LED Resistor Calculator

Why LEDs Need a Resistor An LED (Light Emitting Diode) is not a resistor — it has a fixed forward voltage drop (Vf) and will draw unlimited current without a current-limiting resistor. Too much current burns the LED instantly. Too little current and the LED is dim. The resistor limits current to the LED’s rated operating current (typically 20 mA for standard LEDs).

The Formula R = (V_supply − V_f) / I_LED Where: V_supply = supply voltage (e.g., 3.3V, 5V, 9V, 12V) V_f = LED forward voltage (voltage drop across the LED at operating current) I_LED = desired LED current (typically 10–30 mA) R = resistor value in ohms

Typical LED Forward Voltages Red LED: 1.8–2.2 V (typical 2.0 V) Orange LED: 2.0–2.2 V (typical 2.1 V) Yellow LED: 2.0–2.2 V (typical 2.1 V) Green LED (standard): 2.0–2.4 V (typical 2.2 V) Blue LED: 3.0–3.5 V (typical 3.2 V) White LED: 3.0–3.5 V (typical 3.2 V — white LEDs use a blue chip with phosphor) UV LED: 3.2–3.8 V Infrared (IR) LED: 1.2–1.6 V (typical 1.4 V)

Typical Operating Current Standard 5mm LED: 20 mA (maximum), 10–15 mA for long life High-brightness LED: 30–50 mA Low-current LED: 2–5 mA (some indicator LEDs) Power LED: 350 mA, 700 mA, 1A, or higher (require dedicated drivers)

Resistor Power Rating Power dissipated in resistor: P = I² × R = (V_supply − V_f) × I Use a resistor rated at at least 2× the calculated power. Standard resistor ratings: 1/8 W, 1/4 W, 1/2 W, 1 W, 2 W. For most LED circuits at 20 mA, a 1/4W resistor is sufficient.

Multiple LEDs in Series Series connection: V_total = V_f1 + V_f2 + … + V_fn. Current is the same through all. Advantage: all LEDs have identical current and brightness. R = (V_supply − n × V_f) / I Maximum LEDs in series: n < V_supply / V_f.

Multiple LEDs in Parallel (not recommended) Parallel LEDs have mismatched Vf values — one LED draws more current and burns out. Better to use multiple series strings, each with its own resistor.