LED Resistor Calculator

Subtract the LED forward voltage from the power supply voltage, then divide that result by the desired current (in Amperes). Formula: R = (Vs - Vf) / I.

An LED (Light Emitting Diode) has a very low forward voltage (Vf = 1.8–3.5V depending on colour) and minimal internal resistance — it cannot self-limit current. Without a series resistor, the LED draws unlimited current until it destroys itself. The resistor limits current to the safe operating range, typically 10–20mA for standard 5mm and 3mm LEDs, and 350mA–3A for high-power LEDs.

LED resistor formula: R = (Vs − Vf) / If. Where R = resistor value (Ω), Vs = supply voltage (V), Vf = LED forward voltage (V), If = desired forward current (A). Example: 12V supply, white LED (Vf = 3.2V), 20mA: R = (12 − 3.2) / 0.020 = 440Ω. Select nearest standard resistor value (in this case 470Ω, giving 18.7mA — slightly lower, which is fine).

Typical LED forward voltages (Vf) at 20mA: Infrared — 1.2–1.7V; Red — 1.8–2.2V; Orange/Amber — 2.0–2.2V; Yellow — 2.0–2.4V; Green (standard, GaP) — 2.0–2.4V; Green (high-brightness, InGaN) — 3.0–3.5V; Blue — 3.0–3.5V; White (blue chip + phosphor) — 3.0–3.5V; UV — 3.5–4.2V. Always confirm with the specific LED's datasheet.

For series LEDs: All share the same current. Calculate: R = (Vs − n × Vf) / If, where n is the number of LEDs. Example: 12V supply, 3× red LEDs (Vf=2.0V each), 20mA: R = (12 − 3×2.0) / 0.020 = (12−6)/0.020 = 300Ω. Series wiring is efficient but if one LED fails open-circuit, all go dark. Never share a single resistor across parallel LEDs — slight Vf differences cause unequal currents.

Calculate resistor power: P = (Vs − Vf)² / R, or P = If² × R. Example: 470Ω resistor with 18.7mA: P = 0.0187² × 470 = 0.164W. Use a resistor rated at 2× calculated power for safety — so use a 1/4W (0.25W) or 1/2W resistor. Standard ratings: 1/8W, 1/4W, 1/2W, 1W, 2W, 5W. For high-power LED arrays (hundreds of mA), use a dedicated LED driver IC rather than resistors.