Finish sizes, pipette length & tip geometry
Specifying a dropper is mostly a question of measuring the bottle.
A dropper assembly arrives as one item — collar, pipette and bulb already fitted — but it is dimensioned against two different things at once. The collar has to match the finish; the pipette has to match the inside height of that particular bottle. Change the bottle and keep the finish, and the assembly is still wrong.
The finish covers a narrower range than you expect
Dropper closures concentrate on 18-400, 20-400 and 22-400. In practice an 18-400 lands on 15 mL Boston rounds while 20-400 covers the 30 mL and 60 mL sizes, so the finish is effectively chosen by bottle volume. Because the 400 series gives only about one turn of thread, there is little tolerance for a collar molded to a different series.
Pipette length is measured to the base, not guessed
Stock lengths run in steps around 65, 76, 88 and 89 mm because they correspond to common internal bottle heights. A pipette a few millimeters short strands the last of an expensive serum; a few millimeters long and it bottoms out, so the bulb cannot decompress fully and the draw shortens. The measurement to take is the inside height, not the outside.
Tip form sets the drop, not the bulb
A straight cut releases a fast, larger drop suited to carrier oils. A tapered or fine tip breaks smaller drops and gives more control over an active serum. Round or bulb tips are used where the pipette touches skin, on eye-area and lip products. Since a drop is formed at the tip, changing tip form changes dosage even with an identical bulb and pipette.
Graduations have to be fired on, or they will not last
Marked pipettes for dosed products use ceramic graduations fired into the glass rather than printed ink, which would come away in contact with alcohol or essential oils. Fired marks add a step and a cost per piece, and they are worth it only where the customer is asked to measure. A decorative scale nobody uses is an expense with a compliance risk attached.
Decide whether the collar is plastic or metal-shelled
A plain polypropylene collar is the working choice: light, chemically indifferent and cheap enough to change. An aluminum shell over that collar gives the weight and finish that skincare brands want, and it introduces a second material near the product and near the thread. Anodized shells hold color better than sprayed ones when the bottle is handled daily.
Assemble late, and check the pipette after decoration
Assemblies are usually fitted after the bottle is filled, and a hot-decorated or frosted bottle can move enough at the finish to change how the collar seats. Programs running decorated glass should check assembly and draw on decorated bottles, since the same closure that seals plain glass can stand a fraction proud on a sprayed neck.
