LED Resistor Calculator
Every LED needs a current-limiting resistor. Enter your supply voltage and LED, and this calculator gives you the exact value, the nearest standard resistor to buy, and the wattage rating to use.
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Use a standard resistor of
150 Ω
The supply voltage is not higher than the LEDs' total forward voltage, so they will not light. Use fewer LEDs in series or a higher supply voltage.
Forward voltages are typical values; check your LED's datasheet. Standard values are rounded up to the E12 series, so the real current is slightly below your target.
The LED resistor formula
The resistor has to drop whatever voltage the LEDs do not use, at the current you want:
R = (Vsupply − n × Vf) ÷ I
where Vf is one LED's forward voltage, n is the number of LEDs in series and I is the current in amps (20mA = 0.02A). For a red LED on 5V at 20mA: (5 − 2.0) ÷ 0.02 = 150Ω. Always round up to the next standard value, which keeps the current at or below your target.
Quick answers for common supplies
One LED at 20mA, rounded up to a standard E12 value:
| Supply | Red LED (~2.0V) | White / blue LED (~3.1V) |
|---|---|---|
| 3.3 V | 68 Ω | 10 Ω |
| 5 V | 150 Ω | 100 Ω |
| 9 V | 390 Ω | 330 Ω |
| 12 V | 560 Ω | 470 Ω |
On 3.3V a white or blue LED has very little headroom, so its brightness varies a lot with small changes in supply — a common surprise on microcontroller projects.
How much current should an LED get?
20mA is the familiar maximum for standard 5mm LEDs, but it is rarely needed. An indicator LED is clearly visible at 5–10mA, and running below the maximum makes it cooler and longer-lived. High-power LEDs are a different category: they need a constant-current driver rather than a simple resistor.
Series vs parallel
LEDs in series share one resistor: add their forward voltages, which is what the "LEDs in series" field does. LEDs in parallel should each get their own resistor, because tiny differences in forward voltage make one LED take most of the current. That is why kits like the 81-LED chaser are designed with the resistors they are.
Reading and checking the resistor
Found the value you need? Our resistor color code calculator shows which bands to look for, and a multimeter confirms it before you solder. If an LED stays dark after soldering, check its orientation first — the flat edge and shorter leg are the cathode — and see our common soldering mistakes.