How to Measure the Diameter of Braided Line | Expert Advice

Hi, this is Kyle from APS. I’m going to show you how to measure the diameter of braided line properly with high confidence on both used line and brand new line.

Line Diameters are Nominal Values

Okay, the first thing we need to do is set our expectations around line diameters for braided lines. Line measurements are nominal values. That is to say that the list of diameters are approximations. Things that contribute to the diameter of a line would be the type of fiber being used, maybe a blend of fibers. The braid type, 8-plate, 16-plate, 12-strand.

Then there’s the braid angle, how tight or loose of an angle is it? Then there’s another thing called linear density. That’s basically how much material is found in a given length of line and it goes on. Manufacturers are balancing a lot of things. They may be trying, when they set out to make a line, to achieve a number of different things. Maybe the priority isn’t diameter, it’s a specific strength or a low stretch characteristic, or maybe a handling property. Maybe, lastly, that’s where diameter is in the priority line. It’s a tall order.

Remember that braided line diameters vary and the value that best represents the actual diameter is applied. This can involve rounding a diameter up or down. If you can imagine, they’ve got a finished line, it comes off the braider, and they measure the diameter and it comes in at 11/32”. Well, 11/32” is not a diameter that we commonly use when talking about running rigging.

That’s halfway between 3/8” and 5/16” and they’re going to have to make a call. They’re either going to round it up to 3/8”, and in that instance, we would call it a skinny or a thin 3/8”, ‘because it’s not quite 3/8”, or they round it down and call it a 5/16”. In that case, we commonly call that a fat 5/16”. It would be specified as either a 3/8” or a 5/16”. Story is, expect variation.

New Line ‘Bedding In’

One last thing about a new line, there is … When you get it off the spool, you go and use it for the first time, the diameter will decrease slightly. This is called bedding in. As a new line that’s cycled through load through use, the fibers and the braids and strands start to come into alignment and settle in to each other, shoulder up, and you get that initial bedding in, and you’ll lose just a slight amount of diameter.

Measuring braided Line Diameter bedding in


Measuring the Diameter of New Line

We’re going to start off with new lines right of the spool and we’re going to do two things, we’re going to confirm the diameter that the spool says or somebody told us, or we’re going to be trying to figure out what the diameter of this line is because the label on the spool came off, or the person that handed it to us just doesn’t know.

For this one, I’ve been told this is ½” and I want to confirm that by measuring it myself. This is a 12-strand. I have two challenges here. This is a very fuzzy type of line which makes it bulk up and the construction is such that in a relaxed state it obviously is a larger diameter, not true diameter.

Know that when you’re measuring any line diameter, it is always under a load, not just sitting here in an unloaded state. If you could get a hundred pounds on it, that would be great. I’m comfortable putting 10 or 20 pounds by hand on this and having worked the line down. In this case, I’m trying to work the fuzz down and I’m trying to get the braid to compact to its loaded diameter.

It may not have enough load on it here. It’s a little bit of a challenge to get the calipers on this right now. I’m going to put a little more tension on it and that’s done it. I think if I had 100 pounds on there, it would be right at the ½”, but this is what we call a fat ½”. It’s just a little bit big.

Moving to this, this is a little bit tighter. This is a double braid polyester, but it does have, by design some fuzz in the jacket here. That slightly inflates its diameter. Here, again, I want to work the braided core down, the braided jacket, get any of that fuzz down. Great job, right at the ½”.

Let’s say I didn’t know what diameter this line was. The label on the spool is gone and I’m starting from scratch. I’m going to take the brake off which allows the jaw to freely move. I’m going to go back and load this line and hold it in a loaded position with the jaws running along the lines, so it’s touching as much surface area. That’s my start. Then I’m just going to turn it to 90-degrees and run it down the line. I’m going to take a look at this. This just came in a hair under a ½”. I’m very comfortable and confident I’ve got a half-inch there.

Moving on 7/16” line, some different constructions. I’m going to preset my calipers. Let’s see, we’ve got 1/8, ¼, 3/8, 7/16”, set the brake. This is a double braid polyester, braided core, braided jacket. I need to get both of those braids loaded up and I want to confirm. Very nice. Just touching the nubs on the line. Very confident on that 7/16”.

This is a double braid, but it’s a coated spectra core and the coatings on these usually glue these cores down into a set orientation, so there’s not much constructional bulk in that, but there isn’t a jacket, because the jacket sometimes slide like that and get real thick. I’m going to make sure I’ve got this melt all the way down and well under pressure, under load. Really hit that one. I’m just making slight contact with the nubs of the cover. Very confident, I’ve got 3/8”.

We’re moving down to 5/16”. Take my brake off. I’m going to go down to … And set it again. Two lines here, both with Dyneema cores. They’re glued and constructionally set. This is an all polypropylene, and this is a polypropylene polyester blend. All of these looks really tight. It doesn’t look like I can get any more bulk out of this jacket.

This one looks really fuzzy, but its got a tight jacket. Here we go, I’m going to start with this one, 5/16”. I’m having to put a little pressure on it, but it’s running. I don’t have any flat spots. Yup, right at 5/16”. I’m happy with that.

This … Get any of the fuzz down, get it under load. This is a fat one. We try one more time. Yeah, I’m going to have to put some pressure. I’m on there, I’m just making more contact as I run this down, but it’s making it so. I’d call this a slightly fat 5/16”. In all fairness, this is a European manufactured line. It was done in millimeters, nominal millimeters, and then it was converted over to nominal imperial at the 5/16”. This could be twice removed.

All right. A couple more lines on the new 12-strand. This is a single braid, this is Dyneema, it’s coated. Sometimes you get this stuff and it’s rock-hard, it won’t even fall, because the coating is to thick. That means that it’s been glued and it’s pre-stretched, preset construction mode, and you’re not going to have to milk this out much. This one’s a bit softer, so I’m going to make sure that I put a good amount of pressure on it.

With 12-strands, hi-techs like this, especially the Dyneemas, they’re not always round, so it’s important that you get any flat spots out of the line before you measure it. If this was a used piece of 12-strand, it’s been under load under a strap or something, you’ll see that it’s really flattened out in those areas. It wouldn’t be a good area to measure it.

I’m actually trying to get this completely round right now. I don’t feel any flat spots anymore. I’m going to get it under pressure. I need to change my calipers down to ¼”, set the brake, get back to this. Still feeling round. Initially, there’s a little bit of a struggle. I think I might have caught a little bit of the Spectra. I’m rotating this in case there are any more flats in it. It’s making contact, but it’s going smoothly over. ¼”, I’m comfortable with that.

These are control lines, 8-plate. We got 8-strands of line; or 16-plate, we’ve got 16 plates of line being woven. This is smoother. This is a little nubbier. This is really tight. I don’t feel the cover moving much, but I’m going to make sure that I’ve got it under load. There, perfect. Just making slight contact. With the 8 … This one’s … Yeah, it’s small. It’s a skinny ¼”.

We’ve had some fat ones. We’ve had some okay ones. We’ve had another one that was pretty fat. This one is actually slightly undersized. They are all coming in at their nominal diameters.

How to measure the diameter of new braided lines


Measuring the Diameter of Used Line

Now we’re going into the strange world of measuring used line from a boat. We, through the years, have had some really odd line come through the door where people want it replaced or it’s been shipped to us. Sometimes you’re wondering how it was even in service the week before.

These are some good average examples of line that would come to us for replacement where you might be facing. We need a good area to measure from. With some of these lines, you can see that this has got an irregularity from a knot. This has got some damage to the cover, some flat spots from being under load right there. You can see with this one, it’s been under load there. None of these area would be anything that you would want to measure for an accurate diameter. Find a good area to measure.

What’s the condition of the line? Do you think it’s the same diameter as when it was new? Do you think that it is really damaged by the sun, it’s gotten fuzzy, it’s got a lot of broken fibers in it? It’s got that chalky feeling and it’s very spongy like this one? I can feel that this has come and the braids come together, and this is probably going to present as thicker than it was as a new line.

Is it something like this smaller control line that feels incredibly tight. If anything, parts of the cover have just worn off with use. If anything, this could actually be smaller diameter than when it was new. I like to know which way I’m facing before I make my measurement.

I’m going to start with this. This looks like an 8-plate. It’s soft. It looks like it’s pretty worn. It’s a little fuzzy, so I’m just going to work it down. I’ve got my brake off of my calipers. I’ll get this under load. Okay, I’ve got my calipers the longway, and just a little bit of pressure, and then I’m going to turn it to 90. Everything’s at 90-degrees. What am I looking at? A solid ¼”, and that seems reasonable.

This line feels like it’s got a lot of damaged fibers in it. It feels like it’s been pulled back together and the braid might have inflated a little bit. I’m going to work hard to get the braid angle down with the construction, constructional diameter out of there. I’ve got it under load. Open this up, longways, rotate 90, and off. That’s a good hair over a ¼”, but it’s not getting close enough to 5-16ths.

My suspicion going into this was it was so damaged that the braid was coming back together and it was going to be on the large size. That’s not surprising and I’m confident that was a quarter-inch, or is.

This line, I’m a little suspicious. It feels like it could be undersized. It looks like a quarter inch to me. Close my calipers down. You’ll notice, I’ve got my thumb back here, I’m keeping a slight amount of pressure on this. I’m not pushing. I just don’t want this to get pushed towards me and get an inaccurate reading. This is pretty spot-on, right at a ¼”.

I’ve got three lines, this looks like a 32 carrier. This looks like just a small double braid, a small cousin of a full-sized double braid, and an 8-plate. I’m comfortable with that.

This line … I can’t remember which type it is. We used to sell this a long time ago. Semi-soft, it’s really been used hard. It doesn’t feel like it’s shouldering up from the fibers being sun-damaged. Most of the damage to this seems to be from just really hard use, because the fibers don’t feel chalky. They still feel fairly soft.

I’m going to load this up. I’m going to stay away from that area where I’ve got some abrasion, maybe, from a clutch or a sheave. This feels like a good area right in there, feels round. Open my caliper. Again, longways, rotate running down, just barely touching the fibers. I’m going to come off. This is coming in at, see … Wow! This one’s a little closer to 3/8”.

I’m looking at this, I feel like it’s been worn pretty hard. I’m pretty comfortable that this is a 3/8” inch line. It was more than a hair off of 3/8” towards the 5/16”, but not halfway between. I’m comfortable calling that 3/8”.

12-strand sheets can do some funny things overtime. This one doesn’t feel to sun-damaged. This one has, perhaps, specter in it, that’s really UV stable. It looks old, but it’s actually got some life left in it. My challenge here is to really get the constructional diameter out of it, get it under load, get the caliper on, run it down. This one, I’m rotating to making sure I’m hitting all those different fibers and I’m not measuring a flat spot. That’s a solid, straight-on 3/8”. One, two, three, yup. Super confident I’ve got a 3/8” 12-strand.

Here’s an interesting … This is the same line, just different ends. This must have been a halyard with this being maybe in the mast all the time, or I’m not sure. No, it’s the bitter end. Probably, in a sheet bag, not being used. This looks almost brand new. This looks like it’s had a good hard life. It feels that soft, chalky. Not a lot of the flexibility is gone.

I’m choosing to measure this, which is obviously more like it’s new condition. I can feel, it’s a double braid polyester. Clamp that and pull the core and the jacket, trying to get that constructional diameter down. Open this up, close it up. There we go. Rotate. The line felt round to me. This one’s coming right smack-dab on a 3/8”.

Okay. Last line. I’m going to guess, this is a double braid polyester. It feels soft. It’s spongy. I can tell its got some damage to the fibers, because they feel chalky. I feel like the whole thing is just nested up and it just feels fat to me. I’m going to be challenged to … Oh! I could feel all those waves down here. I want to stay away from all that damaged area. I’ll be measuring up here.

I’m going to guess this is 7/16” by the look of it. Caliper is on, keeping a little pressure. Yeah, it’s a hair over 7/16”, but I’m very confident I’m dealing with a 7/16” line here.

To summarize, when measuring line, make sure that you … Whether it’s single braid, double braid, new or old, that you pull the constructional diameter out of it, the shouldering up, and keep it under tension while you’re putting the calipers on it. The more tension, the truer your measurement.

Secondly, when it comes to measuring used line, before you put the calipers on it, decide whether you think it might come in a little bit high or low. It’s good to be a little suspicious of that.

how to measure thee diameter of used braided line



Lastly, remember that line diameters on braided lines are nominal. They are approximations. If you buy a 7/16” line and you put it up to a diameter gauge, it may go zinging right through there and not touch the sides. It might just touch the sides, and you might not be able to actually get it through there. It’s an approximation.

Braided line diameter are nominal and approximations


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