I have a bunch of LEDs that I want to power using some fixed input voltage. How many LEDs can I put in series, and how big of a resistor should I put in series with them all?

If you want multiple LEDs of the same type in parallel, add the number of parallel 'columns' you want under 'columns'. The total number of LEDs will be rows * columns, and the recommended resistance is for a single resistor in series with everything (see diagram at bottom of page).

If you want to put multiple LEDs of different voltages in series, add their voltages up and enter the bunch as one LED.

Disclaimer: These calculations assume the LEDs draw a constant current, but I'm actually not sure how typical LEDs behave under high or low voltage conditions.

It's assumed that your circuit is set up like the following, if you chose 2 columns and have enough voltage for 3 rows:

Random Tips

• Polarity: If one lead is longer than the other, the longer one is positive (it's the 'anode'). If there's a flat side on the LED, that's the negative ('cathode') side.
• "Never connect an LED directly across a battery or other power source – it will burn out. LEDs must always be connected in series with a resistor." (Starting Electronics, but also my dad). LEDs emit light given a specific current. By putting a resistor in series with the LED, knowing the LEDs approximate voltage drop, you effectively turn a voltage source into a current source. You might happen to give it the right current by providing a specific voltage, but if there's no resistor to absorb any excess power, a small over-voltage could become an over-current and burn out your LED.