The pitch diameter is where the thread groove and thread ridge are equally wide. It is the diameter that actually determines how a nut and bolt fit. Inspectors measure it with thread micrometers or with the three-wire method, laying three precise wires in the grooves and measuring over them.
How to use the Pitch Diameter Calculator
- Major diameter
- Pitch
- Wire diameter (0 = best size)
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the major (nominal) diameter and the pitch; optionally your wire diameter (0 uses the best wire size).
Inputs explained
| Field | What to enter |
|---|---|
| Major diameter | Units: mm, in, cm |
| Pitch | Units: mm, in |
| Wire diameter (0 = best size) | Units: mm, in, cm |
What each result means
- Pitch diameter d₂ = [Major diameter] – 0.649519×[Pitch]
- Minor diameter, external d₃ = [Major diameter] – 1.226869×[Pitch]
- Minor diameter, internal D₁ = [Major diameter] – 1.082532×[Pitch]
- Best wire size = 0.57735×[Pitch]
Example values
How the main result changes when Major diameter changes and everything else stays at the example values above:
| Major diameter (mm) | Pitch diameter d₂ (mm) |
|---|---|
| 5 | 4.0257 |
| 7.5 | 6.5257 |
| 10 | 9.0257 |
| 12.5 | 11.5257 |
| 15 | 14.0257 |
| 20 | 19.0257 |
Formula
d₂ = d − 0.649519 × P
d₃ = d − 1.226869 × P; D₁ = d − 1.082532 × P
Best wire w = 0.57735 × P; M = d₂ + 3w − 0.866025 × P
Worked example
Inputs: Major diameter: 10 mm; Pitch: 1.5 mm; Wire diameter (0 = best size): 0 mm.
Results: Pitch diameter d₂ = 9.0257 mm; Minor diameter, external d₃ = 8.1597 mm; Minor diameter, internal D₁ = 8.3762 mm; Best wire size = 0.866 mm; Measurement over wires = 10.3248 mm.
Frequently asked questions
What is the pitch diameter of M10 × 1.5?
10 − 0.6495 × 1.5 = 9.026 mm.
Why use the three-wire method?
It measures pitch diameter directly with ordinary micrometers and is very accurate.
What is the best wire size?
The wire that touches the flanks at the pitch line: 0.57735 × pitch for 60° threads.