LED Resistor Calculator
Size the series resistor for one LED or a whole array, with real current, power and the parallel-wiring trap spelled out.
Never share one resistor between parallel LEDs
This is the mistake the subject is built around. Two LEDs of the same part number do not have identical forward voltages. Wire them in parallel behind a single resistor and they are forced to the same voltage, so the one with the slightly lower forward voltage takes more of the current. That extra current warms it. Warming lowers its forward voltage further, which takes more current still. The circuit walks itself into one LED blazing and the other dark, and shortly after that into one LED dead.
- Series shares a resistor. Parallel does not. Components in series carry identical current by definition, so one resistor sets the current for the whole string. Parallel branches are independent circuits and each needs its own.
- Leave at least a volt of headroom. Forward voltage varies part to part and falls as the LED warms. A design with 0.2 V across the resistor will have visibly uneven brightness across a batch and will drift as it runs.
- 20 mA is a habit, not a requirement. It comes from indicator LEDs of the 1970s. Modern parts at 2–5 mA are usually bright enough to be annoying in a dark room.
- A resistor wastes the difference as heat. That is fine for an indicator and wrong for anything bright — a one-watt LED on a series resistor throws away more power than it uses. Constant-current drivers exist for that reason.
How to use
- Enter the supply voltage and pick the LED colour, or type a forward voltage from the datasheet.
- Set the forward current — 20 mA is the habit, 2 to 5 mA is usually enough.
- Choose one LED, a series string or parallel branches.
- Read the standard resistor value, how many you need, and the actual current it gives.
Frequently asked questions
Can I put several LEDs in parallel behind one resistor?
No, and this is the mistake the whole subject is built around. Two LEDs of the same part number have slightly different forward voltages. In parallel they are forced to the same voltage, so the lower one takes more current, which warms it, which lowers its forward voltage further, which takes more current still. You end up with one LED blazing, one dark, and shortly afterwards one dead. Every parallel branch needs its own resistor.
Why does a series string share one resistor then?
Because components in series carry identical current by definition — there is nowhere else for it to go. One resistor sets the current for the whole string, which is why series is both cheaper in parts and far more efficient than parallel. The limit is the supply: the forward voltages add up, so a 12 V rail only fits three white LEDs before there is no headroom left for the resistor.
How much headroom does the resistor need?
At least a volt, and more is better. Forward voltage varies between individual parts and falls as the LED warms up. If the resistor only drops 0.2 V, a 0.1 V spread between parts changes the current by half, so a batch of identical LEDs will visibly differ in brightness and each one will drift as it runs. With a couple of volts across the resistor, the same spread barely registers.
Where does the forward voltage come from?
The semiconductor chemistry, not the lens colour. Red and infrared use low-bandgap materials and sit near 1.4 to 2.1 V, while blue, white and ultraviolet sit near 3.2 V. White LEDs are blue dice with a phosphor coating, which is why they carry a blue LED forward voltage rather than something in the middle. The figures here are typical; spreads of half a volt within one colour are ordinary, so the datasheet for your actual part wins.
Is 20 mA the right current?
It is the traditional figure from indicator LEDs of the 1970s and it is often far more than a modern part needs. At 20 mA a current LED in a dark room is genuinely unpleasant to look at. For a panel indicator, 2 to 5 mA usually looks better and saves power. For anything meant to actually illuminate something, go by the datasheet and expect to need a driver rather than a resistor.
Why does the resistor get warm?
Because a series resistor works by throwing away the difference between the supply and the LED as heat. Driving a 2 V LED from 12 V means five sixths of the power goes into the resistor. For an indicator drawing a few milliamps that is irrelevant. For anything bright it is a real waste and the reason constant-current drivers exist — they regulate current without dissipating the surplus.
What if the exact resistor value is not available?
This tool rounds up to the nearest E12 value, which is the series most hobby kits are built from, because rounding up keeps current at or below what you asked for. Rounding down would exceed it. The exact figure is shown alongside so you can see how far the standard part moved things, and the actual current tells you whether that matters.
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