If you have a jar and you’re trying to reverse-engineer the opening to find a lid for it, then this is the post for you. We get a lot of questions like this about lids for various-sized jars—particularly gallon-sized jars. One of the most common questions is:
At first glance, this seems like a simple question. Which brings us to #1.
The reality here is that there’s no universal “gallon lid,” “half-gallon lid,” “quart-sized lid,” or any other volumetric size lid. “Gallon,” for example, simply refers to the volume the container will hold; it says nothing about the shape or the opening size of the top of the jar. Think of a milk jug versus a gallon bucket—both hold a gallon, but their openings are drastically different. So one of the first obstacles is the variety of container opening dimensions.
Even if two jars hold the same amount, their openings may be completely different. So, the real question isn’t, “What size lid do I need for a gallon jar?” Instead, it should be, “What size lid do I need for my specific jar?” And this is where things get interesting.
Every manufacturer has the freedom to design their container differently, choosing the shape, thread style, and opening size they prefer. Once they finalize a design, they create a mold, and all jars from that mold share the same specifications. There are no rules.
Separately, to bring some consistency to the packaging industry, organizations like the Glass Packaging Institute (GPI) and the Society of the Plastics Industry (SPI) have established standard measurements for various container and lid sizes. However, these standards aren’t always followed. Many jars are custom-made, leading to variations in thread styles, neck finishes, and lid compatibility across different brands and manufacturers.
So, if the jar you have was designed to adhere to packaging industry standards, you should be able to find a lid fairly easily—which can’t always be said about custom jars.
For reference, here are some common gallon jars and jugs with different-sized openings.
In addition to opening size, there are also two types of threads to consider: Continuous Thread (C/T) vs. Lug. These are the most common thread types, so for simplicity’s sake, we will stick to these two.
As you can tell by the picture, identifying which type you have is easy. So, this side of things is pretty straightforward—look at the image above, compare it to your jar, and determine what you have.
As discussed a little earlier, there are standard lid and container sizes widely used in the packaging industry that adhere to the GPI packaging standard. When looking at lids and jars that follow these guidelines, you will see them labeled with numbers like 38-400 or 110-400. So, what do these mean?
The first number (e.g., 38) refers to the diameter of the jar’s opening in millimeters. For example, a 110-400 jar has an opening that is 110mm wide (4.33 inches), or a lid has an inside diameter of 110mm wide (4.33 inches).
| Neck Finish | Thread Turns | Characteristics |
|---|---|---|
| 400 | 1 turn | Standard thread turn |
| 410 | 1.5 turns | Slightly more secure fit |
| 415 | 2 turns | Thinner threads, typically taller neck |
| 425 | 2 turns | Commonly on smaller containers like vials |
For example, most of our 1-gallon jars use a 110-400 continuous thread and will be listed on the product detail page. If your jar has this opening size, it will be compatible with any lid labeled 110-400. This makes it easier to find replacement lids and accessories, such as airlock lids for fermentation.
So, the real question from here is: does your jar follow one of these standards?
All right, now that we know how to read sizes on lids, thread types, and jars, here are the steps to correctly measure your current jar to determine what size will be needed.
Before measuring, determine what kind of threading your jar has:
The first number in a lid size corresponds to this diameter. For example, a 110-400 lid fits a jar with a 110mm opening. The threads may be slightly under—for instance, a 110mm jar may show a Fig. A measurement of 108mm. These slight variances are common with glass.
Now, let’s determine the thread type. Take a look at the image above, then compare it to your jar. Does your jar’s threading match either of these types? The second number in the lid size refers to the thread finish, which indicates how many times the threads wrap around the neck of the jar.
Once you have the measurements, thread type, and finish, you can start searching for replacement lids.
The more common the size, the easier it is to find a matching lid. However, if your jar has a rare or one-off opening size, replacement lids might not be available.
If you need a specialized lid but your jar has an unusual opening size, you might need to switch to a more common jar to ensure lid compatibility.
If you already have a jar with a plastic lid and want to convert it into an airlock lid for fermentation, you might consider making your own. We offer a DIY Fermentation Lid Set, which comes with a drill bit and airlock grommets that allow you to drill a hole in any plastic lid, insert a grommet, and add an airlock to create your own fermentation lid.
Another option for turning a jug—possibly a jar—into a fermenter is by using an airlock bung. Bungs are commonly used with gallon jugs that have smaller openings, but if your jar has a small enough opening, it will work. We have bungs that fit openings up to 2.5″. From here, you can add the bung to your jar to seal it, then attach an airlock, and your fermenter is complete.
When it comes to lids, volume doesn’t determine compatibility—size and thread type do. By following the steps above, you can measure your jar’s opening to determine the diameter and identify the corresponding thread type. Once you have this information, you’ll be able to find the right lid, assuming it follows standard packaging industry specifications.
I hope this guide helps and provides all the information you need to find a lid that fits your jar!
Still have questions? Feel free to reach out—we’re happy to help!
