How to Measure a Downstem Size and Length | Step-by-Step Guide
When it comes to getting the perfect bong setup, one part you can’t overlook is the downstem. It might look simple, but it plays a big role in how smooth and clean your hits feel. The key to making it work right? Getting the right size and length. In this quick guide, we’ll show you exactly how to measure a downstem, step by step — so your next session is smooth, filtered, and frustration-free.
What Is a Downstem?
A downstem is a long, tube-shaped piece of glass that connects the bowl of your bong to the main water chamber.
It plays a key role in how your bong functions by creating an airtight seal between glass components. This seal helps pull the smoke from the bowl into the water chamber when you inhale.
Why Accurate Downstem Measurement Matters?
If your downstem is too long, it might not fit properly. It won’t seal well, and worse, it could hit the bottom of the bong and crack the glass. If it’s too short, it won’t reach deep enough into the water. You’ll either need to overfill your bong, which makes it hard to draw, or risk getting splashed during use.
The right size ensures a snug, airtight fit—no air leaks, no wobbling bowl, just smooth, consistent draws. Think of it like Goldilocks: not too loose, not too tight—just right.
How to Measure Your Downstem: Step-by-Step Guide
What You’ll Need:
- A ruler or tape measure
- The bong you’ll be using
- A wooden skewer, pencil, or similar tool
- A marker or pen
Step 1: Understand How Downstem Length Is Measured
When measuring a downstem, the length refers to the distance from the top edge of the ground joint (the frosted part) down to the very tip of the downstem—not the entire piece.
Step 2: Measure the Downstem Length
- Option 1: If You Already Have a Downstem: If you’re replacing an existing downstem, carefully remove it from your bong. Then measure from the top of the ground joint down to the end, just like shown in the reference image.
- Option 2: If You Don’t Have a Downstem to Reference: If your downstem is missing or broken, don’t worry. Grab a wooden skewer, pencil, or similar straight item. Insert it into the joint on your bong until it touches the bottom or inner wall. Then pull it out about ½ to 1 inch to leave some clearance. Mark the spot where the skewer meets the top of the joint. Remove the skewer and measure from the mark to the tip. That’s your ideal downstem length.
Step 3: Measure the Downstem Size (Joint Size)
The “size” usually refers to the joint diameter—the width of the part of the downstem that fits into the bong.
Common sizes include 10mm, 14mm, and 18mm, with 18mm being the most popular and fitting about 95% of bongs.
Method 1: Direct Measurement: You can measure the joint circumference at the frosted end using a tape measure, then use a formula to calculate the diameter. But this can be tricky.
Method 2: Simple Ruler Method: The easiest way is to measure the inner diameter of your bong’s female joint using a ruler,, as shown in the picture. Make sure you’re only measuring the inner opening—not including the glass thickness.
The number you get will likely be close to 10mm, 14mm, or 18mm. For example, if you measure around 14.5mm, you’ll still want a 14mm downstem. If it’s around 19mm, go with an 18mm one.、
Method 3: The Penny Trick: No ruler? Try using a U.S. penny. A penny is about 19mm wide, so:
- If the penny barely fits into the joint opening, it’s probably an 18mm joint.
- If only about ¼ of it fits, you’re likely looking at a 14mm joint.
- If it won’t fit at all, you may have a 10mm joint.
Step 4: Combine the Two Measurements
Now that you’ve measured both the length and joint size, you’re ready to shop. For example, you might need a 4-inch, 18mm downstem, or a 3.5-inch, 14mm one.
Just use your measurements to find the right fit—whether you’re buying online or in-store.
Frequently Asked Questions
What’s the most common downstem size?
Downstem sizes typically include 10mm, 14mm, and 18mm. Among these, 18mm is by far the most common—over 95% of bongs use this size. Chances are, yours does too.
Here’s a quick size reference:
- 10mm ≈ ~⅜ inch
- 14mm ≈ ~½ inch
- 18mm ≈ ~¾ inch
Why Did I Measure 18.8mm Instead of 18mm?
That’s completely normal! Many 18mm joints actually measure around 18.8mm. It’s just a small manufacturing tolerance, not a different size.
Most brands round it to 18mm for simplicity—so if you see 18.8mm, it’s still considered a standard 18mm joint. No need to worry!
How to Measure Downstem Size？
Use a ruler to measure the inner diameter of your bong’s female joint. If you don’t have a ruler, try the coin method mentioned above as a quick size check.
Where Should I Start Measuring the Downstem Length?
You should measure from the very top edge of the frosted joint down to the bottom tip of the downstem.
Do not include the small exposed section above the frosted joint—counting that part will make your measurement too long.
What If I Don’t Have the Original Downstem?
No worries. Use a wooden skewer, chopstick, or straight stick.
- Insert it into the joint until it lightly touches the bottom.
- Pull it back out about ½ inch so it doesn’t sit flush on the bottom.
- Mark the spot where it meets the top of the joint, then measure that section.
- That’s your ideal working length.
Should the Downstem Touch the Bottom of the Bong?
Definitely not. It should sit just above the base, ideally ½” to 1″ off the bottom. This allows for smooth bubbling and prevents cracking or poor airflow.
What If My Measurement Falls Between Sizes?
Round down for a better fit (less risk of breakage).
Round up only if you’re sure it won’t hit the bong’s base.
Are There Standard downstem Lengths?
Yes. Most downstems range from 3 inches to 6 inches, with some common lengths including 3.5″, 4″, 4.25″, and 5″.
Can I Use Any Downstem with My Bong?
Not exactly. You need to match:
- The joint size (10mm, 14mm or 18mm)
- The correct length
Mismatched downstems can lead to awkward angles, air leaks, or poor smoke quality.
What If My Downstem Wiggles or Doesn’t Seal Properly?
A loose fit usually means a size mismatch.
Double-check that both your bong and downstem have matching joint types (male or female) and sizes (like 14mm or 18mm). Also, inspect for dirt or small cracks that might be messing with the seal.
Why Do Some Methods Exclude the Frosted (Ground Joint) Part When Measuring?
This is actually a common measurement approach. The reason is: every brand or shop may use a slightly different standard. Some only measure the clear glass part of the downstem, others include the frosted joint, and a few even count the top lip above the joint.
There’s no single “correct” or official method. What matters most is consistency—make sure you’re measuring the same way the seller does.
What If the Seller Doesn’t List How They Measure Their Downstems?
Be cautious! Without knowing whether the listed size includes the frosted joint, the clear stem only, or even the top rim, you could end up with the wrong fit. Always check the product description or reach out to customer support. A trustworthy shop—like Smoke Meg—will be happy to explain exactly how they define “length,” so you can find the right match without the guesswork.
Why Do Some Listings Say 18mm/18mm or 18mm to 14mm? What Do These Two Sizes Mean?
This is a very common standard! A downstem connects two parts — your water pipe and your bowl.
In the case of 18mm/14mm (also written as 18 to 14mm or 18 by 14mm):
- 18mm refers to the joint size that fits into your bong.
- 14mm is the size that fits your bowl piece inside the downstem.
So, it simply means the downstem goes from an 18mm bong joint to a 14mm bowl joint — totally normal!
How Do I Know My Bowl Size (the Second Number)?
You can check this in two ways:
- Measure the inner diameter of your downstem (not including the glass wall).
- Or measure the outer diameter of your male bowl piece (including the glass wall).
Whatever you measure is your “second number” — the bowl size.
The most common combo on the market is 18mm/14mm, because most bongs have an 18mm female joint, and 14mm male bowls are used more than 95% of the time.
👉 Want a full guide? You can reda the details guide “How to Choose the Right Bong Bowl: Size, Gender, and Measurement Tips“.
The Right Fit Isn’t Just About Size—It’s About Experience
Getting the right downstem might seem tricky at first, but with the right measurements and a bit of guidance, it’s totally doable. Whether you’re replacing a broken one or upgrading your setup, knowing how to measure both length and joint size will save you time, money, and frustration.
At Smoke Meg, we make it easy—clear product info, friendly support, and quality gear that fits just right. Still got questions? Don’t worry, we’ve got your back (and your bong).
Because sometimes, in glass—and in life—it’s the little things that make everything flow smoothly.
