Which LED leg is positive?

The longer leg is positive (anode). The shorter leg is negative (cathode). This applies to standard 3mm, 5mm, 8mm, and 10mm through-hole LEDs from all manufacturers.

How do I tell LED polarity if the legs are cut to the same length?

Look at the flat edge on the bottom rim of the LED housing. The side with the flat edge is the negative (cathode) side. You can also look inside the lens — the larger internal metal piece (the anvil) is the cathode.

What happens if I connect an LED backwards?

The LED will not light up, but it will not be damaged either. Standard LEDs can safely withstand reverse voltage up to about 5V. Simply reverse the connections and it will work.

How do I check LED polarity with a multimeter?

Set your multimeter to diode test mode. Touch the red (positive) probe to one leg and the black (negative) probe to the other. If the LED lights dimly, the red probe is on the anode (positive) and the black is on the cathode (negative). If it does not light, swap the probes.

Do all LEDs have the same polarity indicators?

All standard through-hole LEDs (3mm, 5mm, 8mm, 10mm) use the same three indicators: longer leg = anode, flat edge = cathode, larger internal piece = cathode. Some specialty LEDs like bi-color or RGB may have different pin configurations — always check the datasheet.

How to Tell LED Polarity: A Beginner's Guide

This guide explains how to tell LED polarity for common through-hole LEDs like the 3mm, 5mm, and 10mm sizes. Correctly identifying the positive (anode) and negative (cathode) legs is the most important step when wiring an LED. If you connect it backwards, it will not light up. Luckily, there are three easy visual cues to identify the polarity of your LED.

A diagram showing how to tell LED polarity by looking at the leg length, flat edge, and the internal anvil. — Three ways to identify the polarity of a 3mm, 5mm, or 10mm LED.

Why is LED Polarity Important?

An LED is a "Light Emitting Diode." A diode is an electronic component that only allows electrical current to flow in one direction. Therefore, if you try to send power through it the wrong way, no current will pass, and the LED will not light up. Unlike some components, reversing the polarity will not usually damage a standard LED, but it will prevent your circuit from working correctly. For this reason, knowing **how to tell LED polarity** is a fundamental skill for any electronics project.

Method 1: Check the LED Leg Length

The easiest and most common way to determine polarity is to look at the two legs, or leads, coming out of the LED. On a new, uncut LED, one leg will be longer than the other.

The **longer leg** is the positive (+) side, also known as the anode.
The **shorter leg** is the negative (-) side, also known as the cathode.

This is the most reliable method, but it only works if the legs have not been trimmed to the same length.

Method 2: Find the Flat Edge of the LED

If the legs have been cut, you can look at the plastic lens or housing of the LED itself. If you look at the bottom rim of the LED, you will notice that one side is perfectly round, while the other side has a small flat edge.

The side with the **flat edge** indicates the negative (-) leg, or the cathode.

This method is very useful for LEDs that are already soldered into a circuit board where you cannot see the leg length.

Method 3: Look Inside the LED Lens

Finally, if you cannot use the first two methods, you can look directly inside the clear (or colored) plastic lens of the LED. You will see two small metal parts. One part looks much larger than the other.

The **larger** metal piece inside the lens is called the anvil. This indicates the negative (-) side, or the cathode.
The **smaller** metal piece is called the post. This is the positive (+) side, or the anode.

This internal indicator is a permanent feature of the LED and can always be used to check polarity.

Conclusion on How to Tell LED Polarity

In conclusion, you now know three simple ways for **how to tell LED polarity**. You can check the leg length, look for the flat edge on the housing, or inspect the size of the metal parts inside the lens. By using these methods, you can ensure your LEDs are always connected correctly, allowing your projects to light up as intended.