LED Series Resistor Calculator

Sizing an LED resistor

R = (V_supply − n × V_f) / I

P_resistor = (V_supply − n × V_f) × I

An LED needs a series resistor to limit its current. The resistor drops the difference between the supply and the LED's forward voltage (multiplied by the number of LEDs in series), at the target current. Pick thenext standard value at or above the ideal resistance so you never exceed the rated current, and choose a resistor power rating comfortably above the calculated dissipation.

The supply must be higher than the total forward voltage (n × V_f); otherwise the LEDs can't light and no resistor value will work.

Working backwards from a resistor you have

Switch the mode to Current from resistor to run it the other way: enter a resistor value you already own and it returns the LED current that results, along with the resistor's voltage drop and power. This is handy for checking whether a resistor from your parts bin drives an LED at a safe, sensible brightness. The two directions are the same circuit rearranged — I = (V_supply − n × V_f) / R instead of solving for R.

Worked example: a red LED on 5 V

A red LED (≈2 V forward) at 20 mA from a 5 V supply needs (5 − 2) / 0.02 = 150 Ω — conveniently a standard value. The resistor dissipates I²R = 60 mW, so a common ¼ W part is fine; the LED itself takes 40 mW, for 100 mW total.

Forward voltage is not a fixed number

The largest source of error here is the forward voltage. It varies with colour, with current, with temperature, and between parts from the same reel — a spread of a few tenths of a volt is normal. That matters most when the supply is close to the forward voltage: driving a 3.4 V white LED from 5 V leaves only 1.6 V across the resistor, so a 0.3 V error shifts the current by nearly 20%. Use the datasheet figure at your intended current, and prefer more headroom where you have the choice. There is more on this in the guide tochoosing an LED resistor.