LED Current Limiting Resistor Calculator

Why LEDs Need a Resistor An LED (Light Emitting Diode) is a diode — it has a forward voltage (Vf) that it “uses up,” and then it conducts current with very low resistance. Without a current limiting resistor, the LED draws as much current as the power source can supply. This instantly destroys the LED. A resistor limits the current to a safe value, protecting the LED.

Forward Voltage by Color LED forward voltage depends on the semiconductor material and determines the light color. Red, orange, and yellow LEDs use lower-energy semiconductors: Vf is approximately 1.8–2.2V. Green LEDs span a range: 2.0–3.0V depending on the type. Blue, white, and UV LEDs require higher-energy photons and have Vf of approximately 3.0–3.5V. White LEDs are actually blue LEDs with a phosphor coating — so they have blue-range Vf values. Infrared LEDs are typically 1.2–1.7V.

The Ohm’s Law Calculation The resistor sees the voltage remaining after the LED’s Vf is “used up.” If your supply is 5V and the LED has Vf of 2.0V, the resistor sees 3.0V. With a target current of 20mA (0.020A), the resistor needed is R = V/I = 3.0V / 0.020A = 150 ohms. The resistor dissipates power: P = I²R = 0.020² × 150 = 0.06W. A standard ¼W resistor easily handles this.

E12 Standard Resistor Series Resistors are not manufactured in every possible value — they come in standard series. The E12 series has 12 values per decade: 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82 (and their ×10, ×100, ×1000 multiples). Always round UP to the next higher standard value to keep current at or below the target. Never round down — that sends more current through the LED than it’s rated for.

High-Brightness and Power LEDs Standard 5mm indicator LEDs run at 20mA. High-brightness power LEDs designed for flashlights, grow lights, and automotive applications run at 350mA, 700mA, 1A, or 3A. These LEDs generate significant heat and require a metal heatsink to maintain safe junction temperature. A simple resistor is wasteful (it dissipates excess voltage as heat) — for high-current LEDs, a constant-current LED driver IC is the proper solution.

Multiple LEDs in Series When wiring multiple LEDs in series, the total forward voltage drop is the sum of all individual Vf values. The supply must exceed the total Vf for current to flow. The resistor calculation is the same: R = (Vsupply minus total_Vf) divided by desired_current. All LEDs in series share the same current — so they must be the same type and rating.