 3.0–3.4 V, and white about 3.0–3.6 V. Use the value from the datasheet when you have it; the colour-based estimate is close enough for most hobby circuits.
What wattage resistor do I need?
Work out the power the resistor burns with P = I² × R (or the voltage across it times the current). A typical 20 mA LED resistor dissipates well under a tenth of a watt, so a standard ¼-watt resistor is plenty. Higher currents or bigger voltage drops may need a ½-watt or larger part.
Why is my LED dim even with the right resistor?
Check that your supply voltage is actually what you assumed, that the LED is the right way round (it only conducts one way), and that you have not over-sized the resistor. A loose breadboard connection or a nearly-flat battery is also a common culprit.
What current should I aim for?
Most standard 3 mm and 5 mm indicator LEDs are rated for about 20 mA, which gives full brightness. You can run them at less — 5–10 mA is plenty for an indicator and saves power — but going above the rated current shortens their life or destroys them. When in doubt, design for around 10–15 mA.