What Resistor Do I Need for My LED?

You've got a bag of LEDs, a 9V battery, and an idea. You connect them together and — nothing. Or worse — a brief flash and then permanent silence. Welcome to the most common beginner mistake in electronics.

LEDs don't regulate their own current. They'll draw as much as you give them, right up until they burn out. A resistor is what stands between your LED and an early death. Picking the right one isn't complicated, but the formula trips people up the first time they see it. Let's fix that.

The formula (and what it actually means)

The resistor value you need comes from Ohm's Law applied to your specific circuit:

R = (Vs - Vf) / If

Breaking that down:

Vs is your supply voltage — the battery or power supply you're using. Could be 3.3V from an Arduino pin, 5V from USB, 9V from a battery, or 12V from a wall adapter.

Vf is the forward voltage of the LED — the voltage the LED itself consumes. This varies by colour. Red LEDs are typically around 1.8–2.2V. Green and yellow are similar. Blue and white LEDs need more, usually 3.0–3.4V. When in doubt, use 2.0V for red/yellow and 3.2V for blue/white.

If is the forward current — how much current you want to push through the LED. For standard 5mm LEDs, 20mA (0.02A) is the rated maximum. Running them at 10–15mA gives plenty of brightness and makes them last much longer. For indicator use, 5–10mA is enough.

A worked example

Say you have a red LED, a 9V battery, and you want to run it at 15mA.

Vs = 9V

Vf = 2.0V (typical red LED)

If = 0.015A (15mA)

R = (9 - 2.0) / 0.015

R = 7 / 0.015

R = 466 ohms

You won't find a 466 ohm resistor on any shelf. That's fine — always round up to the nearest standard value. In this case, a 470 ohm resistor is perfect. It's a standard E24 series value and it'll run your LED at just under 15mA, which is exactly where you want it.

If you're powering a blue LED from a 5V Arduino pin at 10mA:

R = (5 - 3.2) / 0.010 = 1.8 / 0.010 = 180 ohms

Nearest standard value: 180 ohm (that one actually lands perfectly). Or go to 220 ohm if you want a touch more safety margin.

Standard LED forward voltages by colour

Skip the maths

If you're wiring multiple LEDs, changing your supply voltage, or just want the answer fast — use the LED Resistor Calculator. Plug in your supply voltage, LED colour, and desired current, and it gives you the exact resistor value plus the nearest standard component.

What happens if you get it wrong

Too low a resistance means too much current. The LED overheats, degrades, and eventually fails — sometimes instantly, sometimes over a few hours of use. The damage is often invisible from the outside.

Too high a resistance is much safer but means a dim LED. For indicator lights this is usually fine. For a flashlight circuit, not so much.

No resistor at all is the most common mistake. Direct connection to a 5V or 9V supply will kill most standard LEDs in seconds.

Wiring multiple LEDs

If you're running several LEDs, you have two options.

In series: the LEDs share the same current but voltages add up. Your formula becomes R = (Vs - (Vf × number of LEDs)) / If. Great for keeping current consistent, but if one LED fails the whole string goes dark.

In parallel: each LED needs its own resistor. Don't share one resistor across multiple parallel LEDs — even tiny differences in forward voltage between LEDs cause uneven current distribution, and the LED with the slightly lower Vf takes all the current and dies first.

The LED Resistor Calculator handles both configurations.