How to Measure Bottle Threads Accurately (3 Foolproof Methods)

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Why Accurate Bottle Thread Measurements Matter

Finding a replacement pump or cap for your threaded bottle starts with knowing the thread dimensions of your bottle neck.

The dimensions include two numbers, one that represents the diameter of the opening, and the second that represents the thread finish (or how many threads go around the neck of the bottle).

Even a difference of a single millimeter or a half thread can determine whether a pump works or doesn’t.

Bottle pumps and caps are manufactured to fit specific neck sizes and thread finishes.

And there’s no “standard” in the industry.

There is no accurate way to guess the thread dimensions.

And any listing that says “fits between X and Y dimensions” is not a good choice for a replacement pump or cap.

That’s how you waste money on your way to solving your problem.

An accurate measurement narrows your search before you spend money ordering replacement pumps.

In this article, I will show you three practical ways to measure bottle threads.

You can use a bottle thread testing kit (which I sell), digital calipers (which I’ll provide links for), or a combination of good ol’ geometry combined with a flexible seamstress’s tape measure. (Don’t worry, I’ll give you the formula.)

Hey, it’s good to know those high school math skills really are useful in real life, right?

What Do Bottle Thread Dimensions Mean?

Bottle thread sizes are commonly written as two numbers separated by a slash or hyphen, such as 24/410 or 24-410.

The first number refers to the outside diameter of the bottle neck in millimeters.

For example, a bottle with a 24/410 neck finish has a threaded neck that measures 24 millimeters across.

The second number describes the thread finish.

This identifies how many threads wrap around the outside of the bottle neck.

But it can also describe certain styles of threads and necks like a neck with a flared top or a neck with a locking notch.

And sometimes, with more threads comes more bottle neck, so a properly fitting cap or pump will be taller for some bottles than others.

All these features relating to the actual threads are summed up in the second number of the thread dimensions.

A 24/400 bottle and a 24/410 bottle may have the same neck diameter, but they do not have the same threading.

This distinction is important because measuring the diameter can help you identify the first number, but it does not confirm the complete neck finish.

You can find all you need to know about how to identify different thread finishes in this article about How to Find the Right Replacement Pump for Your Refillable Bottle.

What Part of the Bottle Should You Measure?

To identify the bottle’s neck size, only measure across the opening to the outside of the threads of the bottle neck.

Do not measure the outside of the neck only.

Make sure to include the threads in your measurement.

This is the measurement used to help identify the first number in a bottle neck size such as 24/410 or 28-400.

This measurement is very difficult to get correct when guessing or using a straight ruler.

But there are 3 foolproof methods that will get you the most accurate measurement of the outside diameter of your bottle neck.

This process will identify the first number in your thread dimension.

For the second number, refer to the article How to Find the Right Replacement Pump for Your Refillable Bottle.

The second number can usually be easily determined by eye as long as you have access to the image in that article that shows what different threads look like.

Here are the three foolproof methods for measuring the outside diameter of your bottle neck.

Let’s start with the method that requires math but very few tools.

Method 1: Measure Your Bottle Neck Diameter Using a Tape Measure and Geometry

This method is arguably the least expensive way to get an accurate measurement of your bottle neck diameter.

If you already have a flexible tape measure at home (the kind used to measure your body for sewing or body measurement tracking) then you won’t need anything except a geometry formula to get an accurate measurement of your bottle neck diameter.

If you don’t already have a flexible tape measure, I own these colorful ones and keep them all over the house for quick measurements.

I’ve also found some well-rated flexible tape measures that retract and even a set that contains one regular flexible measuring tape plus one that retracts.

They’re quite inexpensive and serve many purposes around the home including measuring your bottle neck diameter.

Those are Amazon recommendations.

If you prefer to buy on Walmart, I found these nice options for flexible ten foot tape measure by Dritz and this set of super cute pastel retractable tape measures.

Grab your flexible tape measure and your reading glasses if you need them, and measure around the threads of the bottle neck.

Now here’s the formula for calculating the diameter, where X is the measurement you took.

I know, it’s been a while since you’ve seen geometric equations.

Never fear.

This is an easy one.

The measurement you just took was the circumference of the bottle opening, and it’s represented by the red X in the formula above.

The d stands for diameter, which is what we’re trying to solve for.

And the last item in the formula you’ll recognize as pi.

Remember pi is equal to approximately 3.14.

So let’s say you got 75.5 mm circumference (7.5 cm), you would divide that number by 3.14, resulting in approximately 24 mm.

Like this:

There are two important things to keep in mind about using the tape measure method:

1.      Tape measures usually have a little metal end to them, preventing them from laying perfectly flat on the bottle threads. So don’t worry if you don’t get the exact number you were looking for. Just take note of it and move on to the next consideration.

2.      Bottle caps and pumps don’t usually come in partial measurements. So if you measure your circumference and it reads 80mm like mine did in the images below, just know there is no cap that’s 25.4 mm. There are 24mm and a 28mm pumps available but not that partial in between number.

Here’s are a couple images of how to use a tape measure to get the correct circumference and showing the interference of the metal tab.

If your circumference results in a diameter that is in between two available options, it’s most likely the one that’s closest, but not bigger than your measurement.

 

Ok, so you can see there may be a slight hangup with getting exactly the correct number using this method, but it’s (almost) free and will get you closer than any other manual method.

That takes us on to the next method, which is not free, but affordable and useful.

Method 2: Measure Your Bottle Neck Diameter Using My Essential Bottle Lid Thread Testing Kit

I designed this kit specifically to solve this one problem.

I’ve been selling refillable bottles since 2017, and I’ve heard every question and complaint you can imagine about bottles.

And what’s more, I wrote a thorough article on How to Find the Right Replacement Pump for Your Refillable Bottle.

I wrote it to help people.

And it does.

But it also exposes more questions and problems.

Many people over the years have contacted me to help them find a replacement pump for a bottle they already have and don’t want to replace.

So I created the Essential Bottle Lid Thread Testing Kit with eight different common household cap sizes, individually labeled with the threading dimensions so once you match a cap to your bottle, you know that’s the size of pump or cap you need to look for.

Here’s a look at the eight caps you receive in that kit.