Anchor chain size is determined by the diameter of the wire (the round steel bar) that forms each link. You measure this with calipers at the straightest section of the link, away from the bends. But diameter alone isn’t enough if you need your chain to work with a windlass. You also need to know the pitch (internal link length) and width, because those dimensions determine whether the chain seats properly in the gypsy.
The Three Measurements That Define Chain Size
Every anchor chain link has three critical dimensions: wire diameter, internal link length (pitch), and link width. Wire diameter is the primary number used to “size” chain. When someone says they have 5/16-inch or 8mm chain, they’re referring to the thickness of the steel bar that forms the link. Internal link length and width matter for windlass compatibility, since the gypsy (the notched wheel that grips the chain) is machined to fit a specific link shape.
Here’s how to take each measurement:
- Wire diameter: Place your caliper jaws around the straight side of a link, not at the curved ends. Measure at several links and average the results. Take two readings per link, rotating 90 degrees between them, and average those as well. This two-axis average accounts for any oval wear patterns.
- Internal link length (pitch): Measure the inside opening of a single link from end to end, along the long axis. This is the distance the gypsy pocket needs to span.
- Link width: Measure the widest external dimension across the link. Also check the internal width if your chain runs through a gypsy, since that clearance determines whether links can rotate freely through the windlass.
Tools and Technique
A digital or vernier caliper is the right tool. Tape measures and rulers lack the precision you need, especially when the difference between chain sizes can be less than 2mm. Digital calipers that read in both metric and imperial are ideal, since chain standards use both systems.
Avoid measuring at corroded spots, heavily worn bends, or the galvanized buildup near welds. Galvanizing adds a thin zinc coating that can inflate your reading slightly, so measuring at the straight midsection of the link gives you the truest number. Take readings from at least three or four separate links and average them. For pitch accuracy, a reliable technique is to measure across ten links from outside edge to outside edge, then divide by ten. This smooths out any variation between individual links and gives you a more precise average pitch than measuring a single link.
Common Sizes and What They Mean
Recreational boat chain typically comes in 1/4-inch (6mm), 5/16-inch (8mm), or 3/8-inch (10mm) sizes. The nominal size is a rounded label. Actual wire diameters vary slightly by grade and standard:
- 1/4-inch chain: Actual wire diameter runs about 7.0 to 7.1mm (0.276 to 0.281 inches) depending on grade.
- 5/16-inch chain: Actual wire diameter runs about 8.4 to 8.7mm (0.331 to 0.343 inches).
- 3/8-inch chain: Actual wire diameter runs about 10.0 to 10.3mm (0.394 to 0.406 inches).
Those small differences between grades matter. A Grade 30 BBB (short link) 1/4-inch chain measures 7.1mm, while a G43 High Test 1/4-inch chain measures 7.0mm. That’s close enough to be interchangeable in most cases. But at 5/16 inch, the spread between 8.4mm and 8.7mm can affect how the chain seats in a gypsy machined for one specific standard.
Chain Grade vs. Chain Size
Size and grade are independent specifications. Size is the physical dimension. Grade describes the strength of the steel. The grade number represents one-hundredth (or one-tenth) of the wire’s minimum breaking strength in megapascals. So G30 chain is made from steel rated at 300 MPa, while G70 chain is rated at 700 MPa.
G30 (also called Grade L or proof coil) and BBB are carbon steel chains commonly used in anchor rodes. G43, often sold as “High Test” or “HT,” is also carbon steel but stronger per link. G70 is heat-treated steel, quenched and tempered for significantly higher strength. You’ll sometimes see it called transport chain or trawl chain.
The critical thing to understand is that chains of the same nominal size but different grades can have different link dimensions. Peerless G70, for example, does not share the same link proportions as G43 in the same nominal size. This means you cannot assume that a chain labeled 3/8-inch in one grade will fit a gypsy designed for 3/8-inch in another grade. Always measure the actual links rather than relying on the label.
ISO vs. DIN Standards
If you’re shopping for metric chain, you’ll encounter two common standards: ISO 4565 and DIN 766. For most sizes these standards produce nearly identical link dimensions. At 12mm, they are functionally the same. The one notable exception is 10mm chain, where the dimensions differ enough that using the wrong standard on a gypsy designed for the other can cause jamming. If your windlass manufacturer specifies one standard, stick with it at the 10mm size.
Matching Chain to Your Windlass
The gypsy on your windlass is the most demanding constraint. It’s machined with pockets that grip specific link dimensions, so even small mismatches cause problems: the chain skips, jams, or fails to self-feed. When measuring your existing chain to find a replacement, record all three dimensions (wire diameter, internal length, and internal width) and compare them to the gypsy manufacturer’s specifications.
If you’re replacing worn chain, measure a section that still looks relatively new, typically the portion that sat in the chain locker and rarely deployed. Worn links will read smaller in diameter than new ones, which can lead you to order the wrong size. The international standard for assessing wear calls for measuring the diameter at the point of maximum wear and averaging two perpendicular readings, but for purchasing purposes you want the original, unworn dimension.
When in doubt, bring a short section of your old chain to a chandlery. Comparing it directly against known samples eliminates measurement error entirely and confirms both size and grade compatibility with your gypsy.
Spotting Worn Chain
While you have your calipers out, it’s worth checking whether your existing chain needs replacing. Measure the wire diameter at the most worn point of several links, typically where they bear against each other or against the gypsy. If the diameter has reduced by more than 10 to 12 percent from the nominal size, the chain has lost significant strength. Visible flat spots on link sides, elongated links, or stiff joints between links are additional signs of excessive wear. A chain that no longer seats smoothly in the gypsy has likely worn beyond its original dimensions and should be replaced rather than extended.