How to Measure a Bong Downstem Properly
If you’ve ever tried replacing your bong’s downstem and realized it didn’t fit quite right — too short, too long, or just not sealing properly — you’re not alone. Getting the right size downstem is key to smooth airflow, perfect filtration, and a leak-free hit. Here’s how to measure your bong downstem the right way.
What Is a bong downstem?
A downstem is the glass tube that connects your bowl piece to your bong’s water chamber. It directs smoke down into the water for filtration and cooling. Downstems come in different lengths and joint sizes, so measuring correctly is essential before buying a replacement.
Parts of a Downstem
What You’ll Need
- A ruler or tape measure (in inches)
- Your bong and the old downstem (if you have one)
- A dime, penny, and nickel (optional if measuring joint sizes)
- A pencil (if trying to determine length without old downstem
Step 1: How to measure a bong downstem ground joint size
First you need to determine the ground joint size of your downstem. The easiest way is to use the coin trick on your bong joint and reverse for your downstem size. The most common sizes are:
- 14mm (rare, but present)
- 18mm (most common)
Place a coin over the joint of your bong and see which coin fits the best. If a nickel fits over the joint it is a 18mm, and if a dime fits it is a 14mm. (as pictured) The joints on the bong and downstem ground joint will be the same size, just opposite variations. (male / female)
Step 2: Measure the length of a bong downstem
The downstem length is measured from the bottom of the joint ground (frosted glass area) to the bottom tip of the stem. (as pictured)
*NOTE: Although the industry standard is to measure this way, some companies still measure the complete length of the downstem. Be sure to check with out the manufacturer measures.
Optional (if no downstem present to measure)
- Insert a pen or pencil into the joint until it almost touches the bottom of the water chamber. (pictured)
- Mark where the pencil or straw meets the joint’s bottom edge. (pictured)
- Remove it and measure from the marked spot to end of pen
- That number is your ideal downstem length — typically ranging from 3" to 6".
Step 3: Measure downstem bowl head joint size
- Bowl head joint sizes come in 14mm and 18mm.
- Place a coin over the joint and see which one fits best. Reference to sizes listed below.
- Dime=14mm / Nickel = 18mm
Step 4: Test the fit of your new downstem
When you install your new downstem:
It should sit slightly above the bottom of your bong — not touching it.
The top of the ground joint should sit flush with the bong’s joint.
If it’s too long, it won’t seat properly. Too short, and your smoke won’t filter through enough water.
Every bong is a bit different — even within the same brand. Always measure twice before ordering a replacement downstem, and when in doubt, size slightly shorter rather than longer to ensure a perfect fit.
Need a Replacement?
Check out our collection of replacement downstems and diffused glass pieces to find the perfect fit for your setup.