Sizing a roller chain comes down to three core measurements: pitch, roller diameter, and roller width. Whether you’re replacing a worn chain or selecting one for a new drive system, these dimensions determine which chain you need. Getting them right ensures your chain meshes properly with its sprockets and handles the load without premature failure.
The Three Measurements That Define Chain Size
Every roller chain is identified by three physical dimensions, all measured in inches under the ANSI standard used in North America:
- Pitch (P): The distance from the center of one pin to the center of the next pin. This is the single most important measurement and the basis of the chain’s size number.
- Roller diameter (D): The outside diameter of the small cylindrical rollers that sit between the link plates. This determines how the chain seats in the sprocket teeth.
- Roller width (W): The internal distance between the inner link plates, sometimes called the inner width. This must match the sprocket tooth width.
You can also measure the plate height, plate thickness, and pin diameter for a more complete identification, but pitch, roller diameter, and roller width are enough to match a chain to its ANSI size number in most cases.
How to Measure an Existing Chain
If you’re replacing a chain that’s already installed or sitting on your workbench, start with pitch. Measure from the center of one pin to the center of the adjacent pin. A caliper gives you the most accurate reading, but a tape measure works if you measure across 10 or more pins and divide by the number of pitches (the spaces between pins, not the pins themselves). Measuring across multiple links and dividing reduces error from any single measurement.
For roller diameter, place your caliper across the outside of one roller. For roller width, measure the gap between the two inner plates on a roller link. Compare all three numbers to a standard roller chain size chart to confirm the ANSI chain number.
A few tools make this easier. A standard caliper (digital or dial) handles pitch, roller diameter, and roller width precisely. For checking wear on an installed chain, dedicated chain wear indicators from manufacturers like Tsubaki let you clip a gauge onto the chain and get an instant read on whether the chain has stretched beyond its service limit.
Decoding ANSI Chain Numbers
The ANSI numbering system encodes the pitch directly into the chain number, which makes identification straightforward once you know the trick. The first digit (or digits) of the chain number represents the pitch in eighths of an inch. The last digit tells you the chain type: 0 means standard roller chain, 5 means bushing chain (no rollers).
So a #40 chain has a pitch of 4/8 inch, or 1/2 inch. A #60 chain has a pitch of 6/8 inch, or 3/4 inch. A #80 chain is 8/8 inch, or 1 inch pitch. If you measure a pitch of 1/2 inch, you almost certainly need a #40 chain.
A suffix indicates multi-strand chains. A #40-2 is a duplex (two-strand) version of the #40 chain, and a #40-3 is triplex (three-strand). Multi-strand chains carry more load at the same pitch and speed.
ISO Numbering for European Chains
If you encounter a chain labeled something like 08B-2, that’s the ISO (European) standard. The first two digits represent pitch in sixteenths of an inch: 08 means 8/16, which is 1/2 inch. The “B” designates European standard. The suffix works the same way, so 08B-2 is a duplex 1/2-inch pitch chain. An ANSI #40-2 and an ISO 08B-2 share the same pitch but may differ slightly in roller diameter and plate dimensions, so they aren’t always interchangeable. Check all three key measurements before swapping between standards.
Sizing a Chain for a New Application
When you’re designing a new drive rather than replacing an existing chain, you need to work from horsepower and speed rather than physical measurements. The standard selection process uses four pieces of information: the horsepower your drive must transmit, the RPM of the small (driving) sprocket, the type of load, and the number of sprocket teeth.
Step 1: Calculate Design Horsepower
Start with the actual horsepower your motor or engine delivers to the drive. Then multiply it by a service factor that accounts for load conditions. Smooth, steady loads (like a centrifugal pump driven by an electric motor) use a lower service factor, typically around 1.0 to 1.2. Moderate shock loads (like a conveyor with variable loading) bump the factor up to roughly 1.2 to 1.4. Heavy shock loads (like a rock crusher or punch press) push it to 1.4 to 1.7 or higher. The result is your design horsepower, which represents the capacity the chain actually needs.
Using the raw motor horsepower without a service factor is one of the most common sizing mistakes. A 5-horsepower motor driving a machine with heavy shock loads might require a chain rated for 7.5 or more design horsepower.
Step 2: Use a Selection Chart
Chain manufacturers publish quick selector charts that plot design horsepower on the vertical axis against small sprocket RPM on the horizontal axis. Find where your two values intersect, and the chart tells you the recommended chain pitch. If your design horsepower falls between the single-strand and double-strand columns, the chart also indicates whether a multi-strand chain at a smaller pitch would be a better choice than stepping up to a larger single-strand chain.
A smaller pitch chain running on a multi-strand configuration often provides smoother operation and longer life than a single large-pitch chain, especially at higher speeds. Larger pitch chains handle heavier loads at lower speeds more efficiently.
Step 3: Verify Sprocket Compatibility
Once you’ve identified the chain pitch, make sure your sprocket tooth count falls within the recommended range for that pitch. Fewer teeth on the small sprocket means more articulation per revolution, which accelerates wear. Most guidelines recommend a minimum of 17 teeth on the small sprocket for smooth operation, though 21 or more is preferred for high-speed drives.
Checking for Chain Wear
Roller chain doesn’t literally stretch like a rubber band. What happens is the pins and bushings inside each link wear against each other, creating tiny amounts of slack that add up across every link. The result is a chain that measures longer than it did when new, a condition called elongation.
To check for elongation, measure a section of chain across as many links as practical and compare the measurement to what the same number of links should measure when new. For example, 10 pitches of a #40 chain should measure exactly 5 inches (10 x 0.5 inches). If that same section now measures 5.15 inches, the chain has elongated 3%.
The general industry rule is to replace a roller chain when elongation reaches 3% for most drive applications. Precision applications or drives with fixed center distances may require replacement at 1.5% to 2% elongation because the chain will no longer seat properly on the sprocket teeth. Running a chain past its elongation limit causes it to ride up on the sprocket teeth, which accelerates wear on both the chain and the sprockets and can lead to the chain jumping off entirely.
Common Sizes and Their Typical Uses
A handful of ANSI sizes cover the vast majority of applications:
- #25 (1/4″ pitch): Light-duty, low-power applications like small machinery, robotics, and go-karts.
- #35 (3/8″ pitch): Light industrial equipment, small conveyors, and power transmission on fractional-horsepower motors.
- #40 (1/2″ pitch): One of the most widely used sizes. Found on agricultural equipment, moderate industrial drives, and larger conveyors.
- #50 (5/8″ pitch): Medium industrial drives and equipment requiring more load capacity than #40.
- #60 (3/4″ pitch): Heavy-duty industrial applications, large conveyors, and equipment with significant torque demands.
- #80 (1″ pitch): Heavy industrial machinery, mining equipment, and high-torque, lower-speed drives.
If your application falls between two sizes, always go with the larger chain. The cost difference between adjacent sizes is modest, but undersizing a chain dramatically shortens its life and creates a safety risk.