Almost every guide to chastity cages says to “measure carefully” and then never gives a
number. This page is the numbers: how to take the two measurements that matter, how to convert what
you measured into the ring size a shop actually sells, and what to do when you land between sizes.
The short version: measure the circumference behind the scrotum, divide by 3.14 to get the ring
diameter, and match the cage length to your flaccid length. A 145 mm circumference needs a 46 mm
ring. A 75 mm flaccid length needs a 60–75 mm cage. If you remember nothing else, remember those two
sentences.
Why cage sizing confuses people
Condoms are sold by nominal width — the flat width of the latex, roughly half the circumference.
Chastity cages are sold by ring diameter, and the diameter is not half of anything you can
measure with a tape. So a buyer who knows his condom size still has nothing to map onto a cage ring.
There is a second confusion on top of it: the cage length has nothing to do with the ring
size, and shops list them in the same dropdown. Length is measured on the flaccid penis; the ring is
measured around the whole package. Getting one right and the other wrong produces the two classic
failures — a device that pinches at the base, or one that slides forward.
What you actually measure
- Base ring — the circumference of the penis and scrotum together, taken snugly
behind the scrotum. This is the measurement that causes injury when it is wrong. - Cage length — the flaccid length from the base of the penis to the tip, taken
without pressing into the body and without stretching. - Not girth at mid-shaft, and not erect length. Both are the
wrong measurement, and both are what most guides tell you to take.
Take each one twice: once at room temperature, once after a warm shower. Cold skin retracts and
gives a reading 5–10 mm short. If the two readings differ by more than 5 mm, take a third and use the
largest.
Ring circumference to ring diameter
Manufacturers list rings by diameter, but you measure a circumference. The conversion is
arithmetic — diameter = circumference ÷ 3.14:
| Your measurement (circumference) | Ring to order (diameter) | Note |
|---|---|---|
| 100 mm (10.0 cm) | 32 mm | 很可能过紧 |
| 110 mm (11.0 cm) | 35 mm | 很可能过紧 |
| 120 mm (12.0 cm) | 38 mm | 很可能过紧 |
| 130 mm (13.0 cm) | 41 mm | 常见起点 |
| 140 mm (14.0 cm) | 45 mm | 常见起点 |
| 150 mm (15.0 cm) | 48 mm | 常见起点 |
| 160 mm (16.0 cm) | 51 mm | 常规范围 |
| 170 mm (17.0 cm) | 54 mm | 常规范围 |
| 180 mm (18.0 cm) | 57 mm | 常规范围 |
| 190 mm (19.0 cm) | 60 mm | 偏大，注意滑脱 |
| 200 mm (20.0 cm) | 64 mm | 偏大，注意滑脱 |
If you land between two rows, take the larger ring. A ring that is 2 mm too small is uncomfortable
within an hour; a ring that is 2 mm too large lets the device slide forward, which is its own problem
— but it is the safer error to correct, because you can fix it with a smaller ring rather than with
two weeks of waiting for skin to recover.
A worked example
Say you measure 145 mm around, behind the scrotum, and 75 mm flaccid length.
- 145 ÷ 3.14 = 46.2 → order a 46 mm ring (between the 45 mm and 48 mm rows,
take the larger: 48 mm if the shop only sells whole sizes). - 75 mm flaccid → 60–75 mm cage, not shorter.
- Gap: if the device is adjustable, start at 10 mm — mid-range, and easy to
tighten or widen by a millimetre or two as you learn your own anatomy.
What happens when the ring is wrong
| Ring too small | Ring too large |
|---|---|
| Skin under the ring goes pale, then blue | Device slides down the shaft when flaccid |
| The scrotum feels cold to the touch | A testicle slips through the gap |
| Pain that continues after removal | Chafing at the underside from movement |
| Numbness, or loss of sensation at the tip | The cage shifts during the night |
The first column is a stop-now list. The second is a stop-and-change list. Neither is something to
“get used to”.
Cage length
| Flaccid length | Cage length to look for |
|---|---|
| under 60 mm | 50–60 mm |
| 60–80 mm | 60–75 mm |
| 80–100 mm | 75–90 mm |
| over 100 mm | 90–100 mm, expect to need a custom cage |
Cage length should be close to your flaccid length, not shorter. The common beginner mistake is
ordering a cage well below flaccid size because “it should be restrictive”: the result is that any
partial erection pulls the whole device forward and pinches at the base ring. The restriction comes
from the cage being closed, not from it being small.
The same ladder, published by a manufacturer
Kink3D publishes the flaccid range each of its Cobra cages is built for. The ladder runs continuously
from 3 cm to 15 cm — useful because it shows how narrow each band really is:
| Model | Flaccid length (cm) | Flaccid length (in) |
|---|---|---|
| Baby Cobra | 3-5 cm | 1.25-2 in |
| Cobra N Plus | 6.5-7.5 cm | 2.5-3 in |
| Cobra S | 7.5-9 cm | 3-3.5 in |
| Cobra S Plus | 9-11 cm | 3.5-4.25 in |
| Cobra R | 11-12.5 cm | 4.25-5 in |
| Cobra R Plus | 12.5-14 cm | 5-5.5 in |
| Cobra Bfg | 14-15 cm | 5.5-6 in |
Source: Kink3D sizing guide and individual product pages, read 2026-11-04.
The gap between ring and cage
The gap is the space the scrotum sits in. Too small and it chafes and restricts blood flow; too
large and the testicles can slip through — which is both painful and, in the wrong moment, an
emergency. Most devices ship with a fixed gap; adjustable ones usually run 8–12 mm. If your scrotum
is high and tight, aim at the larger end of that range.
How to take the measurement
- Cut a strip of paper or use a soft tape. Do not use a metal tape measure — the edges cut.
- Wrap it behind the scrotum, over the top of the penis base, snug but not tight.
- Mark where it meets, lay it flat, measure in millimetres.
- Repeat. If the two readings differ by more than 5 mm, take a third and use the largest.
Buying: what the numbers imply
- Adjustable rings cost more but remove the single biggest source of a failed
first purchase. If the shop offers three ring sizes, take the set. - “One size fits all” is a red flag — for the ring in particular, the range of
adult circumferences runs from about 100 mm to 200 mm, which is a 32 mm to 64 mm diameter. No
single ring covers that. - Split rings and hinged rings measure the same way but need a 1–2 mm larger
diameter than a solid ring, because the hinge takes up space. - Custom is worth it above roughly 100 mm flaccid length, where standard cages
top out.
What the numbers do not tell you
Fit is geometry, but tolerance is not. Two people with identical measurements can wear the same
ring for different lengths of time. Start with hours, not days; check for numbness, cold skin or
discolouration and stop immediately if any appear. A ring that fits at 8am may not fit at 8pm.
FAQ
My measurement is exactly between two rings — which one? The larger. Then, if it
slides, buy the smaller as a second ring rather than starting with the tighter one.
Does the ring size change over time? Not from weight change alone. It changes with
temperature, arousal and time of day, which is why the check matters more than the number.
Can I use a cock ring size as a starting point? No. A cock ring sits at the base of
the shaft; a chastity ring sits behind the scrotum, and the circumference is 20–40 mm larger.
How tight should it feel? Snug enough that a finger cannot slip under the ring
without pushing, loose enough that the skin does not blanch. If it leaves a mark that is still
visible after an hour, it is too tight.
Once you have the size, the next questions are material and wear time —
materials compared and
how long you can wear one.