Understand Pipe Size vs. Fitting Size
Many people confuse pipe size with brass threaded fitting size.
Pipe sizes are based on nominal bore, not outer diameter.
A 1/2-inch fitting may not measure 1/2 inch across.
Fittings are sized to match specific pipe dimensions, not actual measurements.
Brass threaded fittings follow National Pipe Thread (NPT) or BSP standards.
Identify the Thread Standard
Brass threaded fitting may follow NPT (US) or BSP (British/metric) thread standards.
NPT threads are tapered and used widely in North America.
BSP threads come in parallel (BSPP) or tapered (BSPT) forms.
Mixing NPT and BSP fittings causes leaks and poor connections.
Use thread gauges or pitch charts to determine the standard.
Measure the Outside Diameter (OD)
Start by measuring the outer diameter of the male threaded fitting.
Use a caliper or precise ruler for best results.
For example, a 1/2-inch NPT male fitting has an OD of about 0.84 inches.
This helps identify nominal sizes on brass threaded fittings.
Always measure in inches or millimeters based on your local standard.
Count the Threads Per Inch (TPI)
Thread pitch is crucial when identifying brass threaded fitting.
Use a thread gauge to count the threads per inch (TPI).
For NPT, a 1/2-inch fitting usually has 14 TPI.
BSP fittings have metric equivalents and may differ slightly.
Correct pitch ensures compatibility and prevents leak-prone mismatches.
Match with a Thread Size Chart
After measuring diameter and TPI, compare data to a thread size chart.
Many plumbing supply stores offer free charts for brass threaded fittings.
Online charts list nominal sizes, thread count, and actual measurements.
Match your readings with a fitting size in the chart.
Charts eliminate guesswork and prevent incorrect ordering or installation.
Check Female Fitting Inner Diameter
For female brass threaded fitting, measure the inner diameter (ID).
This helps confirm the nominal size, especially without a matching male end.
Ensure threads are clean and undamaged before measuring.
A 3/4-inch female fitting ID may be near 0.98 inches.
Measuring the ID helps complete size verification accurately.
Use Sample Fittings for Comparison
A simple method is comparing unknown brass threaded fittings with labeled ones.
Bring a known-size fitting and try matching threads.
This is useful in workshops, repair centers, or hardware stores.
Ensure threads align smoothly without force or gaps.
This practical method reduces reliance on tools and improves field identification.
Consider Application Requirements
Choosing the right size goes beyond just measurement.
Consider pressure ratings, material compatibility, and system use.
For gas, water, or steam systems, wrong-sized brass threaded fittings can be dangerous.
Always verify system specifications before finalizing size selection.
Professional guidance is useful for high-risk or high-pressure systems.
Conclusion
Accurately sizing brass threaded fitting ensures leak-free, efficient, and safe system operation.
Use measurements, thread standards, and reliable charts for correct identification.
Avoid mixing NPT and BSP types without proper adapters.
A few simple tools and careful steps make size determination straightforward.
When in doubt, consult a plumbing expert or supplier.
