🔩 Pitch Diameter Calculator
Calculate thread pitch diameter
Nominal thread diameter
1 / threads per inch (TPI)
How to Use This Calculator
Select Thread Type
Choose thread type (Unified, Metric, ACME, or Buttress). Most common are Unified (UNC/UNF) and Metric threads.
Enter Major Diameter
Input major diameter (nominal thread diameter) in inches. For example, 1/2" = 0.500", 3/8" = 0.375".
Enter Thread Pitch
Input thread pitch in inches (1 / threads per inch). For example, 13 TPI = 1/13 = 0.0769", 20 TPI = 1/20 = 0.050".
Calculate and Review
Click "Calculate Pitch Diameter" to see pitch diameter, minor diameter, thread height, and thread details. Use this for thread design and measurement.
Formula
Pitch Diameter: PD = D - 0.6495 × P
Minor Diameter: MD = D - 1.299 × P
where: D = major diameter, P = thread pitch
Example 1: 1/2-13 UNC (D = 0.500", P = 1/13 = 0.0769")
Step 1: Pitch Diameter = D - 0.6495 × P = 0.500 - 0.6495 × 0.0769 = 0.450"
Step 2: Minor Diameter = D - 1.299 × P = 0.500 - 1.299 × 0.0769 = 0.400"
Step 3: Thread Height = 0.866 × P = 0.866 × 0.0769 = 0.067"
Example 2: M12×1.75 (D = 12 mm = 0.472", P = 1.75 mm = 0.069")
Step 1: Pitch Diameter = D - 0.6495 × P = 0.472 - 0.6495 × 0.069 = 0.427"
Step 2: Minor Diameter = D - 1.299 × P = 0.472 - 1.299 × 0.069 = 0.382"
About Pitch Diameter Calculator
The Pitch Diameter Calculator is an essential tool for machinists, mechanical engineers, and thread designers who need to calculate pitch diameter (PD) for threaded fasteners. This calculator implements standard pitch diameter formulas for unified, metric, ACME, and buttress threads, helping you determine thread dimensions and ensure proper thread engagement.
When to Use This Calculator
- Thread Design: Calculate pitch diameter for thread design and manufacturing
- Thread Measurement: Determine pitch diameter for thread inspection
- Thread Analysis: Analyze thread dimensions and tolerances
- Fastener Selection: Verify thread dimensions for fastener selection
- Educational Use: Learn and understand pitch diameter calculations
Why Use Our Calculator?
- ✅ Multiple Thread Types: Supports Unified, Metric, ACME, and Buttress threads
- ✅ Accurate Formulas: Uses standard pitch diameter equations
- ✅ Complete Dimensions: Shows pitch diameter, minor diameter, and thread height
- ✅ Easy to Use: Simple inputs for major diameter and thread pitch
- ✅ Time Savings: Instant calculations eliminate manual math
Understanding Pitch Diameter
Basic Principle: Pitch diameter (PD) is the diameter of an imaginary cylinder that passes through the threads at a point where the width of the thread equals the width of the space. It's calculated using PD = D - 0.6495 × P, where D is major diameter and P is thread pitch. Pitch diameter is critical for thread engagement and fit.
Thread Dimensions: Major diameter is the outer diameter of the thread, minor diameter is the inner diameter (root diameter), and pitch diameter is between them. Thread height is the height of the thread triangle. All dimensions are critical for proper thread function.
Thread Classes: Thread classes (1A, 2A, 3A for external, 1B, 2B, 3B for internal) specify tolerances. Class 1 = loose fit, Class 2 = standard fit, Class 3 = close fit. The calculator shows thread class for reference but doesn't modify pitch diameter calculation.
Tips for Best Results
- Accurate Measurements: Use actual major diameter and pitch from thread specifications
- Correct Thread Type: Select appropriate thread type for accurate calculations
- Verify with Standards: Check thread standards (ANSI, ISO) for exact values
- Consider Tolerances: Account for thread class tolerances in design
- Check Thread Engagement: Ensure proper thread engagement based on pitch diameter
Frequently Asked Questions
What is pitch diameter?
Pitch diameter (PD) is the diameter of an imaginary cylinder that passes through the threads at a point where the width of the thread equals the width of the space. It's calculated using PD = D - 0.6495 × P, where D is major diameter and P is thread pitch. Pitch diameter is critical for thread engagement and fit.
How do I calculate pitch diameter?
Pitch diameter = Major Diameter - (0.6495 × Thread Pitch). For example, 1/2-13 UNC (D = 0.500", P = 0.0769"): PD = 0.500 - 0.6495 × 0.0769 = 0.450". The calculator does this automatically for unified, metric, ACME, and buttress threads.
What is the difference between major and pitch diameter?
Major diameter is the outer diameter of the thread (nominal thread size), while pitch diameter is the diameter at the point where thread width equals space width (typically 0.6495 × pitch smaller than major diameter). Minor diameter is the inner diameter (root diameter), typically 1.299 × pitch smaller than major diameter.
How do I convert threads per inch to pitch?
Thread pitch (in inches) = 1 / Threads Per Inch (TPI). For example, 13 TPI = 1/13 = 0.0769", 20 TPI = 1/20 = 0.050". For metric threads, pitch is specified directly in millimeters (e.g., M12×1.75 has pitch = 1.75 mm).
Does pitch diameter vary by thread class?
Pitch diameter calculation is the same for all thread classes, but tolerance limits vary. Class 1 = loose fit (larger tolerances), Class 2 = standard fit (medium tolerances), Class 3 = close fit (tight tolerances). The calculator shows thread class for reference but uses standard pitch diameter formula.