LED Resistor Calculator

LED current-limiting resistor

An LED is a non-ohmic diode: connect it directly to a battery and it draws excessive current until it burns out. A series current-limiting resistor drops the excess voltage and sets the current. The formula is R = (Vsource − VLED) / ILED. The forward voltage V_LED depends on the chip color: red 1.8–2.2 V, yellow 2.1–2.5 V, green 2.0–3.5 V, blue/white 3.0–3.5 V, UV 3.3–4.2 V. Typical current is 5–20 mA for indicator LEDs, 30–50 mA for high-brightness, hundreds of mA for power LEDs (which require a constant-current driver instead). Always check the dissipated power P = R · I² and pick a resistor rated above it (1/4 W, 1/2 W, 1 W). Example: red LED at 2.0 V / 20 mA on a 5 V supply → R = (5 − 2) / 0.02 = 150 Ω, dissipating P = 150 · 0.02² = 60 mW (a 1/4 W resistor is plenty).

Applications: Arduino, signage and decoration

Used in Arduino blinky projects, status indicators on boards, panel lights, flashlights, holiday decorations and signage. LEDs in series share the same current and add forward voltages (two red LEDs need 4 V before any current flows); LEDs in parallel are best avoided because tiny manufacturing differences cause unequal current sharing — one LED hogs the current and burns first. For multiple parallel branches, use one resistor per branch or a dedicated constant-current driver. Commercial LED strips already include resistors per cluster, so no external resistor is needed.

FAQ

Can I skip the resistor? Only with constant-current drivers or batteries whose internal resistance limits current (a coin cell directly on a small LED works because the cell can't supply enough current). On any regulated supply, no resistor means a burned LED.

What if my exact resistor value doesn't exist? Pick the next higher standard value (E12/E24 series). Slightly less current means slightly dimmer LED — never blown.

Why does V_LED vary with color? The bandgap of the semiconductor (GaAs for red, GaN for blue) sets the photon energy and the forward voltage drop.

Series or parallel for multiple LEDs? Series is more efficient (one resistor, same current). Parallel is risky without per-branch resistors.