Measuring
The importance of choosing the right SNIFFA for the aroma or oil you plan to use it with can't be over-stated.
Too small and you won't be able to even start screwing it down onto the bottle, too large and the SNIFFA will just spin around without gripping the bottle's cap threads.
So please don't just guess, take the time to meaure the bottle, you can use everything from a simple ruler to a standard metal tape measure but please measure, it literally takes less than a minute.
The above pic shows this bottle to be 18mm. Don't include the threads, measure from the outside of the bottle to the opposite side, don't measure the internal diameter of the hole, it's the external diameter excluding threads we're after.
There are three sizes in common use and for now SNIFFA only supports those three.
We've colour-coded the three sizes to keep it as simple as possible and most customers will want the green one as it's the most commonly used but don't just order that and hope for the best - MEASURE IT DUDE!
Note that SNIFFA does not support aluminium bottles because the wall of the bottle is too thin to form a reliable seal with the rubber sealing grommet inside the cap section.