18mm vs 19mm Bong Joints: What’s the Real Difference?

If you’ve ever tried to buy a new bowl, banger, or downstem for your bong, you’ve probably run into a confusing question: Is my bong an 18mm or a 19mm joint? Some brands say 18mm, others say 19mm. Some even list 18.8mm. So what’s actually correct?

The short answer: They all refer to the same size.

It might seem strange that three different measurements can describe one standard joint size, but that’s exactly what happened as the smoking industry evolved. In the world of scientific glass—the same style of fittings used long before they appeared on bongs—the official measurement is 18.8mm. When these fittings were adopted for smoking pieces, the terminology became more casual. Some people rounded down to 18mm, while others rounded up to 19mm. Over time, both became common, and both stuck.

This means if you see a bong listed as 18mm and another listed as 19mm, they’re talking about the same joint size, and the accessories are completely cross-compatible. An 18mm bowl will fit a 19mm bong, and vice versa, because both are designed around the true 18.8mm taper. You don’t need special adapters, and you don’t need to worry about choosing the “wrong” one when the numbers are only different because of rounding.

So why do some companies use one term while others use another? Often it comes down to tradition. Certain glassblowers and brands have always used “19mm,” especially those who learned from older scientific styles. Other brands prefer “18mm” because it sounds cleaner and simpler. Some shops stick with whatever wording was printed on the original packaging years ago. Meanwhile, the true measurement—18.8mm—is usually only mentioned by glass artists who like to stay as close to the technical standard as possible.

Most customers never even hear the number 18.8mm, but it’s the reason both terms work. Whether someone says 18mm, 19mm, or 18.8mm, they’re all describing the large-size joint used on bigger bongs and water pipes, as opposed to the smaller 14mm fittings used on compact pieces. If you've ever wondered why your 18mm bowl fits perfectly into a piece labeled “19mm,” now you know: the difference isn’t in the glass—it’s just in the wording.

So when shopping for a new bowl or downstem, don’t stress about choosing between 18mm and 19mm. They’re the same size, made to the same standard, and fully interchangeable. As long as your piece uses the large joint size, anything labeled with any of these measurements will work just fine. Understanding the numbers can make shopping easier—and now you can confidently choose the parts you want without second-guessing your gear.