To identify a roller chain’s size, you need three measurements: pitch, roller diameter, and internal width. These three numbers, taken with a simple caliper, will tell you the exact chain number and standard you’re working with. Here’s how to take each measurement and match it to the right chain.
The Three Measurements That Matter
Every roller chain is defined by three critical dimensions. Get these right and you can identify virtually any standard chain.
Pitch is the distance from the center of one pin to the center of the next pin. This is the single most important measurement because it determines the base chain number. Since pin centers can be hard to eyeball, a practical shortcut is to measure from the inner edge of one pin to the inner edge of the next. Or, for better accuracy, measure across multiple links (say, 10 pins) and divide by the number of pitches to get an average.
Internal width is the gap between the two inner plates of a roller link. Place your caliper jaws inside the link, perpendicular to the chain’s length, and measure the space between the plates. This dimension separates chains that share the same pitch but belong to different standards.
Roller diameter is the outside diameter of the cylindrical roller that sits on the bushing and contacts the sprocket teeth. Close your caliper around a single roller to get this number. Together with pitch and internal width, it confirms both the chain size and the manufacturing standard.
Decoding ANSI Chain Numbers
Most roller chain sold in North America follows the ANSI (American) numbering system, and the numbers aren’t random. The first digit (or digits) represents the pitch in eighths of an inch. A #40 chain has a pitch of 4/8 inch, which is 1/2 inch. A #60 chain is 6/8 inch, or 3/4 inch. The final digit tells you the chain type: 0 means it’s a roller chain, while 5 indicates a bush chain (no rollers).
Here are the most common ANSI sizes and their key dimensions:
- #35: 3/8″ pitch, 0.184″ internal width, bush chain (no separate roller)
- #40: 1/2″ pitch, 0.309″ internal width, 0.312″ roller diameter
- #50: 5/8″ pitch, 0.370″ internal width, 0.400″ roller diameter
- #60: 3/4″ pitch, 0.495″ internal width, 0.469″ roller diameter
- #80: 1″ pitch, 0.620″ internal width, 0.625″ roller diameter
If your measurements fall close to one of these but not exactly, you’re likely dealing with a worn chain or a different standard entirely.
ANSI vs. British Standard Chains
This is where people run into trouble. Two chains can share the same pitch but have different roller diameters and internal widths depending on whether they follow the ANSI standard (common in North America) or the British/ISO standard (common in Europe and much of the rest of the world). British Standard chains use a “B” designation, like 08B, 10B, or 12B.
Consider a 1/2″ pitch chain. An ANSI #40 has a roller diameter of 7.95 mm and an internal width of 7.85 mm. A British Standard 08B-1 has a roller diameter of 8.51 mm and an internal width of 7.75 mm. Those differences are small enough to look identical at a glance, but the chains are not interchangeable. Mixing standards will cause premature wear or outright failure on the sprocket.
The same pattern holds at 3/8″ pitch (ANSI #35 vs. 06B) and 5/8″ pitch (ANSI #50 vs. 10B). The takeaway: pitch alone doesn’t identify a chain. You need all three measurements to confirm which standard you’re matching.
Heavy Series and Double Pitch Variants
If you see an “H” after a chain number (like 60H or 80H), that designates a heavy series chain. The pitch, roller diameter, and internal width are the same as the standard version. What changes is the link plate thickness, which is bumped up to match the next larger chain size. A 60H chain, for instance, uses plates that are 1.80 mm thick instead of the standard 60’s 1.46 mm. This gives it higher fatigue strength without changing the sprocket compatibility. You can identify an H-series chain by measuring the plate thickness with a caliper and comparing it to standard specs.
Double pitch chains are another common variant, especially on conveyors. They use the same roller diameter and internal width as a standard ANSI chain, but the pitch is exactly twice as long. Their numbering adds 2000 to the base chain number with a “C” prefix. So a double pitch version of a #60 chain becomes C2060. An “H” suffix (C2060H) means it also has heavier link plates. If your pitch measurement comes out to 1-1/2″ but the rollers match a 3/4″ pitch chain, you’re looking at a double pitch chain.
How to Check for Wear
Chain doesn’t literally stretch like a rubber band. What happens is the pins and bushings wear against each other, and the cumulative effect across many links makes the chain longer. This is called elongation, and it’s the primary reason chains fail or skip on sprockets.
To check for elongation, measure across as many links as practical and compare the total length to what the nominal pitch predicts. For example, 10 pitches of a #40 chain should measure exactly 5 inches when new. If that same span now measures 5.15 inches, the chain has elongated about 3%.
For standard roller chain, replacement is typically recommended at around 2.5% to 3% elongation. Beyond that point, the chain no longer seats properly on the sprocket teeth, which accelerates wear on both the chain and the sprocket. If you’re replacing a chain, it’s worth measuring the old one first. If it’s past 3%, inspect the sprockets too, because a new chain on worn sprockets will wear out faster than it should.
Putting It All Together
Grab a caliper (digital or dial, either works) and take these steps in order:
- Measure pitch: Pin center to pin center, or across multiple links divided by the number of pitches.
- Measure internal width: Inside the roller link, between the two inner plates.
- Measure roller diameter: Across the outside of one roller.
Compare all three numbers against an ANSI or British Standard size chart. If the pitch matches but the roller diameter and width don’t line up with ANSI values, check the B-series (British Standard) chart. If the pitch is double what you’d expect for the roller size, it’s a double pitch chain. If the plates look unusually thick for the chain size, measure them and check for an H-series match.
One last detail: multi-strand chains (duplex, triplex) use the same base dimensions as single-strand chains. The chain number just gets a suffix like “-2” or “-3” to indicate the number of strands. Your three core measurements stay the same regardless of strand count.