4L to A-Section Belt Conversion: What's the Difference and Are They Interchangeable?

The cross-section is identical. The length numbering is completely different. That combination trips up a lot of people.

Same Profile, Different Number System

A 4L belt and an A-section belt are the same shape. Top width 1/2 inch, depth 5/16 inch, 40 degree groove angle. They fit the same sheaves. Hold them side by side and measure the cross-section dimensions match exactly.

The 4L designation was developed for fractional horsepower drives: HVAC fans, washing machines, small compressors, lawn equipment. Same profile as the industrial A-section, different length convention. Manufacturers needed a separate designation for the light-duty market and chose to use outside circumference instead of inside circumference for the length number. That decision is the source of nearly every 4L conversion mistake.

The Length Numbers Don't Correspond Directly

A 4L belt encodes outside circumference in tenths of an inch. An A-section belt encodes inside circumference in whole inches. The same physical belt gets two different numbers depending on which system you're using.

A 4L460 has a 46.0 inch outside circumference. The inside circumference of that belt is roughly 44 inches, which makes it an A44. Not an A46. Order an A46 thinking it matches a 4L460 and you'll get a belt about 2 inches too long.

The conversion: take the 4L number, divide by 10 for outside circumference in inches, subtract roughly 2 inches for inside circumference. That gives you the A-section number.

Some common conversions:

4L200 is roughly an A18. 4L220 is roughly an A20. 4L250 is roughly an A23. 4L310 is roughly an A29. 4L460 is roughly an A44. 4L590 is roughly an A57.

The 2-inch offset isn't exact for every belt. Belt construction and manufacturer tolerances shift it slightly. When the application is critical, measure the outside circumference of your old belt directly rather than calculating from the part number.

When This Substitution Works

On light-duty drives it works reliably. HVAC fan drives, residential furnace blowers, washing machine drives, small air compressors. If the original equipment shipped with a 4L belt, an A-section of the correct length fits the sheave and handles the load. A-section belts are often easier to find at industrial supply counters and come in more brand options.

When to Be Careful

4L belts are built for fractional horsepower loads, typically under 1 HP. A-section belts cover the same profile but are available in heavier industrial constructions rated for 1-10 HP. Putting a heavier A-section on a fractional horsepower appliance drive is fine. Running a light 4L on a high-load industrial drive is not.

One more thing worth stating clearly: 4L and 5L are not interchangeable. The 5L is the FHP equivalent of the B-section, not the A-section. A 5L belt will not fit an A-section sheave. This catches people who grab the wrong FHP belt at a supply house.

Finding the Right Belt

Search any 4L or A-section part number on Partmatch to see cross-references across Gates, Dayco, Bando, Continental, and Optibelt with pricing.

4L200 converts to approximately A18. 4L310 converts to approximately A29. 4L460 converts to approximately A44. 4L590 converts to approximately A57.

Or search the A-section equivalent directly: A44, A57, A29.

Search any V-belt part number on Partmatch to find cross-references across Gates, Dayco, Bando, Continental, and Optibelt with pricing.