How to Calculate an LED Current-Limiting Resistor
Formula and worked examples for choosing an LED series resistor, including supply voltage, forward voltage, desired current and power rating.
The basic formula
The series resistor is R = (Vs − Vf) / If, where Vs is supply voltage, Vf is the LED forward voltage (about 1.8–2.2V for red, 3.0–3.4V for blue/white), and If is the desired forward current (often 5–20mA for indicators).
Choose the nearest standard value and verify the resistor power P = (Vs − Vf) × If, selecting a resistor rated above it.
Practical tips
When driving multiple LEDs, decide between series (same current, needs higher voltage) and parallel (each needs its own resistor). Never wire LEDs directly across a supply without a current-limiting element.
For microcontroller GPIO driving, confirm the pin can source/sink the chosen current; use a transistor or driver for higher-power LEDs.
Frequently Asked Questions
What resistor for a 5V supply and 2V red LED at 10mA?
R = (5−2)/0.010 = 300Ω; use a standard 300Ω or 330Ω resistor.
Can multiple parallel LEDs share one resistor?
Not recommended—forward-voltage mismatch causes unequal current; give each LED its own series resistor.