LED Resistor Calculator

Stop Guessing at LED Resistor Values

Wire an LED straight to a battery and it either barely glows or burns out in seconds — LEDs need a current-limiting resistor sized to your exact supply voltage and LED specs, and "close enough" isn't good enough when 20mA of headroom is the difference between a working circuit and a dead LED. This calculator takes your supply voltage, the LED's forward voltage and current rating, and how you're wiring your LEDs, then gives you the exact resistor value, the nearest standard part you can actually buy, its power rating, and the color bands to look for on the resistor itself.

How to use it

• Enter your supply voltage — a coin cell, a 9V battery, a USB 5V rail, whatever you're powering the circuit from.
• Pick your LED color from the preset list (or type a custom forward voltage if you know it from a datasheet) and set the forward current, usually 20mA for a standard 5mm LED unless the datasheet says otherwise.
• Choose how the LEDs are wired: a single LED, several in one series string, several in parallel each with their own resistor, or series strings wired in parallel with each other.
• Read off the resistor value, its recommended wattage, and the color bands — then hit copy to save the full breakdown.

Real-world use cases

Adding a status LED to an Arduino or Raspberry Pi project running off a 5V or 3.3V rail, wiring up a strip of individual indicator LEDs on a control panel where each one needs its own resistor, figuring out how many white LEDs you can chain in series off a 12V supply before you run out of headroom, or double-checking a resistor value from an old schematic before you solder it in.

Frequently asked questions

Why does my LED need a resistor at all?

An LED's voltage barely changes as current increases, so without something limiting the current, it climbs until the LED overheats and fails — usually within seconds on a fresh battery. The resistor is what turns your fixed supply voltage into a safe, steady current.

What forward current should I use if I don't have a datasheet?

20mA is the standard safe default for a generic 5mm through-hole LED. High-brightness or high-power LEDs can be rated much higher (350mA or more), so check the part if you have one — using 20mA on a high-power LED will just make it dimmer than it could be, not damage it.

Why does the calculator round my resistor value up instead of down?

Resistors only come in standard values (the E12 series shown here), and rounding up gives you a slightly higher resistance — which means slightly less current than your target, which is the safe direction to round. Rounding down would push more current through the LED than you calculated for.

Can I put LEDs of different colors in the same series string?

You can, but every LED in a series string carries the same current, and the string's resistor is only sized for one forward voltage — so mixing colors with different forward voltages (say, a red LED at 1.8V and a blue LED at 3.2V) throws off the total voltage math. It's cleanest to keep each series string to one LED color, or calculate the string's resistor by summing each LED's actual forward voltage.

Is it safer to use a bigger resistor than the calculator recommends?

A bigger resistor is always electrically safe — it just means less current and a dimmer LED. Going smaller than calculated is what risks overdriving the LED, so if you're between two standard values, sizing up is the low-risk choice.