Anchor chain size is measured by its diameter, not its length or weight, and the two numbers that matter most are the inside link length and the wire diameter of the link itself. The standard reference point is the diameter of the round steel bar the link is made from — that is the number stamped on chain, quoted in catalogs, and matched to your windlass gypsy. If you measure the wrong part of the link, or measure a worn or stretched section, you will get a number that does not match any standard size.
What Does “Anchor Chain Size” Actually Mean?
Chain size refers to the diameter of the steel rod used to form each link. A “10 mm chain” has links made from 10 mm bar stock. This is the industry convention, and it is what every windlass manufacturer, chain supplier, and classification table uses.
It is not the outside width of the link. It is not the length of the link. It is not the weight per meter. Those are separate specifications that also appear on chain data sheets, but when someone says “chain size,” they mean bar diameter.
This matters because your windlass gypsy is machined to accept one specific chain size and pitch. A gypsy cut for 10 mm ISO chain will not reliably grip 10 mm chain built to a different link geometry, even though both are nominally “10 mm.” The diameter and the link proportions work together.
How Do You Measure the Diameter of a Chain Link?
Use a caliper, not a tape measure. A tape measure cannot resolve the difference between 8 mm and 9 mm, and that difference determines whether your chain fits your windlass.
Here is the process:
- Find a link that sits straight, not one at a bend or twist.
- Measure across the round part of the bar — the crown of the link — not across the weld or the stud.
- Take three measurements at different points on the same link and average them.
- Repeat on two or three other links in different parts of the chain.
The crown of the link is the curved outer section. On a stud link chain, avoid measuring anywhere near the stud, because the stud is a separate piece of metal and will give you a false reading. On a welded link, avoid the weld flash, which can sit slightly proud of the surrounding bar.
A caliper reading of 9.6 mm on a chain sold as 10 mm is normal. Chain is manufactured to a tolerance, and slight variation is expected. What you are looking for is whether the chain is close to a standard size or has worn well below it.
Why Does Link Length and Pitch Matter Too?
Two chains can share the same bar diameter and still not fit the same gypsy. That is because link length — the distance from the inside of one end of a link to the inside of the other — varies between chain standards.
ISO chain and DIN chain are the two common standards, and they do not have identical link proportions. A gypsy matched to one will not necessarily seat the other, even at the same nominal diameter. This is a genuinely common source of fitment problems, and it is often missed because buyers focus only on the millimeter number.
If you are replacing chain, measure the inside length of a link as well as the bar diameter. Compare both against the windlass manufacturer’s specification, not just against the chain you are removing. If the old chain was the wrong size to begin with, matching it will just reproduce the problem.
How Do You Measure Chain That Is Already Worn?
Worn chain measures smaller than its original size, and stretched chain measures longer. Both changes affect fit and strength.
Chain wears where links rub against each other, and it wears fastest at the crown of the link and at the contact points. If your caliper reads noticeably under the nominal size across multiple links, the chain has lost material. There is no single universally accepted wear limit that applies to every chain type and application, so the practical approach is to compare your measurement against the manufacturer’s stated wear allowance for that specific chain.
Stretch is a different problem. Chain does not stretch like a rubber band — the links deform permanently under load. Once that happens, the link geometry changes and the chain may no longer sit correctly in the gypsy. A chain that has visibly elongated links or that jumps in the gypsy should be assessed by a qualified rigger rather than measured and reused.
What Tools Do You Need to Measure Anchor Chain?
A digital or dial caliper is the main tool. A steel rule works for measuring inside link length. A tape measure is only useful for overall length, not for diameter.
| Measurement | Tool | What It Tells You |
|---|---|---|
| Bar diameter | Caliper | Nominal chain size and wear |
| Inside link length | Caliper or steel rule | Chain standard and gypsy fit |
| Outside link width | Caliper | Gypsy groove fit |
| Overall length | Tape measure | Rode length only |
If you do not own a caliper, most chandleries and rigging shops will measure chain for you at no cost. For a one-time check, that is often the simplest route.
What Are the Common Mistakes When Measuring Anchor Chain?
Measuring the weld is the most frequent error. The weld sits on the outside of the link and can be slightly thicker or thinner than the parent bar. A reading taken there will be wrong in either direction.
Measuring a single link is the second mistake. Chain varies link to link, and one reading tells you very little. Take several and look at the range.
Assuming the number on the old chain is correct is the third. Chain gets replaced with whatever was available, and mismatched chain is common on older boats. Measure what you have, then confirm against the windlass specification.
Confusing diameter with grade is the fourth. Grade refers to the strength of the steel — the letter and number designation on the chain — and it is a separate specification from size. A chain can be the correct diameter and the wrong grade, or the correct grade and the wrong diameter. Both matter, and they are measured or read differently.
How Do You Confirm You Have the Right Size?
Once you have a reliable diameter and link length, check them against the windlass manufacturer’s published chain specification. Most windlass makers list the exact chain sizes and standards their gypsies accept, and some publish a compatibility table.
If your measurement falls between two standard sizes, it usually means the chain is worn, the measurement was taken incorrectly, or the chain is a non-standard product. None of those situations should be resolved by guessing. A chain that does not seat properly in the gypsy can slip under load, and that is a safety issue, not just an inconvenience.
When in doubt, take a short section of your chain to a rigging shop and have it measured and identified against known standards. It takes minutes and removes the guesswork entirely.
Frequently Asked Questions
What part of an anchor chain do you measure for size?
Measure the diameter of the round bar that forms the link, taken at the crown of the link rather than at the weld or stud. That diameter is the number used to describe chain size.
Can I measure anchor chain with a tape measure?
No, a tape measure cannot resolve the small differences between standard chain sizes. Use a caliper for diameter and a caliper or steel rule for inside link length.
Does chain size mean the same thing as chain grade?
No. Size is the bar diameter, while grade describes the strength of the steel. A chain can have the correct diameter and the wrong grade, so both need to be confirmed.
Why does new chain of the same size not fit my windlass?
Because link length and pitch differ between chain standards such as ISO and DIN, even when the bar diameter matches. Check the windlass manufacturer’s specified chain standard, not just the millimeter size.
