The longer leg of a standard through-hole LED is usually the positive leg, called the anode (+). The shorter leg is the negative terminal, called the cathode (−).

The easiest rule to remember is:

Long leg = Anode = Positive (+)
Short leg = Cathode = Negative (−)

How to Identify the Positive Leg of an LED

There are several ways to determine LED polarity, especially if the legs have already been trimmed.

1. Look at the Leg Length

For a new 3 mm or 5 mm through-hole LED, compare the two metal legs.

The longer leg is normally positive (+).

The shorter leg is normally negative (−).

This is the fastest method when the LED is new and its leads have not been cut.

2. Check the Flat Side

If both legs have already been trimmed to the same length, inspect the plastic LED body.

Most standard round LEDs have a small flat edge on one side.

Flat side = Cathode (−)

Therefore, the opposite side corresponds to the anode (+).

3. Check Inside the LED

Transparent LEDs often allow you to see the internal metal electrodes.

In many conventional through-hole LEDs:

Smaller electrode = Anode (+)
Larger electrode = Cathode (−)

The larger internal metal structure is commonly called the anvil.

4. Test the LED With a Multimeter

A digital multimeter provides a useful way to confirm polarity.

Set the multimeter to diode-test mode and connect the probes to the LED.

When correctly forward biased:

Red probe → Anode (+)
Black probe → Cathode (−)

The LED may glow slightly, depending on the multimeter and the LED's forward-voltage requirement.

Why Does LED Polarity Matter?

LED stands for Light Emitting Diode. Because it is a diode, it is designed primarily to conduct current in one direction.

During normal operation:

Positive (+) → Anode → LED → Cathode → Negative (−)

If the LED is installed backwards, it normally will not illuminate. Excessive reverse voltage can also damage an LED, so correct polarity is important in circuit design.

What About SMD LEDs?

SMD LEDs such as 2835, 3030, 3535 and 5050 do not have long and short legs, so their polarity must be identified differently.

Manufacturers may use:

• A notch

• Dot

• Line

• Triangle

• Chamfered corner

• Other package markings

Importantly, there is no single visual marking rule that applies to every SMD LED manufacturer and package. For PCB design and SMT production, always check the manufacturer's datasheet to confirm the anode and cathode orientation.

Quick Answer

If you are holding a normal new through-hole LED:

Long leg → Positive (+) → Anode

Short leg → Negative (−) → Cathode

If the legs have been cut, look for the flat side, which usually marks the cathode (−), or confirm the polarity using a multimeter.

For SMD LEDs, always refer to the package markings and manufacturer's datasheet.