How to Measure a V-Belt
How to Measure a V-Belt
When the part number on your V-belt is worn off or unreadable, you need three measurements to identify the correct replacement: top width, depth, and outside circumference. These three dimensions tell you the cross section (which determines the sheave groove it fits) and the length (which determines the part number). This guide walks through each measurement with the conversion formulas for every section family.
If you would rather skip the measuring, send us a photo of the belt showing the cross section and any remaining markings, along with a ruler or coin for scale. We identify V-belts every day and can usually confirm the correct part number within the hour. Email sales@texasbelting.com or call 888-203-2358.
Tools You Need
- Ruler or caliper for measuring top width and depth
- Flexible tape measure for measuring outside circumference (a fabric sewing tape works well)
- The removed belt (measuring while the belt is still on the machine is much less accurate)
If you cannot remove the belt, see the "Measuring On the Machine" section at the bottom of this page.
1 Measure Top Width (Determines Cross Section)
Lay the belt flat on a surface. Measure straight across the widest (top) surface with a ruler or caliper. This is the top width.
| Top Width | Possible Sections | How to Distinguish |
|---|---|---|
| 1/4" (6.4 mm) | 2L | Only one option at this width |
| 3/8" (9.5 mm) | 3L, 3V, 3VX | 3L is shallower (7/32" deep, FHP). 3V/3VX are deeper (5/16"). 3VX has cogs. |
| 1/2" (12.7 mm) | A, AX, 4L | All are 5/16" deep. A has fabric cover. AX has raw edge + cogs. 4L is lighter FHP construction. |
| 5/8" (16 mm) | B, BX, 5L, 5V, 5VX | Measure depth. B/BX = 13/32". 5V/5VX = 17/32" (deeper). 5L = 3/8" (shallower, FHP). |
| 21/32" (16.7 mm) | 5L | Only one option at this width (some 5L measure slightly wider than 5/8") |
| 7/8" (22 mm) | C, CX | C has fabric cover. CX has raw edge + cogs. Both 17/32" deep. |
| 1" (25.4 mm) | 8V, 8VX | 8V is wrapped. 8VX has cogs. Both 7/8" deep. |
| 1-1/4" (32 mm) | D | Only one option at this width. 3/4" deep. |
| 1-1/2" (38 mm) | E | Only one option at this width. 1" deep. |
2 Measure Depth (Confirms Cross Section)
With the belt still lying flat, measure from the top surface straight down to the bottom point of the V. Do not measure along the angled sidewall. Measure through the center of the belt.
| Width + Depth | Section | Type |
|---|---|---|
| 1/4" x 3/16" | 2L | FHP |
| 3/8" x 7/32" | 3L | FHP |
| 3/8" x 5/16" | 3V / 3VX | Narrow Wedge |
| 1/2" x 5/16" | A / AX / 4L | Classical or FHP |
| 5/8" x 3/8" | 5L | FHP |
| 5/8" x 13/32" | B / BX | Classical |
| 5/8" x 17/32" | 5V / 5VX | Narrow Wedge |
| 7/8" x 17/32" | C / CX | Classical |
| 1" x 7/8" | 8V / 8VX | Narrow Wedge |
| 1-1/4" x 3/4" | D | Classical |
| 1-1/2" x 1" | E | Classical |
Wrapped vs. Cogged: Check the Underside
Once you know the cross section, flip the belt over and look at the inside (bottom) surface:
- Smooth bottom with fabric cover on all surfaces = Wrapped (A, B, C, D, E, 3V, 5V, 8V)
- Molded notches (cogs) on the bottom, raw rubber sidewalls = Cogged (AX, BX, CX, 3VX, 5VX, 8VX)
Cogged and wrapped belts of the same section are interchangeable on the same sheave. If replacing a wrapped belt, you can upgrade to cogged for better performance without changing sheaves.
3 Measure Outside Circumference (Determines Length/Part Number)
Wrap a flexible tape measure around the outside (top) surface of the belt. Pull snug but do not stretch the belt. Record this measurement as the outside circumference (OC).
Now convert the OC to a part number using the correct formula for your section family:
| Section Family | Part Number Formula | Example |
|---|---|---|
| A / AX | Part # = OC minus 2 | OC 70" → A68 or AX68 |
| B / BX | Part # = OC minus 3 | OC 103" → B100 or BX100 |
| C / CX | Part # = OC minus 4 | OC 124" → C120 or CX120 |
| D | Part # = OC minus 5 | OC 245" → D240 |
| E | Part # = OC minus 7 | OC 367" → E360 |
| 3V / 3VX | Part # = OC x 10 | OC 35.5" → 3V355 or 3VX355 |
| 5V / 5VX | Part # = OC x 10 | OC 106.0" → 5V1060 or 5VX1060 |
| 8V / 8VX | Part # = OC x 10 | OC 170.0" → 8V1700 or 8VX1700 |
| 2L, 3L, 4L, 5L | Part # = OC x 10 | OC 59.0" → 4L590 |
4 Account for Belt Stretch
A used V-belt stretches during its service life. A belt that started as a B68 may now measure 69" or 70" inside length. When your calculated part number falls between two standard sizes or does not match any standard size, round down to the nearest standard length.
Typical stretch by belt age:
- Lightly worn (under 6 months): 0 to 1/2" stretch
- Moderately worn (6 months to 2 years): 1/2" to 1-1/2" stretch
- Heavily worn (2+ years or visibly glazed/cracked): 1" to 3" stretch
Example: Your B-section belt measures 104" OC. That gives B101 (104 minus 3 = 101). There is no standard B101. The belt has likely stretched from B100 (standard). Order B100.
Measuring a V-Belt On the Machine
If you cannot remove the belt, you can still identify it:
Width and Depth
Find an accessible straight section of belt between sheaves. Measure the top width with a ruler pressed against the top surface. For depth, you may need to pull the belt slightly away from the sheave to get a caliper on it. This is usually enough to determine the section.
Length (Using Sheave Diameters and Center Distance)
If you cannot wrap a tape around the belt, measure the drive dimensions instead:
- Measure the outside diameter (OD) of both sheaves
- Measure the center distance between the two shaft centers
- Use the belt length formula: L = 2C + 1.57(D + d) + (D - d)² / (4C)
Where L = pitch length, C = center distance, D = large sheave OD, d = small sheave OD.
This formula gives an approximate pitch length. Converting to the correct part number requires adjusting for the difference between pitch length and inside length (for classical) or outside circumference (for narrow wedge). Texas Belting can do this conversion for you. Call 888-203-2358 with the sheave ODs and center distance.
Fastest Method: Send Us a Photo
If measuring is impractical or you are not confident in your measurements, send us a photo and we will identify the belt. Here is what to include:
- Cross section view. Photograph the end of the belt so the V shape is visible. Place a ruler, coin, or other object of known size next to it for scale.
- Any remaining markings. Even partial part numbers, brand logos, or date codes help narrow the identification.
- Top surface measurement. If you can measure the top width, include it.
- Outside circumference. If you can wrap a tape around the belt, include the OC measurement.
Email the photo and measurements to sales@texasbelting.com or submit through our contact form. Our team identifies belts every day and can usually confirm the correct part number within the hour during business hours.
V-Belt Measurement Cheat Sheet
| Section | Width | Depth | OC Conversion | Example (OC → Part #) |
|---|---|---|---|---|
| A / AX | 1/2" | 5/16" | OC minus 2 | 70" → A68 |
| B / BX | 5/8" | 13/32" | OC minus 3 | 103" → B100 |
| C / CX | 7/8" | 17/32" | OC minus 4 | 124" → C120 |
| D | 1-1/4" | 3/4" | OC minus 5 | 245" → D240 |
| E | 1-1/2" | 1" | OC minus 7 | 367" → E360 |
| 3V / 3VX | 3/8" | 5/16" | OC x 10 | 35.5" → 3V355 |
| 5V / 5VX | 5/8" | 17/32" | OC x 10 | 106.0" → 5V1060 |
| 8V / 8VX | 1" | 7/8" | OC x 10 | 170.0" → 8V1700 |
| 4L (FHP) | 1/2" | 5/16" | OC x 10 | 59.0" → 4L590 |
Frequently Asked Questions
A ruler or caliper for width and depth, and a flexible tape measure (like a fabric sewing tape) for outside circumference. Remove the belt from the machine before measuring if possible. If the belt cannot be removed, measure the sheave diameters and center distance instead and call Texas Belting for a part number calculation.
Inside length is measured around the inner (bottom) surface of the belt. Outside circumference (OC) is measured around the outer (top) surface. OC is easier to measure with a tape because the top surface is smooth and accessible. Classical belt part numbers (A, B, C, D, E) are based on inside length. You convert from OC by subtracting a factor (A = 2, B = 3, C = 4, D = 5, E = 7). Narrow wedge and FHP part numbers use OC multiplied by 10 directly.
Measure the depth. B is 13/32" (10.5 mm) deep. 5V is 17/32" (13.5 mm) deep. The 5V is noticeably deeper even to the naked eye. Also check the sheave groove: B grooves are wider and shallower, 5V grooves are narrower and deeper. B and 5V are not interchangeable. See our V-Belt Size Chart for the full cross section comparison.
The belt has likely stretched from use. Round down to the nearest standard part number. For example, if your B-section belt calculates to B101 or B102, order B100. V-belts stretch over their service life, so the measured length of a worn belt is always longer than its original part number. If you are unsure, call Texas Belting at 888-203-2358 with your measurement and we will confirm the correct size.
You can measure the top width and depth on an accessible span between sheaves. For length, it is more accurate to measure the sheave outside diameters and center distance, then calculate the belt length using the formula L = 2C + 1.57(D+d) + (D-d)^2/(4C). Texas Belting can do this calculation for you if you provide the sheave and center distance measurements.
Yes. Send a photo of the belt cross section (end view showing the V shape) with a ruler or coin for scale, plus any remaining part number markings. Include the top width and OC measurement if you have them. Email sales@texasbelting.com or submit through the contact form. We typically confirm the correct part number within the hour during business hours.
Width and depth need to be accurate to within about 1/16" to correctly determine the section. For outside circumference, accuracy to within 1" is usually sufficient because standard belt lengths are spaced 2" to 5" apart in most sections. The stretch adjustment (rounding down to the nearest standard size) accounts for minor measurement variation.
Related Pages
Can't Figure Out Your Belt?
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