Thread Size Charts · Free Tool
Pipe Thread Identifier (NPT, BSP or Metric)
Identify an unknown thread on a fitting, gauge or transmitter: measure the outside diameter and the thread pitch, and get the closest NPT, BSP and metric matches.
Measure ODCount TPINPT or BSPMetric pitch
mm
mm
Best matchG or R 1/2" (BSP)
Second match1/2" NPT
Your pitch1.814 mm (14 TPI)
Closest standard threads
| Thread | Nominal OD (mm) | Pitch (mm) |
|---|---|---|
| G or R 1/2" (BSP) | 20.96 | 1.814 |
| 1/2" NPT | 21.34 | 1.814 |
| G or R 3/4" (BSP) | 26.44 | 1.814 |
| 3/4" NPT | 26.67 | 1.814 |
| M24 × 2.0 | 24.00 | 2.000 |
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How to Identify a Thread
1. Measure the outside diameter across the crests (male) with a caliper
2. Count threads over 1 inch (TPI) or measure the pitch with a thread gauge
3. Taper threads get smaller towards the end: compare the first and last thread
2. Count threads over 1 inch (TPI) or measure the pitch with a thread gauge
3. Taper threads get smaller towards the end: compare the first and last thread
Worked example
OD 20.9 mm with 14 TPI matches 1/2" BSP (20.955 mm). An OD of 21.3 mm with 14 TPI and a visible taper is 1/2" NPT.
Key insight: NPT and BSP 1/2" and 3/4" both have 14 TPI; the 60° vs 55° thread angle, checked with a thread gauge, is the only sure way to tell them apart.
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Frequently Asked Questions
How can I tell NPT from BSP?
Check the thread angle with a gauge (NPT 60°, BSP 55°) and the diameter; 1/2" NPT is about 21.3 mm, 1/2" BSP about 20.96 mm.
How do I count threads per inch?
Place a ruler along the thread and count the crests in 25.4 mm, or use a thread pitch gauge.
Is a thread tapered or parallel?
Measure the diameter at the first and last full thread; a taper thread is smaller at the end.
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