Technical Guides
How to Measure a V Belt Correctly
Correct belt measurement guide for inside (Li), outside (La) and pitch (Lp) measurements. Practical steps for workshops, mechanics and buyers.
Table of contents
- What Do Li, La, and Lp Mean?
- Li (Inside Length)
- La (Outside Length)
- Lp / Ld (Pitch or Datum Length)
- Methods to Measure a V Belt
- Method 1: Read the Printed Belt Code
- Method 2: Flexible Tape Method
- Method 3: String Method
- Method 4: Compare with Existing Belt
- Common Belt Sections
- Classical Sections
- Wedge Sections
- Narrow Sections
- Common Measurement Mistakes
- Inch vs MM Measurements
- Practical Examples
- Example 1
- Example 2
- Important Warning
- FAQ
- Internal Links
- Visual placeholders
Correct belt measurement is important because many buyers confuse inside length (Li), outside length (La), and pitch length (Lp/Ld). Ordering the wrong length or wrong section can cause slipping, improper tension, reduced belt life, and machine problems.
What Do Li, La, and Lp Mean?
Li (Inside Length)
Measured from the inner surface of the belt.
La (Outside Length)
Measured from the outer surface of the belt.
Lp / Ld (Pitch or Datum Length)
Measured near the belt’s pitch line where the belt effectively runs inside the pulley.
Different manufacturers and belt standards may print different length types on the belt. Always confirm which length the marking refers to before ordering.
Methods to Measure a V Belt
Method 1: Read the Printed Belt Code
The easiest method is to read the code printed on the belt.
Examples:
- A45
- B72
- SPA1250
- 5V900
However, old belts may become faded or damaged.
Method 2: Flexible Tape Method
Use a flexible measuring tape around the belt circumference. This gives an approximate outer length (La).
Method 3: String Method
Wrap a non-stretch string around the belt and measure the string length. Transfer the string to a tape to read the length.
Method 4: Compare with Existing Belt
If another identical belt is available, compare width, section, and length markings.
Common Belt Sections
Classical Sections
- Z
- A
- B
- C
- D
- E
Wedge Sections
- SPZ
- SPA
- SPB
- SPC
Narrow Sections
- 3V
- 5V
- 8V
Different sections have different top widths and heights, so section identification is critical.
Common Measurement Mistakes
- Confusing Li and La
- Measuring stretched belts
- Ignoring belt section
- Ordering only by inches
- Using damaged belts for measurement
Inch vs MM Measurements
Industrial and automotive belts may use either inch-based or metric measurements depending on manufacturer and application.
Example:
- A45
- SPA1250
Both systems are common in Pakistan.
Practical Examples
Example 1
A belt marked A45 may not have the same outside length as its pitch length.
Example 2
A worn stretched belt may measure longer than its original specification.
Important Warning
Always confirm:
- printed belt marking
- belt section
- actual application
before ordering replacement belts.
FAQ
Internal Links
See the practical conversion tool: /tools/belt-length-converter/
Related guides:
Related products:
Contact: /contact/
Visual placeholders
- Li / La / Lp diagram
- V belt cross-section comparison (Z, A, B, SPA, 3V etc.)
- Step-by-step measuring photo examples
What to confirm before inquiry
- • Old belt marking
- • Profile or section
- • Machine or vehicle application
- • Quantity and delivery city
Frequently asked questions
Why do two belts with similar lengths fit differently?
Because Li, La, and Lp measurements are different and belt sections also affect effective running length.
Can I measure a belt in inches and order in mm?
Yes, but conversion must be accurate and the correct belt section must also be confirmed.
Why is my old belt longer?
Used belts can stretch and wear over time.
Why does the belt section matter?
Different sections have different widths, heights, and pulley compatibility.
Related products
Need help matching the correct size?
Send the exact belt marking to GM Traders and we will help confirm the right reference before supply.