How to Measure Thread Pitch: NPT vs BSPT vs BSPP vs Metric

When working with pneumatic fittings, identifying the correct thread type is critical. Even threads that look similar can leak or fail if they don’t match.

This quick guide shows you how to measure thread pitch and tell the difference between NPT, BSPT, BSPP, and metric threads.

Step 1: Tapered or Parallel?

Start by checking the shape of the thread:

• Tapered threads get narrower toward the end
  (NPT, BSPT)
• Parallel threads stay the same diameter all the way across
  (BSPP, Metric)

Quick check:
Look at the thread or measure both ends with calipers. If the size changes, it’s tapered.

Step 2: Measure the Thread Pitch

Thread pitch = spacing between threads.

• NPT & BSP: measured in threads per inch (TPI)
• Metric: measured in millimeters

Best tool: Thread pitch gauge
Just match the teeth to the thread until it fits perfectly.

Step 3: Measure the Diameter

Use calipers to measure:

• Outside of a male thread
• Inside of a female thread

Important:
Thread sizes are nominal. A 1/4" fitting won’t measure exactly 1/4".

Step 4: Identify the Thread Type

Now that you have measured the thread type, pitch, and diameter, you can use the reference charts below to identify your fitting.

Match your measurements as closely as possible. Remember that most pipe thread sizes are nominal, meaning the labeled size may not match the exact physical diameter.

NPT

• Tapered
• Common in North America
• Seals on the threads

Pipe Size	OD in inches
1/16" NPT	0.313"
1/8" NPT	0.405"
1/4" NPT	0.540"
3/8" NPT	0.675"
1/2" NPT	0.840"
3/4" NPT	1.050"
1" NPT	1.315"
1-1/4" NPT	1.660"
1-1/2" NPT	1.900"
2" NPT	2.375"

 

BSPT

• Tapered
• Similar to NPT, but not compatible

Nominal Size	OD in mm
1/8" BSPP	9.728 mm
1/4" BSPP	13.157 mm
3/8" BSPP	16.662 mm
1/2" BSPP	20.995 mm

 

BSPP

• Parallel
• Needs an O-ring or washer to seal

Nominal Size	OD in mm
1/16" BSPT	7.723 mm
1/8" BSPT	9.728 mm
1/4" BSPT	13.157 mm
3/8" BSPT	16.662 mm
1/2" BSPT	20.995 mm

 

Metric

• Parallel
• Measured in mm (example: M10 x 1.5)
  
  

NPT vs BSP: Don’t Mix Them

They may look similar, but they are different:

• Different thread angles
• Different pitch
• Different sealing methods

Bottom line: They will leak if mixed.

Tools You Should Have

• Thread pitch gauge
• Calipers
• Thread chart (for quick reference)

These make identification much faster and more accurate.

Common Mistakes

• Assuming threads are the same because they look similar
• Not checking pitch
• Forcing mismatched fittings together

Final Thoughts

Measuring thread pitch only takes a few minutes, but it can save you from leaks, damage, and wasted time.

If you’re unsure, it’s always better to double-check before installing.