Pitch Diameter Calculator
Calculate ISO metric thread pitch diameters for threaded fasteners
Thread Specifications
The nominal thread diameter (outer diameter for external threads)
Distance between adjacent thread crests (P)
Higher numbers = looser tolerance (easier machining)
Fundamental deviation from basic size
Pitch Diameter Results
⚠️ Threading Safety Warning
• This calculator is for ISO metric threads (60° thread angle) only
• Always verify calculations with engineering drawings and specifications
• Improper threading can lead to fastener failure and structural collapse
• Consider material properties, stress concentrations, and fatigue factors
• Use appropriate measuring tools: thread pitch gauges, micrometers, ring/plug gauges
• Follow manufacturing tolerances strictly for safety-critical applications
Example: M20 × 2.5 - 6h Bolt Calculation
Given Specifications
Thread Designation: M20 × 2.5 - 6h
Major Diameter (d): 20 mm
Thread Pitch (P): 2.5 mm
Tolerance Class: 6h (Grade 7, Position h)
Thread Type: External (bolt/screw)
Pitch Diameter Calculation
• Basic Pitch Diameter: d₂ = d - 0.6495 × P
• d₂ = 20 - 0.6495 × 2.5 = 20 - 1.624 = 18.376 mm
• Fundamental Deviation (es) for h position = 0 mm
• Tolerance (Td2) = 1.0 × (90 × 2.5^0.4 × 20^0.1) / 1000 = 0.224 mm
• Maximum: d₂max = 18.376 + 0 = 18.376 mm
• Minimum: d₂min = 18.376 + 0 - 0.224 = 18.152 mm
Result: Pitch diameter range 18.152 - 18.376 mm
Key Formulas
Basic Pitch Diameter
d₂ = d - 0.6495 × P
External Thread Limits
d₂max = d₂ + es
d₂min = d₂ + es - Td2
Internal Thread Limits
D₂max = D₂ + EI + TD2
D₂min = D₂ + EI
Tolerance Grades
Fine (4-6)
Precision applications, close fits
Medium (7-8)
General engineering, standard fits
Coarse (9-10)
Easy assembly, loose fits
Common Threads
Values shown are basic pitch diameters
Understanding Pitch Diameter in Threading
What is Pitch Diameter?
Pitch diameter is one of three critical thread dimensions (along with major and minor diameters). It's the diameter where the thread thickness equals the space between threads. This diameter determines how threads engage and is crucial for proper fastener assembly and strength.
Basic Calculation Formula
d₂ = d - 0.6495 × P
- •d₂ = pitch diameter
- •d = major diameter (nominal size)
- •P = thread pitch (distance between threads)
- •0.6495 = constant derived from 60° thread angle
Tolerance System
ISO metric threading uses a sophisticated tolerance system with grades (4-10) and positions (e, f, g, h for external; G, H for internal) to ensure proper fit between mating threads.
Critical Applications
Structural Bolting
Precision pitch diameter ensures proper thread engagement and load transfer.
Manufacturing
Quality control requires pitch diameter verification for thread acceptance.
Assembly Operations
Proper thread fit prevents cross-threading and ensures reliable connections.