Measuring gas pipe size isn’t as straightforward as you would think. Gas pipe sizes are standardized on the NPT (National Pipe Thread) standard. NPT sizes are smaller than the actual thread size. Additionally, NPT threads are tapered to in order to seal against leakage of gas. As the fittings are tightened the threads are compressed against each other providing a stronger seal the more they are tightened.
The image below shows a 1/2″ NPT pipe thread against a measuring tape. As you can see, the actual measured size is slightly more than 3/4″. Note that since the threads are tapered the diameter at the end of the nipple measures less than the diameter at the base. At the bottom of this article you’ll find a table which provides actual outside thread diameter sizes for common NPT pipe sizes.
Since NPT pipes and fittings are common, you can easily find fittings for your fire pit at your local home improvement or hardware store.
|Pipe Size (NPT)||Outside Diameter|