How to Read Fan Belt Numbers

by Elizabeth Falwell
Auto Engine image by Andrew Breeden from

A "fan belt" is also called a "v-belt" because it is shaped like the letter "V." This belt is made of rubber, and powers everything from your vehicle's water pump to the alternator. When this belt breaks, stretches or frays, it is necessary to replace it immediately to ensure your vehicle runs correctly. The first step in replacing a fan belt is knowing what size it is. To do that, you have to be able to read the numbers on it.

Reading Standard Automotive Fan Belt Numbers

Step 1

Locate the number on your existing fan belt. If the belt is still under your vehicle's hood, you may need a flashlight to see it.

Step 2

Read the first two characters on the belt. Standard automotive belts start with "3L" or "4L." If your [fan belt]( begins with this number/letter combination, continue on to Step 3. If it doesn't, skip to Section 2.

Step 3

Understand the meaning behind "3L" and "4L." Both symbolize the width of the belt. A "3L" belt measures 9.5 millimeters across, while a "4L" belt measures 12.5 millimeters.

Step 4

Read the next characters on the fan belt. These should be numbers, not letters or a combination of the two. These numbers represent the outside length of a fan belt. The number might be 420; this means the outside circumference of the fan belt is 42 inches.

Combine the information from Steps 3 and 4 to figure out exactly what width and length of fan belt you need. A belt with the number "3L420" on it means it is 9.5 millimeters in width and 42 inches in length. This is the information you'll need when searching for a replacement.

Reading Classic Fan Belt Numbers

Step 1

Look at the first character on the fan belt; if it's a letter instead of a number, you have a classic fan belt.

Step 2

Understand what the letter means. The letters run A through E, and symbolize the size of the belt. An "A-belt," which is the most common, is also the smallest. An E-belt is the largest.

Step 3

Read the number that follows the letter. This number represents the length of the inside of the fan belt (as opposed to the number in the standard automotive belt, which represents the outside circumference). For example, if the number is 46, that means the inside circumference of the belt is 46 inches.

Combine the letter and the numbers to understand what kind of fan belt you need. A belt that read "B44" is a fan belt with a "B" width that measures 44 inches long on the inside of the belt.


  • An "A" belt is roughly equivalent to a "4L" belt, and they can be interchanged.
  • The outside of a 4L fan belt is roughly two inches longer than the inside. This means a 4L420 fan belt is the same as an A40 fan belt. The outside of 3L fan belt is one and a half inches longer than the inside.
  • Some belts may also have the letter "X" included. The "X" means the belt has notches on it. Belts without an "X" are smooth.
  • All the belts described above have a 40-degree angle (this is the part that makes the belt look like the letter "V"); belts that have the letter "V" included in the product number have a 30-degree angle instead of the standard 40 degrees.
  • It is recommended you change your fan belts every three to four years, regardless of how they look. This is because some damage may be invisible to the untrained eye.


  • If you still aren't sure what type of fan belt you need for your vehicle, check with an automotive specialist. Installing the wrong type of belt could cause serious damage to your engine.

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