LED Resistor Calculator

LED series resistor: sizing the current-limiting R

Wire an LED straight to a supply with nothing in series and it burns out within milliseconds. That's why you need a series resistor to hold the current in check. The math is R = (Vsource − VLED) / ILED. Forward voltage depends on color: red ≈ 2 V, yellow/green ≈ 2.1–2.4 V, blue/white ≈ 3–3.5 V, and UV ≈ 3.3–4.2 V. Indicator LEDs usually run somewhere between 5 and 20 mA. Example: 5 V supply, red LED (2 V), 20 mA → R = (5 − 2) / 0.02 = 150 Ω. The resistor also dissipates power, P = (V_source − V_LED) · I, so give yourself headroom and pick one rated for at least twice that.

Applications: Arduino, LED strips, decoration

The classic Arduino blinky puts a series R on a GPIO pin (220–470 Ω at 5 V, 150–330 Ω at 3.3 V). 12 V LED strips are different: each segment already has its own resistor, so as long as you cut at the marked points you don't add anything. Building your own decoration from bare 3 mm/5 mm LEDs off a 9 V battery or USB 5 V? Work out R separately for each LED color.

FAQ

Can I parallel LEDs with one resistor? Avoid it. Even tiny differences in V_f let one LED grab most of the current and cook itself. Give each LED its own resistor, or string them in series behind a single R.

How do I pick the resistor wattage? Work out P = (V_s − V_LED) · I and double it for safety. A common 20 mA / 3 V drop comes to about 60 mW, so a 1/4 W resistor has plenty to spare.

Can I run an LED below 20 mA? Sure. Modern LEDs already look bright at 5–10 mA, and running them softer keeps them cooler and stretches their life.