Hidroteknik Calculators
Thread Pitch & Size Identification
Two measurements are enough to identify an unknown thread: the major diameter (with calipers) and the pitch (with a pitch gauge). These are matched against standard tables.
⚙ Open the Hydraulic Calculator →Basic Relationship
Metric (60°) minor diameter
Pitch of an inch thread
D: major diameter (mm), P: pitch (mm)
Units
| Quantity | Symbol | Unit |
|---|---|---|
| Major diameter | D | mm |
| Pitch | P | mm |
| Thread count | TPI | threads per inch |
Worked Example
Given: Calipers read an outer diameter ≈ 20.0 mm; the pitch gauge shows P = 2.5 mm.
1) 20 mm + 2.5 mm pitch → standard match: M20 × 2.5 (metric coarse)
2) Minor diameter ≈ 20 − 1.0825×2.5 = 17.3 mm (as a check)
Frequently Asked Questions
How do I measure the pitch?
The easiest way is a pitch (thread) gauge: find the blade that fits, and it is stamped with the pitch (mm) or TPI. Alternatively, measure across 10 threads and divide by 10.
How do I tell metric from inch?
If the pitch is a round mm value (1.5, 2, 2.5) it is usually metric. If it fits a "threads per inch" value (e.g. 19 TPI) it is a BSP/NPT/UN thread.
What is the difference between BSP and NPT?
BSP can be parallel or tapered and has a 55° thread angle; NPT is tapered and 60°. Mixing them causes leaks.