Thread Calculator: UN, Metric, NPT, BSP, ACME

UN Thread Calculator

Calculate dimensions and tolerances for Unified National threads (UNC, UNF, UNEF).

Calculation Results

Major Diameter Tolerance
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Pitch Diameter Tolerance
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Minor Diameter
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Tap Drill Size
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Advanced Options +
Calculated Pitch Diameter
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Metric Thread Calculator

Calculate dimensions and tolerances for ISO metric threads (M series).

Calculation Results

Major Diameter
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Pitch Diameter
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Minor Diameter
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Tap Drill Size
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NPT Thread Calculator

Calculate dimensions for National Pipe Taper threads (NPT).

Calculation Results

Outside Diameter
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Pitch Diameter
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Threads Per Inch
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Taper Per Foot
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BSP Thread Calculator

Calculate dimensions for British Standard Pipe threads (BSPP & BSPT).

Calculation Results

Major Diameter
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Pitch Diameter
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Threads Per Inch
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Minor Diameter
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ACME Thread Calculator

Calculate dimensions for ACME trapezoidal threads used in power transmission.

Calculation Results

Major Diameter
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Pitch Diameter
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Minor Diameter
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Thread Depth
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Thread Types & Standards

Our calculator supports the most common thread standards used in engineering and manufacturing:

  • UN Threads (Unified National): Used primarily in the United States, Canada, and the UK. Includes UNC (coarse), UNF (fine), and UNEF (extra fine) series.
  • Metric Threads: Standardized under ISO 68-1, used worldwide with coarse and fine pitch options.
  • NPT Threads (National Pipe Taper): Tapered threads used for pipe fittings in North America.
  • BSP Threads (British Standard Pipe): Parallel (BSPP) and tapered (BSPT) threads used internationally.
  • ACME Threads: Trapezoidal threads designed for power transmission and lead screws.

Thread Formulas

Key formulas used in thread calculations:

Pitch Diameter (D) = Major Diameter – 0.6495 × Pitch
Minor Diameter = Major Diameter – 1.08253 × Pitch
Thread Height (H) = 0.866025 × Pitch
Over Wires Measurement (M) = D + 3G – 0.866025 × Pitch
Where G is the wire diameter

Frequently Asked Questions

What is the difference between UNC and UNF threads? +

UNC (Unified National Coarse) threads have a larger pitch (fewer threads per inch) and are used for general purpose applications where rapid assembly/disassembly is needed. UNF (Unified National Fine) threads have a smaller pitch (more threads per inch) and provide better tensile strength and vibration resistance.

How do I calculate the correct tap drill size? +

For most applications, the tap drill size can be calculated as: Major Diameter – Pitch. For example, a 1/4-20 UNC thread would require a tap drill of 0.25″ – (1/20)” = 0.20″ or a #7 drill. Our calculator automatically provides the recommended tap drill size based on the thread parameters.

What are thread classes and how do I choose the right one? +

Thread classes (1A/1B, 2A/2B, 3A/3B) define the tolerance and fit of threaded fasteners. Class 1 is the loosest fit for easy assembly, Class 2 is the standard for most commercial applications, and Class 3 is the tightest fit for precision applications. The “A” designation is for external threads, “B” for internal threads.

Thread Glossary

Major Diameter: The largest diameter of a thread, measured from crest to crest on external threads or root to root on internal threads.

Minor Diameter: The smallest diameter of a thread, measured from root to root on external threads or crest to crest on internal threads.

Pitch Diameter: The theoretical diameter where the thread width equals the space between threads. This is the most critical dimension for thread fit.

Pitch: The distance from one thread crest to the next. In TPI (threads per inch) systems, pitch = 1/TPI.

Lead: The distance a thread advances in one complete revolution. For single-start threads, lead equals pitch.