How to Use the LED Series Resistor Calculator
An LED is a diode, so without something limiting the current it will draw far more than it can handle and burn out almost instantly. That's why a series resistor sits between the power source and the LED to hold the current to a safe value. Enter your supply voltage, the LED's forward voltage and current, and how many LEDs are wired in series, and this calculator returns the resistor value you need plus the power (wattage) that resistor has to dissipate.
The math is straightforward: whatever voltage the LEDs don't consume (forward voltage times the number in series) is dropped across the resistor, and dividing that voltage by your target current gives the resistance (Ohm's law). The resistor's power dissipation is that same voltage times the current — if you use a resistor rated below this wattage, it can overheat and fail.
This covers everything from lighting a single LED off a 5V microcontroller like an Arduino to sizing a resistor for several LEDs in series on a battery. If there's no standard resistor at exactly the calculated value, picking the nearest equal-or-higher value from the E12/E24 series is the safe choice.
Frequently Asked Questions
Subtract the LED's total forward voltage (forward voltage times the number of LEDs in series) from the supply voltage, then divide by the LED's forward current.
Round up to the nearest standard value (E12/E24 series). A slightly higher resistance is safe since it slightly reduces LED current; a lower value risks overdriving the LED.
If the resistor's wattage rating is lower than the power it actually dissipates, it can overheat and fail — always choose a rating comfortably above the calculated power.