Nearly every sizing question we get is the same question wearing different clothes. Someone has a specific object in their hand and wants to know whether it will go in. The comparison table answers it, but only once you know which two numbers to read.
Almost every Cielo is one of two bores. The slim holders measure half an inch across on the inside, which is 12.7mm. The wide holders measure three quarters of an inch, which is 19.05mm. Everything else about fit is length. So the method is short. Measure your object across its widest point and check it against 12.7 or 19.05, then measure its length and check that against the chamber depth on the sizing guide.
Here is how that works out for the things people actually carry.
Tablets and capsules
The Petite holds about four aspirin-sized tablets in a chamber 0.8 inches deep. The Slim Single Chamber holds about nine at 1.8 inches, and the Slim Extra Tall about thirteen at 2.6 inches. Width is what rules out capsules and larger tablets from the slim holders, not capacity, which is why the Compact Wide exists at the same 0.8 inch depth but a three-quarter-inch bore.
If you carry two things that should not touch, the dual chambers open at opposite ends and seal separately. That is the whole argument for carrying a triptan and an anti-nausea tablet in one holder, or a prescription and an over-the-counter remedy.
Hearing aid batteries
Every standard size clears the slim bore, which surprises people who assume they need the wide one. Size 10 is 5.8mm across. Sizes 312 and 13 are both 7.9mm. Size 675, the largest standard cell, is 11.6mm. All four drop into a 12.7mm chamber.
Size 675 has about a millimeter of room on each side, so if that is what your aids take, the three-quarter-inch bore is noticeably easier to load and unload with cold fingers. Wax guards and domes are smaller than any battery, so they ride along in the same chamber. That is what the hearing aid battery holder page walks through.
A security key
A YubiKey 5 NFC measures 18 by 45 by 3.3mm. The 18mm rules out the slim bore and the 45mm, which is about 1.8 inches, rules out any short chamber. The Wide Single Chamber is 19mm across and 2 inches deep, so a full-size key clears it with room left over. The Nano models are small enough that neither number matters.
Worth saying plainly, steel blocks NFC and there is no way to plug in through a sealed case. This works for the backup key you store, not the one you tap a dozen times a day.
Earplugs
Foam and silicone plugs compress, which is why they behave differently from anything rigid. A pair fits the Slim Single Chamber, and the Compact Wide takes a pair plus a spare set with its wider mouth. The seal matters more here than the dimensions, because the thing that ruins a pair of earplugs is pocket lint rather than a bad fit.
A wedding ring
This is the one that catches people out, because a ring is measured by its inside rather than its outside. A US size 10 has an inside circumference of 62.1mm, which works out to an inside diameter of 19.8mm before you add the metal of the band itself. The bore is 19.05mm.
So an adult band is wider than the chamber. It goes in at an angle rather than lying flat, and it seats and seals either way. If you want certainty before you order, measure across the outside of your band and compare it to 19.05mm, then look at the ring holders.
Matches
Matches are a length problem rather than a width one. Diamond's strike-anywhere kitchen matches come in two inch and two and three eighths inch lengths. UCO Stormproof matches run 2.75 inches. The Wide Extra Tall chamber is 2.8 inches, so kitchen matches have real room and stormproof matches only just clear.
A Chamber Extension adds 0.8 inches of chamber, which takes it to 3.6 inches and gives stormproof matches margin instead of luck. There is no striker built in, which matters if you carry the safety kind. A waterproof match case is only half the job if the striker strip stays home, so cut it off the box and keep it inside with the matches.
How to check anything else
Two measurements settle it. Take the widest point across your object and compare it to 12.7mm for a slim holder or 19.05mm for a wide one. Then take its length and compare it to the chamber depth, remembering that an Additional Chamber or a Chamber Extension adds 0.8 inches to any holder that takes one.
Anything that clears both numbers will seal, because the O-ring does not care what is behind it. That is why the same object that protects a nitroglycerin bottle also protects a spare key, and why the list of things people carry keeps growing past what we designed for. There are more of them on the other uses page.