Measuring a water jug neck so the replacement cap actually fits
Replacement caps are one of the few purchases where getting it wrong is guaranteed rather than likely, because a cap is either the right diameter or it is not. The good news is that the measurement is easy, most US jugs use one of a small number of sizes, and once you know yours you never have to think about it again.
Measure the outside of the neck, across the opening
What matters is the outer diameter of the neck at its widest point, measured across the centre. That is the dimension a cap has to fit over.
A ruler across the opening is accurate enough. If you have calipers, measure the outside of the threaded or ridged section rather than the inside of the hole.
55mm covers the large majority of US jugs
Three and five gallon jugs sold for water coolers in the United States overwhelmingly use a 55mm neck. That is why most replacement caps are sold at that size and why they fit most jugs people own.
It is still worth measuring rather than assuming, because imported and older jugs sometimes differ, and a cap two millimetres out will not seal.
Snap-on and screw-on are different fittings
Some jugs take a cap that snaps over a lip; others have a thread. They are not interchangeable, and a snap cap on a threaded neck will sit on top without sealing.
Look at the neck: a visible spiral means threaded, a plain raised ring means snap-on. Listings should state which, and where they do not, ask before buying.
Silicone seals better than rigid plastic here
A silicone cap has enough give to conform to small imperfections in the neck, which is what makes the difference between a seal and a slow drip. Rigid plastic relies on precise dimensions and is less forgiving.
For a jug that gets moved or tipped, silicone is the more reliable choice for exactly that reason.
Clean the neck before fitting a new cap
Mineral deposit builds up on a jug neck and stops a new cap seating fully. People frequently conclude a cap is the wrong size when the neck simply needs cleaning.
A wipe with vinegar removes the deposit and often solves an apparent fitting problem at no cost.
Buy more than one
Caps are small, cheap and easily lost, and a jug without a cap is awkward to store and easy to contaminate. Multi-packs exist because single caps disappear.
Keeping spares means the failure of one is a two-second fix rather than an order and a wait.
Caps on our shelf
The jug caps here state their diameter and whether they are snap-on or threaded, which are the two facts that determine fit. On our own shelf that range currently runs from $4.49 for the GentleJet 3-Pack Reusable 5 Gallon Water Jug Caps – Blue Silicone, Non-Spill up to $6.99 for the Boao 5 Sets Silicone Water Bottle Carriers with Keychain Clips, Black, Silver, Yellow, Purple, Green.
Each of these sits in Bottle Accessories; there are 21 products in that group today. The figures below are the charged prices, and nothing further is added at payment.
- GentleJet 3-Pack Reusable 5 Gallon Water Jug Caps – Blue Silicone, Non-Spill — $4.49
- PurYupearl 6 Pcs Multicolor Silicone Water Bottle Buckle with Carabiner Clip — $4.99
- Chill Oasis Water Bottle Caps for Cirkul Lids – 2 Pack, White — $4.99
- Iconikal 2-Pack Water Bottle Holder with Rubber Grip & Aluminum Carabiner — $5.69
- PandaEar 6-Pack Silicone Water Bottle Caps for Kids & Toddlers – No-Spill Spout Adapter — $6.98
- Boao 5 Sets Silicone Water Bottle Carriers with Keychain Clips, Black, Silver, Yellow, Purple, Green — $6.99
Related reading
Two other pieces on this site cover ground next to this one. A boot solves noise and dents, and introduces one problem of its own if you never take it off. And on a related note, a leaking bottle almost always has one of four causes, and three of them cost nothing to fix.
- Silicone bottle boots: what they protect and when they cause problems
- Why bottle lids leak, and which part is usually at fault
Mistakes when measuring
Measuring the inside of the opening rather than the outside of the neck is the most common, and it produces a figure several millimetres too small. The cap has to fit over the neck, so the outside diameter is the relevant one.
Measuring at an angle rather than across the true centre is the other. A slightly diagonal measurement reads larger than the actual diameter, which leads to ordering a cap that will sit loose and leak.
When a cap is not the problem
If a new, correctly sized cap still weeps, the neck itself is usually at fault. Jugs get chipped at the rim in handling, and a chip that is barely visible is enough to break the seal all the way round.
Run a finger carefully around the rim. A nick you can feel means no cap will seal reliably on that jug, and continuing to buy caps will not solve it. That jug is better retired for anything that needs to be carried or tipped.
Before you order
Measure the outer neck diameter, check whether it is threaded or snap-on, clean off any mineral deposit, and then match those two facts to the listing. Doing that takes under a minute and removes essentially all the risk of ordering the wrong part.
Parts go out from Los Angeles, CA at a flat $3.95, free past $29. Unused returns are accepted for 30 days. Write to [email protected].