Sizing and Measuring for a Ball-Trap Device

The basics of comfort and fit

Aug 28, 2025

Measuring for your first cage is something no one ever seems to get right. Unfortunately, plenty of people screw up measuring for their fifth cage.

There are four numbers you need to know to get a perfect fit every time,

Anyone spot anything unusual about that list? Well, the first two are measurements of your penis. The second two are measurements of the cage. Anyone can talk about their length and girth. Only chastity users will talk about what their ring size is or what ball gap they need.

Following from this, if this will be your first cage, don’t worry about ring size and ball gap. Let’s just worry about length and girth. Why, because you can’t measure the other two. And any site or guide that tells you that you can is lying or written by someone who has never worn a cage.

Buying Your First Chastity Device

Newbie Sub

Jul 17

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In this guide, I am going to write about each measurement. Because I feel it’s important to understand why things are the way they are, I am going to go into some detail behind each measurement. But if you’re sick of reading my verbose writing, I’ll include a TL;DR at the end of each section.

But before we start, in order to know if a cage fits, you need to know what “fit” means. Guys, it’s simple. A cage should be as comfortable as an old pair of tennis shoes… or a wedding ring. Anyone who tells you that you have to get used to it or that pain is part of it is wearing a cage that doesn’t fit and just doesn't know any better1. And to add to that, for most of us, a cage should be able to be worn without straps or anything else – certainly a plastic cage and for most of us, even a heavy steel cage.

And I would like to credit u/McQueen455 and u/DanBalt for their excellent guides in the r/chastitytraining wiki for being the basis of this as well as William and Mistress Mature Metal for explaining some of this to me and finally KD of Kink3D for a few conversations on sizing. 90% of this is the regurgitation of their ideas with my practical experience. 10% is my own secret sauce.

What does comfortable mean in chastity?

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May 3

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So, that being said, here’s Newbie Sub’s Measuring Guide.

Length

Unfortunately, there’s no single way to measure a cage’s length. The depicted line is one way to do it but there are others. The tricky part for cage artisans is converting any kind of penis measurement into a cage measurement. The line above, how does that correspond to anything you could measure? Imagine a penis in that cage. The line starts at the top of the shaft but then ends at the middle of the shaft. And also, my penis naturally hangs lower than that – if you measure a penis held more straight out, you will get a much shorter length than if you measure one as it naturally hangs along the top of the shaft. The length of that line is 96 mm. The length of my penis held outright, parallel to the ground is 105 mm. When I bend over and look between my legs, I see my penis is still touching the end of the cage. That’s about what you want for length. You can go shorter (you can go much shorter) but you really don’t want to go longer. Why? Because sometimes if you pull back from the end of the cage any time you pick a pen off the floor, you won’t go back into place. And when it comes time to go the bathroom, you’re going to make a bit of a mess. To demonstrate this, take a toilet paper tube and if necessary cut it so it’s a bit shorter than your flaccid length. Now put yourself in the tube and bend over. Do you notice that your glans retracts into the tube?

Now let’s look at how Kink3D converts a penis measurement into a cage measurement.

They use the same definition of a cage’s length but they don’t actually tell you a cage’s length. Well, they do, but you have to dig for it. Why? Because they tell you in their sizing guide what size penis goes in what size cage. They’ve removed the lengths from their names. It’s a R+ or an N, not a 105 or a 50 mm cage.

Interestingly, and I don’t know why Kink3D does this, they want you to measure as you hang. Using that, I measure 125 mm, 20 mm longer than I measure held parallel to the ground. And then they give you this handy chart to figure out which size you should wear.

With my 125 mm, I am right smack dab in the middle between an R and R+. Let’s compare the length of my dialed in home-made cage with the length of an R and R+ and see if everything adds up.

Looking at the Cobra Specifications Page, the Cobra R is 90 mm and the R+ is 105 mm. My dialed in cage is 96 mm. So what does that mean? Well, it means I can measure my length held parallel to the floor and take off some number, about 10 mm, and that works out pretty well or I can measure my length as I hang and take off about 30 mm. Basically Kink3D’s sizing chart with his measuring method and my dialed in cage with my measuring method pretty much line up. Huzzah!

Now, I personally prefer measuring parallel to the ground as I can do that with ruler. Measuring as I hang requires string and it leaves open the question of how closely do I follow the glans to the meatus (the tip).

TL;DR

So, if you have a cage that’s measured the way Kink3D and I are measuring our cages, take your shortest flaccid length as measured several times throughout the day with your penis held parallel to the ground and subtract some amount, somewhere between 10 and 20 mm. It won’t be uncomfortable if your cage is a bit too short.

Girth

This one is easy guys. I won’t even need a TL;DR. In general, a cage should match your flaccid girth. If you’re going off the shelf, just don’t let it be smaller than your flaccid girth.

A few caveats.

If it’s a more closed cage, you’re going to want a few more millimeters because you’ve got to get yourself into it. I wear a 38 mm in a Jail Bird, a very open cage. In the Xenomorph, I wear 40 mm. If it didn’t have the two cutouts at 120 and 240 degrees, I would wear 42 mm.

One of these two cages is the Xenomorph. Guess which one.

If you prefer a shorter cage, you’ll want to add a few more millimeters or you’ll just have skin poking out everywhere.

So, to measure, take a receipt, yes a receipt2, and wrap it around your flaccid penis and mark it. Now divide by 3.14159265359 or something like that (should probably change the PIN on my phone now).

If you are getting a Cherry Keeper or custom, you can actually get an oval cage – penises are usually wider than they are “tall”. You already know your average diameter from measuring your circumference and dividing by the former PIN on my phone. Measure your width as best you can – a caliper is nice for this. Your “height” would be what it would take to make the average work out. So, if you’re a perfect oval…

\(diameter_{measured} = \frac{height + width_{measured}}{2}\)

which means according to Algebra I

\(height = 2 \times diameter_{measured} - width_{measured}\)

(Yay, I got to write LaTeX expressions. Eat it, Reddit Markdown).

In reality, I have found little substantive difference in comfort between an oval cage and a round cage. I just don’t mess with ovals anymore.

Ring Size

Oh, Christ. Ring size. There are so many takes on this and it’s ultimately personal preference. But it’s the single most important factor on how your cage is going to fit. Too large a ring and you’ll need straps. Too small and you’ll find pressure from erections uncomfortable or worse, you’ll end up in the hospital.

I’m going to give you my take, a briefer, more procedural version than my ring sizing article, with the caveat that hey, this is based on my experience and my anatomy. You may find that it doesn’t work for you. In fact, it may be dangerous for you.

Don’t measure your ring size. You can’t. Don’t. Anyone who tells you different wants you to just buy a cage off their site and not think too much about it.

Either get a cage with multiple rings or even better get sizing rings.

Reasonably priced sizing rings– hey, it’s my blog, I can push my own shit.

Start with a relatively large one, one that is easy to get into (note: how to get into a ring – it’s not exactly intuitive). It should fit pretty loosely. You should easily be able to get a finger between your ring and your shaft. And here comes the fun part, the reason my ring sizing procedure is best there is. You have a keyholder? Go get her.

Now, tell her how important this is – that you need her help to repeatedly give you the firmest erection you’ve ever attained. And this is another reason to start with a relatively large ring as you’ll see in a moment.

After your keyholder brings you fully erect, how do you look? Are you distinctly veiny? Slightly veiny? Normal? Can you return to flaccid without climaxing?

We’re going to repeat this procedure until you start to get veinier than normal but we always want to be able to return to flaccid without climaxing. And if you started with an overly large ring, well, we’re going to be here for a while. What a shame.

Eventually, you will start to find your erection is quite a bit firmer than usual and you’re a bit veiny. Now we’re close. This is the point where I stop giving advice because we’re now in personal comfort / risk tolerance area. If the ring is making you veiner than normal, it is restricting blood flow. This is what cock rings do. Now, a properly sized cock ring or rather as people who use cock rings like to size them, there is no way to go flaccid except by climaxing. That is not what we want here. We’re going to sleep in this thing. We want something that is snug enough that it can hold up a heavy steel cage when flaccid but loose enough that you don’t have to climax to return to flaccid. For me, the zone where I find I start getting veiny starts at 52 mm. I have gone all the way down to 45 mm with returning to flaccid. But at 45 mm, it took noticeably longer and frankly, it wasn’t comfortable. My guess is a 43 mm is probably a full blown cock ring for me. In chastity, I wear a 48 mm ring.

A view showing my 48 mm ring’s size compared to my cage’s 40 mm diameter (I measure 38 mm).

Let me spend some time describing how I feel in my ring when fully erect. With a 48 mm cuff ring, a ring that covers 10 mm of my shaft, it is tight and I really feel the pressure. When waking up in the morning with my morning wood, it is borderline uncomfortable (we aren’t pain-desensitized during nocturnal erections unlike arousal-based erections). In a 48 mm 1/4” round ring (a la Mature Metal), it is quite uncomfortable and regularly wakes me up at night – not painful but I don’t want to be woken up by every erection. I consequently wear a 51 mm Mature Metal ring.

So, is this safe at all? All I can say is “so far”. KD, of Kink3D, as well as Mature Metal believes that the presence of the cage actually helps – that it further reduces your ability to grow. They are saying that with the cage on, my 48 mm ring is even less like a cock ring. I think they’re right. So, why not do this experiment with the cage on? KD thinks that measuring with just a ring doesn’t work very well for this reason. I disagree. Use this method to find that cock-ring zone and then throw the cage on which should give you an even better safety margin. But if you try to figure out your ring size with the cage on, well that brings me to my next measurement.

TL;DR

Sorry, not here. You really need to read it. I know I write a lot but this is the single most important aspect of cage fit. Go read it. But I promise the next one will have a TL;DR.

The Ball Gap

Guys, this is why I do this. I suffered so much because of my sensitivity to normal-sized ball gaps, I have made it my life’s mission to save everyone I can from that suffering.

If you don’t know this, go watch The Blues Brothers.

The reason I told you to figure out your ring size without the cage is because of the ball gap. A picture being worth a 1,000 words… (I’ll do both)

Behold! The Ball Gap. I learned CAD just so I could make diagrams for my blog.

Actually, there are lots of ball gaps. I can measure it at any point between the ring and the cage I want. But the one that seems to be most determinative of comfort is the ball gap at the 6 o’clock position (or 12 o’clock if you have one of those cool 24-hour time clocks).

Just looking at this picture, you can see what all goes into the ball gap. The ring’s size, the cage’s diameter, the length of the junction, the angle between the cage and the ring (5 degrees in this case – if you notice, the top of the ring is a straight line – it’s lacking curvature – that’s at a slight angle to the cage), and finally the ring’s geometry3.

Listen, most people don’t need to worry about the ball gap. Off the shelf cages usually target a 10 – 14 mm ball gap, something like that. And some majority of men, I suspect a significant majority, find that comfortable. And I suspect it’s that significant majority that kept giving me bad advice when I was suffering from ball gap issues.

Those of us who are sensitive are usually sensitive to too small a gap. What does it feel like? I am most confident in its symptoms while erect, especially nocturnal erection. It feels like the bottom of your scrotum is on fire. Which is why so many people think it has to do with friction. It doesn’t and if you think about it, that explanation doesn’t even make sense4. Some argue that it’s simply from the scrotum being pulled. And that’s less easily dismissed. But I would ask them to just wear the ring and pull on it like an erection would.

I am increasingly convinced that the root cause is entrapment (pinching) of the pudendal nerve, a large nerve cluster that has branches that run right through the ball gap.

From https://my.clevelandclinic.org/health/diseases/24438-pudendal-neuralgia:

What is pudendal neuralgia?
Pudendal neuralgia (or pudendal nerve entrapment) is when you experience chronic pelvic pain from damage or irritation to your pudendal nerve. If you have pudendal neuralgia, this nerve is injured or compressed and causes stabbing, burning [emphasis added] or shooting pain. The pain can be felt in your buttocks, perineum and genital region, and is more severe when sitting.

Sound familiar?

I think other symptoms of a too-small gap, but perhaps unrelated to nerve entrapment, include pinching. Basically, the test is this:

Is the ring comfortable without the cage? If it is, you need a larger ball gap.

Can you have too large a gap? For most of us, no. But ball escape, you ask? Nope, not an issue. I’m 38 mm flaccid with a 48 mm ring. How exactly is a ball getting through? But if you’re uncomfortable with such a small ring, then yeah, you need the ball gap to keep Li’l Lefty in place.

But some people are actually sensitive to too large a gap. And I do not know this from experience but purely from asking Mistress Mature Metal. It’s described as a pain in the stomach, the kidneys, or the back. I theorize that it’s from the testicles getting pulled into the gap and getting squeezed – makes sense but I really don’t know. And I’ve only seen three people complain of this. And I’ve read a lot of posts.

TL;DR (as promised):

Burning sensation? Pinching? Any ring discomfort at all? Try just the ring. If the sensation goes away, it’s not the ring. It’s the ball gap. You most likely need a larger one as almost everyone that has ball gap issues needs a larger one. Pain in the stomach, kidneys, or back indicate you need a smaller ball gap. Easy way? Use a larger ring. Better way? Get a curved ring like in my diagrams or get a Cherry Keeper with a positive ring / cage angle.

Cherry Keeper's Secret Sauce

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Aug 15

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We Made It

I knew at some point, I’d have to write a one-stop-shop sizing guide. I know it was a lot to read but it was even a lotter to write. There are a few things I didn’t mention like cages with headlocks or sizing for flat cages, perhaps a future post, but this will cover 90% of the cages out there and I think 90% of the anatomies5.

But listen, even with The Best Sizing Guide Known to Man in your hands (this likely isn’t it), there’s still some trial and error, some guesswork. The ball gap is certainly that. And you just can’t measure for your ring. Even without those, you’re going to mess stuff up. But hopefully this with other well-written guides out there, you’ll have a pretty good shot of finding 24 / 7 comfort within your first three cages.

Here is some further reading (ha) that goes into some of these topics individually and some other posts that might make sense.. If you get your first cage following this advice and something still isn’t quite right, I recommend reading some of this.