To measure a motor belt, you need two things: the belt’s width (which tells you the type) and its length. If you have the old belt, wrap a cloth tape measure around the outside, then subtract a correction factor based on the belt type to get the correct replacement size. If the old belt is gone, you can calculate the length from the pulleys and the distance between them.
Identify the Belt Type by Width
Before measuring length, figure out what kind of belt you’re dealing with. The width across the top of the belt determines its profile, and each profile has different depth and performance characteristics. The three most common standard V-belt profiles are:
- A belt: 1/2 inch wide, 5/16 inch deep
- B belt: 5/8 inch wide, 13/32 inch deep
- C belt: 7/8 inch wide, 17/32 inch deep
For lighter-duty motors, you may find fractional horsepower (FHP) belts instead. These use a different naming system: 3L (5/16 inch wide), 4L (3/8 inch wide), and 5L (1/2 inch wide). A 5L belt and an A belt share the same top width but have different depths, so they’re not interchangeable. Measure both width and depth to be sure.
If the belt has notches or grooves cut into the underside, that’s a cogged design, often marked with an “X” after the letter (AX, BX). These run cooler and flex more easily around small pulleys, but they use the same sizing numbers as their smooth counterparts.
Measuring an Existing V-Belt
If you still have the old belt, even a worn or broken one, measuring it directly is the fastest approach. Use a cloth tape measure, not a rigid steel one, and wrap it around the outside circumference of the belt. If you don’t have a cloth tape, wrap a piece of thin string around the belt, mark it, and then lay the string flat against a ruler or steel tape.
The number you get is the outside length, but belt replacement sizes are based on a shorter measurement called the effective length or datum length. This represents the length of the belt at the point where it actually rides in the pulley groove, not at the outer surface. To convert your outside measurement to the correct replacement size:
- A belts (1/2 inch): subtract 2 inches from the outside measurement
- B belts (5/8 inch): subtract 3 inches from the outside measurement
So if you measure 48 inches around the outside of an A belt, you need a belt labeled A46. For a B belt with the same 48-inch outside measurement, you’d order a B45. The number after the letter is always the corrected length in inches.
Calculating Belt Length Without the Old Belt
When the original belt is missing or too damaged to measure, you can calculate the replacement length using the two pulleys and the distance between their shafts. This formula works for any simple two-pulley setup:
Belt length = 2C + π × (D + d) ÷ 2
Where C is the center-to-center distance between the two pulley shafts, D is the diameter of the larger pulley, and d is the diameter of the smaller pulley. Use 3.1416 for π, and measure everything in inches.
For example, if your pulleys are 12 inches apart center-to-center, the large pulley is 6 inches in diameter, and the small one is 3 inches, the calculation is: 2(12) + 3.1416 × (6 + 3) ÷ 2 = 24 + 14.14 = 38.14 inches. You’d round to the nearest available belt size, which would be a 38-inch belt in whatever profile matches your pulleys.
This formula gives you the pitch length, which is the length at the line inside the belt that doesn’t stretch or compress when the belt bends around a pulley. It’s the standard measurement used in belt catalogs, so the number you calculate should match the number on the replacement belt’s label.
How to Measure Your Pulleys
Getting an accurate belt length calculation depends on measuring your pulleys correctly. Start by measuring the outside diameter, which is simply the distance across the widest part of the pulley face. A ruler or calipers across the top of the pulley will give you this number.
For more precise belt selection, you want the pitch diameter rather than the outside diameter. The pitch diameter is slightly smaller because it represents where the belt actually sits down in the groove, not the outer rim. On a new, unworn pulley, the difference is small enough that the outside diameter often works fine for the formula above. But if you’re replacing a belt on a system with significant wear on the pulleys, the grooves may be deeper than they should be, which throws off the fit. Worn pulleys chew through new belts quickly, so if the groove walls feel polished or the belt sits noticeably below the rim, replacing the pulley is worth considering.
To measure center-to-center distance, measure from the center of one pulley shaft to the center of the other. If you can’t access the shafts directly, measure from the nearest edge of one pulley to the far edge of the other, then subtract one pulley radius from each side.
Measuring Serpentine and Ribbed Belts
Some motors, especially in automotive and HVAC applications, use multi-ribbed (serpentine) belts instead of V-belts. These are flat belts with several small lengthwise grooves running along the inside surface. Sizing them requires a different approach.
First, count the number of ribs (the raised ridges between the grooves). Then measure the distance between two adjacent rib peaks to determine the pitch. A pitch of about 3.56 mm identifies the belt as a PK profile, which is the most common type in automotive use. The belt’s width should equal the groove pitch multiplied by the number of ribs, though measuring width directly is less reliable than counting ribs because wear can change the edges slightly.
For length, serpentine belts are measured the same way as V-belts: wrap a cloth tape around the outside and check against the manufacturer’s part number. Many serpentine belts are labeled with a number like 6PK2120, which tells you it has 6 ribs, PK-profile pitch, and a 2120 mm effective length.
Measuring Timing Belts
Timing belts (also called synchronous belts) have teeth on the inside surface that mesh with grooves on the pulleys. They’re used when precise speed ratios matter, because they don’t slip. Sizing a timing belt requires three measurements: width, pitch, and tooth count.
To find the pitch, lay the belt flat with the teeth facing up. Use calipers to measure across 10 teeth, placing the caliper tips in the grooves between teeth for accuracy. Divide that measurement by 10 to get the distance between tooth centers. Common pitches include 5 mm (often called HTD 5M) and 8 mm (HTD 8M).
Count the total number of teeth on the belt. The belt’s total length equals the pitch multiplied by the number of teeth. So a belt with 120 teeth at a 5 mm pitch has a 600 mm pitch length. Width is straightforward: just measure across the flat surface of the belt with a ruler. Timing belt part numbers typically encode all three values, making reordering simple once you’ve identified them.
Tips for an Accurate Measurement
A few common mistakes can lead to ordering the wrong size. If you’re measuring an old belt that’s been running for years, keep in mind that V-belts stretch with use. The measurement you take off a worn belt may be slightly longer than the original size. When in doubt, round down to the next available size rather than up, since a new belt needs to be tensioned properly and a too-long belt won’t grip the pulleys well.
Temperature also matters. A belt that’s been sitting in a cold garage will measure slightly shorter than one at room temperature. Let the belt warm up to indoor temperature before measuring for the most reliable number. And always lay broken belts as flat as possible before measuring end to end, then add a small amount for the break point if the ends overlap cleanly.
If you can find any markings on the old belt or the motor’s nameplate, use those first. Manufacturers stamp the belt size directly on the rubber in most cases. Look for a number like A42, B68, or 4L350 (where 350 means 35.0 inches). A legible stamp saves you the trouble of measuring entirely.