LED resistor calculator
Every LED needs a resistor in series or it pulls too much current and burns out — often taking the Arduino pin with it. Enter your supply voltage, LED colour and the current you want, and this works out the resistor to use.
Your circuit
An Arduino pin is 5 V. A 3.3 V board or a 9 V battery pack changes the answer.
15 mA is bright and safe. An Arduino pin should never be asked for more than 20 mA.
Use this resistor
220 Ω
- Exact value needed
- 200 Ω
- Actual LED current
- 13.6 mA
- Power in the resistor
- 41 mW
A standard 1/4 W (250 mW) resistor is fine for anything on this page. Round up rather than down — a slightly bigger resistor only makes the LED a little dimmer.
How the maths works
Ohm's law: resistance = voltage across the resistor ÷ current through it. The LED eats a fixed voltage (its forward voltage), so the resistor has to soak up whatever is left over.
R = (supply voltage − LED forward voltage) ÷ current in amps
For a red LED on a 5 V Arduino pin at 15 mA: (5 − 2) ÷ 0.015 = 200 Ω, so you reach for the nearest standard part — the famous 220 Ω resistor in every starter kit.
Common questions
- Can I always just use 220 Ω?
- On a 5 V Arduino, yes for red, yellow and green LEDs. Blue and white LEDs have a higher forward voltage, so they end up dimmer but still safe.
- Does the resistor go before or after the LED?
- Either side. Current is the same all the way round the loop, so the resistor limits it wherever you put it.
- What if I skip the resistor?
- The LED draws as much current as the pin can give, gets very hot and burns out — and it can damage the Arduino pin permanently.
- How do I read the colour bands?
- First two bands are digits, the third is how many zeros to add, and gold is the tolerance. Red-red-brown is 2, 2, one zero = 220 Ω.