You found the right bottle and ordered 500 caps to match. Then the first cap spins on, stops a quarter-turn short, and leaks the moment the bottle tips over. Most of the time, that's a neck finish mismatch.
This glass bottle neck finish guide is for product makers and buyers who keep seeing codes like 18-400 or 20-400 on product pages and want to know what they mean before placing a bulk order. Once you can read the code, picking a cap, dropper, or pump that actually fits takes about ten seconds.
What Is a Glass Bottle Neck Finish?
The neck finish is the threaded top of the bottle. That covers the opening, the threads around it, and the height of the threaded section. It's the part your closure actually grabs.
Every cap, dropper, sprayer, and pump is made to match one specific finish. When it matches, the closure threads down smoothly and presses its liner flat against the bottle's lip. When it doesn't, nothing else about the bottle matters. Glass color, shape, and capacity won't fix a cap that can't seat.
How to Read a Neck Finish Code
Most bottles sold in the US use the GPI system, a standard set by the Glass Packaging Institute. It's written as two numbers separated by a dash or a slash: 20-400, 28/400, 24-410. Each number tells you something different.
The First Number: Diameter in Millimeters
The first number is the outside diameter of the threads, in millimeters. A 20-400 neck measures about 20 mm across the outside of its threads. A 28-400 neck measures about 28 mm. The inside of the cap is sized to match.
Two millimeters sounds like nothing. It isn't. An 18 mm dropper won't thread onto a 20 mm neck, and a 20 mm dropper on an 18 mm neck just spins.
The Second Number: Thread Style, Not Thread Count
This is where most people get tripped up. The second number isn't how many threads the neck has. It's a style code that describes how far the thread wraps around the neck and how tall the threaded section is.
400: One full thread turn on a short neck. It's quick to cap and the most common finish on Boston round bottles.
410: One and a half turns on a slightly taller neck. You'll see it on many lotion and treatment pump bottles.
415: Two turns with thinner threads and the tallest neck of the three. It's common on narrow bottles where the closure needs extra grip.
So a 24-400 bottle and a 24-410 bottle share a diameter but not a neck height or thread pattern. Their caps aren't interchangeable.
Why the Neck Finish Decides Which Closure Fits
A mismatched closure fails in a few predictable ways. If it's too small, it won't thread on at all. If it's too large, it spins without catching. The sneaky case is the right diameter with the wrong thread style. The cap threads partway, feels tight in your hand, and still never presses the liner down against the lip. That's the one that turns up as a leak inside a shipping box three states away.
Dispensing closures add another variable: length. Droppers, pumps, and sprayers all reach down into the bottle, so they're sized for a specific bottle height as well as a specific finish. The 28-400 trigger sprayers we carry show this well. Both fit a 28-400 neck, but the 8 oz version uses a 130 mm dip tube and the 16 oz version uses a 160 mm tube. The finish is the same, the bottle depth isn't, so the part isn't either.
If you're still deciding which kind of closure suits your product, start with this rundown of glass bottle closure types. Then come back here to match the neck.
Common Neck Finishes and the Bottles That Use Them
Here's how neck finishes map across the wholesale glass bottles in our catalog. Consider it a glass bottle thread size guide you can keep open whenever you reorder closures.
| Bottle Style | Size | Neck Finish |
|---|---|---|
| Boston Round | 0.5 oz | 18-400 |
| Boston Round | 1 oz and 2 oz | 20-400 |
| Boston Round | 4 oz | 22-400 |
| Boston Round | 8 oz and 16 oz | 28-400 |
| Packer | 60 ml | 33-400 |
| Packer | 75 ml to 120 ml | 38-400 |
| Packer | 150 ml to 250 ml | 45-400 |
| Packer | 300 ml to 500 ml | 53-400 |
| Euro | 5 ml to 100 ml | DIN 18 |
| Syrup | 30 ml to 250 ml | DIN 28 |
GPI 400 Finishes: Boston Rounds and Packers
Every Boston round and packer bottle in that table uses a 400 finish. That keeps things simple. The thread style never changes, only the diameter does, so once you know a bottle's size you know which closures fit it.
The 18-400 glass bottle is the 0.5 oz Boston round, the size beauty brands reach for in sample and travel kits. The 20-400 neck finish covers both the 1 oz and the 2 oz, so caps and sprayers carry across. Droppers are the exception to watch. They're listed separately for each size because the glass pipette has to reach the bottom of a taller bottle. If you'd rather skip the matching altogether, our glass dropper bottles are Boston rounds that ship with the right dropper already included.
Packers go wider, from 33 mm up to 53 mm. That bigger opening is what lets capsules, gummies, and powders pour in and out without jamming. If that's your product, here's what packer bottles are typically used for.
DIN Finishes: Euro and Syrup Bottles
DIN finishes come from a German metric standard, not the US GPI system. You'll mostly see them on Euro dropper bottles and pharmaceutical-style syrup bottles. Our Euro bottles all use DIN 18, which pairs with droppers, orifice reducers, and treatment pumps. Our amber syrup bottles use DIN 28 across all seven sizes.
Here's the trap. A DIN 18 neck and an 18-400 neck share a number, but they come from different standards. Don't assume their closures swap. Buy closures listed for the exact finish on your bottle, and you'll never have to find out the hard way.
How to Check a Neck Finish Before You Order
1. Read the product name. Every bottle we sell puts its finish right in the title, as in "1 oz Amber Glass Boston Round Bottle 20-400 Neck Finish." That's the number to trust.
2. Measure if you're matching a bottle you already have. Use calipers or a millimeter ruler across the outside of the threads. That's your first number. Round to the nearest standard size.
3. Count the thread turns. Trace one thread from where it starts to where it ends. One full wrap is a 400. One and a half is a 410. Two is a 415.
4. Match both numbers on the closure. The diameter and the thread style both have to agree. Getting one of the two right isn't half a fit. It's a leak.
5. Test before you scale. Fill a few bottles with your actual product, cap them, and leave them upside down overnight on a paper towel. A dry towel in the morning tells you more than any spec sheet.
Glass Bottle Neck Finish FAQ
Will a 400 cap fit a 410 bottle?
Not properly. The diameter matches, but a 410 neck is taller with more thread, so a 400 cap won't engage the way it's designed to and may not seal. Match both numbers in the code, not just the first one.
What's the difference between 18-400 and 20-400?
Two millimeters of diameter. Both use the same single-turn thread style, but the closures aren't interchangeable. In our Boston round line, 18-400 is the 0.5 oz bottle and 20-400 is the 1 oz and 2 oz bottles.
What is a 405 neck finish?
A 405 is a glass-only variation of the 400 finish. It has a slight dip in the threads where the two halves of the glass mold meet, which keeps the cap from rubbing at that seam. Standard 400 closures fit 405 glass.
Do child-resistant closures follow the same neck finish codes?
Yes. Child-resistant caps and droppers are built for specific finishes just like standard closures. We carry child-resistant droppers in 20-400 for 1 oz and 2 oz Boston rounds and in 22-400 for the 4 oz size.
How can I test a closure fit before buying a full case?
Order a few units first. Our glass bottle samples let you check the bottle, closure, and your actual product together. That's a lot cheaper than finding a mismatch across 500 units.
Every bottle and closure at The Bottle Depot lists its neck finish up front. Find yours, then shop caps and closures sized to fit, shipped from our US warehouse in 3 to 5 days.
