LED Resistor Calculator

LEDs have a very low dynamic impedance near their forward voltage. Without a current-limiting resistor, the current through an LED increases rapidly with any small increase in voltage, causing it to overheat and fail instantly. The resistor limits current to the LED's rated forward current (typically 20mA for standard LEDs).

R = (Vs - Vf) / If. Where Vs is the supply voltage, Vf is the LED's forward voltage, and If is the desired forward current. Example: 5V supply, red LED (Vf = 2.0V), 20mA target: R = (5 - 2.0) / 0.020 = 150Ω.

Typical forward voltages: Red = 1.8-2.2V, Orange/Yellow = 2.0-2.2V, Green = 2.0-3.5V, Blue = 3.0-3.5V, White = 3.0-3.5V (white LEDs use a blue LED chip). High-brightness and special LEDs may vary; always check the datasheet.

Power dissipated = (Vs - Vf) × If. Select a resistor rated at twice the calculated power for safety margin. Use at least a 0.25W resistor (the standard size) unless the power is very low. For high-current LEDs (>50mA), use 0.5W or 1W resistors.

Yes, in series: use one resistor with the sum of forward voltages subtracted from supply. R = (Vs - n×Vf) / If. In parallel: each LED should ideally have its own resistor since small Vf differences cause unequal current sharing. Never connect LEDs in parallel without individual resistors.