V-Belt Size Chart & Cross-Reference Guide

You’re holding a worn V-belt with a code stamped on it, and you’ve got fifteen minutes to find the replacement before a line goes down. That code is its own little language: a letter for the cross-section, a number for the length. Read it wrong and you order the wrong part, wait days for a swap, and watch a machine that was fine yesterday sit dark. We’ve been on both ends of that phone call in our factory, and nine times out of ten the belt didn’t fail first — the part number in the purchasing inbox did.

Here’s the thing: a size chart and a cross-reference guide aren’t the same tool. A size chart tells you the physical cross-section you need. A cross-reference tells you which manufacturer’s belt lines up with which. This guide does both, using the classical and narrow section dimensions the industry standardized on decades ago, so you can decode what you’ve got, measure what you don’t, and order a replacement that actually fits — without relying on a vendor’s catalog to be kind to you.

Key Takeaways

• V-belt codes split into a section letter and a length number: A38 means A section, 38-inch nominal inside length.
• Classical sections (Z, A, B, C, D, E) follow standardized top-width and height dimensions across every manufacturer.
• Narrow sections (SPZ, SPA, SPB, SPC) map to US 3V, 5V and 8V equivalents, which keeps cross-Atlantic sourcing honest.
• Li, La and Ld are three different ways to measure a belt’s length — mixing them up is the single most common cross-reference mistake we see.
• An old, worn belt measures short. If the belt is stretched, measure the pulley groove instead of trusting the belt.
• When the cross-reference table isn’t enough, the rubber compound matters more than the dimensions — especially for heat and oil exposure.

Table of Contents

• How V-belt numbering works
• Classical sections chart
• Narrow sections & US equivalents
• Cross-referencing between brands
• How to measure an old belt
• When cross-reference is NOT enough
• FAQ

How V-belt numbering works

Strip a V-belt code down and you’re left with two pieces: the letter and the number. The letter is the section — it fixes the cross-section geometry, meaning the top width and the height of the belt. The number is the length. In the classical inch system the number is the nominal inside length in inches, so A38 is an A-section belt with a 38-inch nominal inside length. Simple enough, until you meet the metric system, where the same belt might be labeled in millimeters instead. An A-section equivalent in the metric world still shares the same top width and height, but the length number means a different unit, and that’s where a quick glance turns into a wrong order.

What trips people up is that the number isn’t always the same physical measurement. Different standards measure a belt at the inside, at the pitch line, or at the outside, and the difference between them is exactly why two belts with the same code from two factories can sit differently on the same pulley. We’ll get into Li, La and Ld in the cross-referencing section, because that’s the part that causes the most headaches. For now, just remember that the letter is geometry and the number is length — the rest is calibration.

Other drive components use their own numbering logic, and it pays to know which family you’re dealing with before you start measuring. If the belt in your hand came off a continuously variable transmission, the code system is a different animal entirely — our guide to CVT belt numbers walks through that separately. And if you’re fitting something to a car or a tractor rather than an industrial drive, the same letter-and-number logic still applies, but you’re more likely to shop by application code than by raw section. The automotive belt and agricultural belt sections of our catalog both start from the same section geometry you’ll see in the charts below.

Classical sections chart

Classical sections are the workhorses of power transmission, and they’ve been standardized long enough that the dimensions below are identical whether the belt is made in Hebei, Ohio or Düsseldorf. Top width is the widest point across the belt’s face; height is the thickness from the flat back down to the bottom of the V. Use these as the fixed reference, not whatever a local supplier happens to print on the box.

Start at the bottom and work up, because undersizing a section is how belts burn up in a season. Z section (10 x 6 mm) is what you’ll find inside small appliances and light equipment where a fractional horsepower motor does the work. It’s cheap, it’s light, and it dies fast if you push it past its intended load.

A section (13 x 8 mm) covers fans and pumps — the most common belt in light industrial use, and the one most home shops reach for first. B section (17 x 11 mm) is the default for general industrial drives: compressors, blowers, conveyors moving moderate loads. If you’re outfitting a shop floor and you aren’t sure what you need, this is where most standard industrial machinery lands, and it’s where our industrial belt range is deepest.

From there the sections get serious. C section (22 x 14 mm) handles heavy industrial work — big compressors and line shafts where a failure stops more than one machine. D section (32 x 19 mm) shows up on heavy machinery and mills, the kind of drive that earns its keep by running continuously. E section (38 x 23 mm) is the old heavyweight: very heavy duty, and honestly rare today, because two narrower belts will usually do its job more flexibly.

One question we hear a lot is whether to run a cogged version of the same section. A cogged belt has notches across the underside, and it runs cooler and more flexible than a solid wrapped belt at the same section. If you’re comparing the two for an industrial application, the trade-offs are laid out in our breakdown of classical vs cogged V-belts, and the surface treatment story — wrapped versus raw edge — is worth a read in wrapped vs raw-edge belts before you commit to a shortlist.

Narrow sections & US equivalents

Narrow belts were developed to squeeze more horsepower out of the same center distance. The V is deeper relative to its width, which lets the belt wedge into the groove harder, grip better, and transmit more power than a classical belt of similar width. That’s why you’ll see a compact drive using a narrow belt where an older design needed a full-size classical section. If you’re retrofitting a machine, the physics works in your favor — but only if you match the section correctly.

SPZ (9.7 x 8 mm) lines up with the US 3V (3/8″ x 5/16″), and SPB (16.3 x 13 mm) matches 5V (5/8″ x 17/32″). At the top, SPC (22 x 18 mm) pairs with 8V (1″ x 7/8″). The odd one out is SPA (12.7 x 10 mm), which has no exact US twin — it sits between 3V and 5V, so don’t force it into either groove. We’ve seen people try to substitute SPA with a 5V and grind a groove edge off in a month.

For the length on a narrow belt, the same Li/La/Ld discipline applies, and the US systems add their own letter conventions on top of it. The number after 3V, 5V or 8V is in tenths of an inch of pitch length, so a 3V475 is a 3V belt with a 47.5-inch pitch length. Again: different measurement basis from a classical A38, and a mistake waiting to happen if you assume the number means the same thing across both families.

Under genuinely high load, you’ll sometimes see operators asking whether a narrow belt on its own is enough or whether a banded (multiple-groove) belt is the smarter call. That’s a real engineering trade-off, and we wrote up the comparison in narrow vs banded belts under high load. Short version: a banded belt shares the load across several ribs and survives shock loads better, but it costs more and needs perfectly aligned pulleys.

Cross-referencing between brands

Cross-referencing means taking a belt code from one manufacturer and finding the equivalent from another. The good news: the section geometry is a universal standard, so an A38 from any reputable maker fits the same groove. The confusing news: each brand writes its code slightly differently, and the number isn’t always measured on the same basis. Here’s the quick map.

The numbering system is an industry-wide convention, not a brand’s invention. An A38 means A section with a 38-inch nominal inside length, and any legitimate manufacturer’s A38 is interchangeable on the same drive. Gates puts it behind the Hi-Power II line name, Optibelt wraps it in the VB series, while Continental and Dayco just stamp the section code directly. Same belt, different branding.

Now the trap. Inside length (Li), pitch length (La, sometimes written Ld) and outside length (Lc) are three different measurements of the same belt. Li is measured at the inner surface, La at the pitch line where the belt’s neutral axis runs, and Ld or Lc at the outside. For a classical A-section belt the difference between inside and pitch length runs roughly an inch per standard increment, and if one manufacturer’s A38 is an inside length while another brand’s number is a pitch length, the two belts can sit differently even though the code looks identical.

That’s the mistake that costs a downtime call. When you’re matching across brands, find out which length basis the code uses — most catalogs state it in the fine print — and if in doubt, measure the drive rather than trusting the code. This is also the moment to be honest about what you’re buying. A cheap import stamped with the same code isn’t automatically the same belt, and if you’re sourcing in volume, our belt quality inspection checklist for importers walks through what to verify before a container ships rather than after it lands.

How to measure an old belt

Measuring the belt you took off the machine feels obvious, and it’s exactly where people go wrong. A V-belt that’s been running for years has stretched and worn, so its code no longer matches reality. The belt can measure a half-inch to an inch longer than its original size, and if you order by that worn measurement, the new belt will be too long, sit loose in the groove, slip under load, and glaze over within weeks. Don’t trust the old belt — trust the drive it was running on.

The string method is the honest way to do it. Wrap a piece of string or a flexible tape around the outside of the two pulleys exactly where the belt rides, keeping it tight enough to follow the belt’s path without stretching it. Mark the point where the ends meet, lay the string flat, and measure it. Add the small allowance for how far the belt sits into the groove, then round to the nearest standard length. That gives you a usable starting point for ordering.

If the belt is missing entirely, measure the pulleys instead. For a V-groove pulley, wrap the string around the bottom of the groove — the narrowest point — to get the outside circumference of the belt path, then subtract the depth the belt sits into the groove on both sides to estimate the effective length. It’s less precise than measuring a matched belt, but it’s far better than guessing, and it’s the same math a decent drive designer runs on paper. Record both pulley diameters and the center distance; that pair of numbers lets anyone with a belt length calculator confirm your result.

When you do order the replacement, don’t mix an old belt and a new belt on the same multi-groove drive. New belts stretch slightly in their first hours, and running a fresh belt beside a settled one puts all the load on the fresh one. Fit a matched set, then re-tension after the first shift. And when the new belt does come out of the box, give it a fair start — proper storage and handling habits matter more than people think, and our belt storage guide covers the temperature and hanging methods that keep stock from going bad before it’s ever fitted.

When cross-reference is NOT enough

A cross-reference table matches dimensions. It does not match rubber. Two belts with identical codes can still be completely different parts if the compound is different, and compound differences show up in exactly the environments where a generic replacement gets punished.

Heat is the first one. A standard CR or SBR belt will harden and crack near a furnace, an oven exhaust, or any drive that runs hot hour after hour. If your application sees sustained high temperature, the section is only half the answer — you need a heat-resistant compound, and we break down the choices in how to choose heat-resistant belts. Related to that, the material itself matters: EPDM handles heat and weathering far better than CR in most industrial settings, and the two aren’t interchangeable even when the code matches. The material comparison in EPDM vs CR rubber belts shows why the same A-section can fail in one plant and run for years in another.

Oil and chemical exposure is the second environment that ignores the size chart. A belt that lives around cutting fluid, grease or solvent needs an oil-resistant compound, and the marking on the belt can tell you more than the code. When you see EPDM stamped on a belt and you don’t know what it stands for, it’s worth a minute — what EPDM means on a belt explains the letters that actually tell you what the rubber is made of.

Then there’s the load profile itself. If your drive absorbs shock loads, starts under full load, or runs at a speed that’s pushing the edge of the section’s rating, matching dimensions alone will leave you replacing belts too often. That’s when the belt becomes a design question, not a catalog question, and a manufacturer who can match the compound to your actual running conditions is worth more than the best cross-reference chart. That’s a conversation we have every week in the factory, and it’s the reason we keep 130+ proprietary rubber formulations instead of one “universal” belt.

FAQ

What does A38 mean on a V-belt?

A38 is an A-section belt with a 38-inch nominal inside length. The letter fixes the cross-section geometry (top width 13 mm, height 8 mm) and the number fixes the length, so an A38 from any manufacturer that follows the standard fits the same drive.

Can I replace a classical V-belt with a narrow belt?

Only if you change the pulleys to match. Classical and narrow sections have different groove geometry, so a narrow belt will sit at the wrong depth in a classical groove and slip. Replacing sections means replacing or machining the pulleys, not just swapping the belt.

What’s the difference between Li, La and Ld on a V-belt?

They’re three length measurements of the same belt. Li is the inside length, La is the pitch length measured at the neutral axis of the belt, and Ld (or Lc) is the outside length. The difference between them is small but real, and mixing up the basis when cross-referencing between brands can leave you with a belt that doesn’t tension correctly.

Why does my new belt run loose when I measured the old one carefully?

Because the old belt was stretched. A worn belt measures longer than its original size, and ordering to that stretched measurement gives you a belt that’s too long. Measure the drive or the pulley path rather than the worn belt, or use the belt’s original code.

Is a cross-reference chart enough when buying belts in bulk?

It’s the right starting point for dimensions, but not for compound quality. Two belts with the same code can differ in heat resistance, oil resistance and durability depending on the rubber. Verify compound ratings and quality control before committing to a large order rather than relying on the code alone.

Final takeaway

The V-belt code is a compact language, and once you read it — letter for section, number for length, and the Li/La/Ld basis underneath — you’ll stop gambling on replacements. Start with the classical and narrow charts to fix the section, use the brand map for the equivalent, and measure the drive rather than the worn belt. Then, when the environment is hot, oily or brutal, stop looking at dimensions and start looking at the compound.

That last step is where a factory with real compounding experience earns its keep, and it’s the part we can’t hand you in a table. If you’re replacing a belt in a demanding application, or sourcing belts in volume for a product you build, talk to someone who can match the rubber to the job — contact our team and we’ll work it out from the actual running conditions. And if you’re designing your own equipment and want belts made to your spec, our OEM/ODM program exists exactly for that.

About Longyi Rubber Products Factory

Longyi Rubber Products Factory (brand: LYBELT) is a Chinese belt manufacturer based in Hebei, Xingtai, founded in 1999. We hold IATF 16949 certification plus ISO 9001, 14001 and 45001, and we work from 130+ proprietary rubber formulations to build automotive belts, industrial belts, agricultural belts, ATV/UTV belts and motorcycle/scooter belts. Beyond standard products, we offer OEM, ODM and private label services for customers who need belts made to their own spec.

If you’re sizing a drive, replacing a worn belt, or setting up a supply for a product you manufacture, contact our team and we’ll help you match the section, the length and the compound to your actual application.