Roller chain size comes down to three measurements: pitch, roller diameter, and inside width. Get these three numbers right, and you can match any chain to its correct ANSI or ISO designation, order an accurate replacement, and confirm it will fit your sprockets. All you need is a caliper and a flat surface.
The Three Dimensions That Define Chain Size
Every roller chain, whether it drives a motorcycle, a conveyor, or an industrial machine, is classified by the same trio of measurements.
- Pitch: The distance from the center of one pin to the center of the next pin. This is the single most important number. Common pitches include 3/8 inch (9.525 mm), 1/2 inch (12.70 mm), 5/8 inch (15.875 mm), and 3/4 inch (19.05 mm).
- Roller diameter: The outside diameter of the small cylindrical rollers that sit between the inner link plates. These rollers are what contact the sprocket teeth.
- Inside width: The gap between the two inner link plates, measured at the roller. This dimension determines whether the chain seats properly on a given sprocket.
Even a small mismatch in any of these three numbers means the chain won’t mesh with your sprockets correctly. A chain with the right pitch but the wrong roller diameter will wear the sprocket teeth unevenly, and a chain with the wrong inside width may not drop onto the sprocket at all.
How to Measure Pitch
Lay the chain flat on a workbench and pull it taut so there’s no slack between the links. Using a caliper, measure from the center of one pin to the center of the adjacent pin. That distance is one pitch.
For better accuracy, measure across several links at once and divide. Span four to six links and count the number of pin-to-pin spaces (pitches) in that span. If you measure 3 inches across 3 pitches, for example, the pitch is 1 inch. Averaging over multiple links reduces the error from any single worn or slightly misaligned pin.
A digital caliper works best because pitch values often land on precise fractions. The difference between a 1/2-inch pitch and a 5/8-inch pitch is only 1/8 inch, which is easy to misjudge with a tape measure.
How to Measure Roller Diameter and Inside Width
With the chain still flat, place your caliper jaws around one roller and measure its outside diameter. Pick a roller that isn’t visibly damaged or worn flat on one side. If you suspect wear, measure two or three rollers and compare readings.
For inside width, open the caliper jaws and fit them between the two inner link plates at the roller. You’re measuring the internal gap, not the outer width of the chain. This measurement needs to match the sprocket tooth width, so precision matters here. Even a difference of half a millimeter can indicate a different chain series.
Decoding ANSI and ISO Chain Numbers
Once you have your three measurements, you’ll use them to match a standard chain designation. Two numbering systems dominate the market: ANSI (American) and ISO/BS (International/British). Chains with identical pitch can have different roller diameters and inside widths depending on which standard they follow.
Take chains with a 12.70 mm (1/2-inch) pitch. An ANSI #40 chain (designated RS40-1) has a roller diameter of 7.92 mm and an inside width of 7.95 mm. The ISO equivalent, 08B-1, has a roller diameter of 8.51 mm and an inside width of 7.75 mm. Those differences are small enough to look identical at a glance, but large enough to cause fitment problems on the wrong sprocket.
At 19.05 mm (3/4-inch) pitch, the gap is even more obvious. The ANSI #60 chain has a 11.91 mm roller diameter and 12.70 mm inside width, while the ISO 12B-1 has a 12.07 mm roller and 11.68 mm inside width. Swapping one for the other will cause premature wear or outright failure.
The simplest way to tell which standard you’re dealing with is to look at the side plates. Both ANSI and ISO standards require manufacturers to stamp the chain series designation onto the link plates. If you can read a marking like “40” or “08B,” you already know what you have and can skip straight to ordering. If the markings are worn away, your caliper measurements become the only reliable identification method.
Identifying Heavy-Series and Multi-Strand Chains
Some chains carry an “H” suffix in their designation (like 60H). Heavy-series chains share the same pitch, roller diameter, and inside width as their standard counterparts, so they fit the same sprockets. The difference is thicker side plates, which give the chain higher fatigue resistance for demanding loads. If you measure the three core dimensions and they match a standard size but the plates look noticeably thicker, you likely have a heavy-series chain. Measure plate thickness with your caliper and compare it to the standard specification for that chain number to confirm.
Multi-strand (or “multiplex”) chains are two, three, or more single-strand chains connected side by side on common pins. To identify them, measure the same three core dimensions on one strand, then measure the overall pin length. The pin length tells you how many strands the chain supports. A duplex (two-strand) chain has a longer pin than a simplex (single-strand) of the same pitch, and a triplex is longer still.
Checking for Wear While You Measure
Measuring a chain you’ve already been running gives you a chance to check whether it needs replacing. Roller chains stretch over time as the pins and bushings wear, and that elongation throws off sprocket engagement.
The standard method is to measure a set number of links and compare the total length to the maximum allowable value. For a chain with a 1/2-inch pitch, you’d measure 120 links. If that span reaches 62 inches or more, the chain has stretched beyond its service limit and should be replaced. A new 120-link section of 1/2-inch pitch chain would measure exactly 60 inches (120 times 0.5), so 62 inches represents roughly 3.3% elongation.
For a 1-inch pitch chain, 120 links should not exceed 123.5 inches. For a 3/4-inch pitch, the threshold is 93 inches over 120 links. Larger pitch chains use fewer links for the test: an 80-link span for 1-1/2-inch pitch (max 123.5 inches), or a 40-link span for 3-inch pitch (max 123.5 inches).
If you don’t have enough chain to measure that many links, you can scale down proportionally, but measuring across more links gives a more reliable average. A chain that’s borderline on elongation may still function, but it will accelerate sprocket wear. If you’re already pulling the chain off to measure it, replacing a worn one at the same time saves a second round of downtime.
Quick Reference for Common ANSI Sizes
These are the most frequently encountered ANSI roller chain sizes. All dimensions are nominal values for new chain.
- #25: 1/4-inch pitch, 0.130-inch roller diameter. Lightweight, often found on small equipment and automation.
- #35: 3/8-inch pitch, 0.200-inch roller diameter, 3/16-inch inside width. Common on small conveyors and low-power drives.
- #40: 1/2-inch pitch, 5/16-inch roller diameter, 5/16-inch inside width. One of the most widely used sizes for general industrial applications.
- #50: 5/8-inch pitch, 0.400-inch roller diameter, 3/8-inch inside width. Mid-range power transmission.
- #60: 3/4-inch pitch, 15/32-inch roller diameter, 1/2-inch inside width. Heavy industrial drives, agricultural equipment.
- #80: 1-inch pitch, 5/8-inch roller diameter, 5/8-inch inside width. High-load applications.
Notice the pattern in ANSI numbering: the first digit(s) multiplied by 1/8 give you the pitch in inches. A #40 chain has a pitch of 4/8, or 1/2 inch. A #60 is 6/8, or 3/4 inch. An #80 is 8/8, or 1 inch. Knowing this shortcut lets you estimate pitch from the chain number alone, which is useful when you can read a partial stamping on a worn plate.