3mm LED Resistor Calculator for Arduino

By Official Hobby Components Online Store | Buy Electronic Kits, Components, Boards, Shields, Modules | Published: 2026-10-17

Category: How-to Guides

Learn how to calculate the correct current-limiting resistor for a 3mm LED used with Arduino, including a step-by-step process and common pitfalls.

Quick Answer

For a standard 3mm green LED running on a 5V Arduino pin, use a 220 Ω resistor. This gives about 10 mA forward current, which is bright enough for indication and well within the LED's safe range. The exact value depends on the LED's forward voltage and your supply voltage.

Why a Resistor Is Necessary

An LED is a current-driven device. If you connect it directly to an Arduino output pin without a resistor, the LED will draw as much current as the pin can supply, often exceeding its maximum rating and destroying the LED or the pin. A current-limiting resistor sets the current to a safe level, protecting both the LED and the microcontroller.

The Resistor Calculation Formula

Use Ohm's law: R = (Vsupply - Vforward) / Iforward. For a 3mm green LED, typical forward voltage is about 2.0 V and desired forward current is 10 mA (0.01 A). With a 5 V Arduino supply, the calculation is (5 - 2) / 0.01 = 300 Ω. A standard 220 Ω resistor gives slightly more current (about 13.6 mA), which is still safe for most 3mm LEDs rated for 20 mA.

Choosing the Right Resistor Value

The exact resistor value depends on your LED's forward voltage and the desired brightness. For a 3mm green LED on 5 V, values between 150 Ω and 330 Ω work well. Lower values (e.g., 150 Ω) give more current and brighter light, while higher values (e.g., 330 Ω) reduce current for dimmer indication. Always check the LED's datasheet for maximum forward current.

Step-by-Step: Calculate and Connect Your LED Resistor

Determine LED Forward Voltage: Look up the forward voltage (Vf) for your specific 3mm LED. For a typical green LED, Vf is around 2.0 V. If you don't have a datasheet, assume 2.0 V for green, 1.8 V for red, and 3.0 V for blue or white. Common mistake: Using the wrong Vf will give an incorrect resistor value. When in doubt, measure with a multimeter.
Choose Desired Forward Current: Decide how bright you want the LED. For indicator use, 10 mA is typical. For maximum brightness, use the LED's rated maximum (often 20 mA for 3mm LEDs). Never exceed the maximum. Checkpoint: If the LED is too dim at 10 mA, try 15 mA. If too bright, reduce to 5 mA. Always stay below the maximum.
Apply Ohm's Law: Use the formula R = (Vsupply - Vf) / I. For a 5 V Arduino, a 2.0 V green LED, and 10 mA: R = (5 - 2) / 0.01 = 300 Ω. Choose the nearest standard resistor value (e.g., 330 Ω or 220 Ω). Common mistake: Forgetting to convert milliamps to amps. 10 mA = 0.01 A. Using 10 instead of 0.01 will give a wildly wrong result.
Select and Connect the Resistor: Pick a resistor from your kit (e.g., a 220 Ω or 330 Ω 1/4 W resistor). Connect one end to the Arduino output pin and the other to the LED's anode (long leg). Connect the LED's cathode (short leg) to GND. Checkpoint: Ensure the resistor is between the pin and the LED, not after the LED. The order doesn't change the current, but it's a good habit.
Test and Verify: Upload a simple blink sketch to the Arduino. The LED should light up at your chosen brightness. If it doesn't light, check polarity and connections. If it's too dim or bright, adjust the resistor value. Common mistake: Reversing the LED polarity. The anode must connect to the resistor side, cathode to GND. If unsure, the longer leg is usually the anode.

Recommended Products

3mm Green LED's (Pack of 100): Standard high quality 3mm LED in a coloured package. Suitable for power and status indicators, back-lighting, message displays, etc. Supplied in bags of 100 quantity. Having a pack of 100 green LEDs means you can prototype multiple projects without reordering. The 3mm size fits standard breadboards and perfboards.
1/4W Wire Ended Resistors - Choose Value and Quantity: These 1/4 watt wire ended resistors are available in a variety of values and are sold individually. Ideal for hobby electronics projects, each resistor features wire leads for easy through‑hole mounting. You can buy exactly the resistor values you need for your LED circuit, and the 1/4 W rating is more than enough for typical LED currents. The wire leads make soldering or breadboarding simple.