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Thread Pitch Calculator

Calculate thread pitch, TPI, tap drill sizes, and thread dimensions for metric (ISO), UNC, UNF, and BSP standards. Convert between pitch and threads per inch.

TPI = 25.4 / Pitch(mm)  |  Pitch(mm) = 25.4 / TPI

Convert between thread pitch (mm) and threads per inch (TPI)

Enter either value to calculate the other.

If provided, minor and pitch diameters are calculated
Result:
ISO Metric Thread Standard (M-series)

Look up standard metric thread dimensions by size

Result:
Common Metric Threads
Size Major ∅ (mm) Coarse Pitch Fine Pitch Coarse Tap Drill Fine Tap Drill
Unified National Thread Standard (UNC / UNF)

Look up standard UNC and UNF thread dimensions

Result:
Common UNC / UNF Threads
Size Major ∅ (in) UNC TPI UNF TPI UNC Tap Drill UNF Tap Drill
Tap Drill = Major ∅ − Pitch  (~75% thread)

Calculate the recommended tap drill size for a given thread

Fill pitch or TPI
%
Typical: 75% (standard), 50% (easy tapping in hard materials)
Result:

How It Works

What is Thread Pitch?

Thread pitch is the distance between adjacent thread crests, measured in millimeters for metric threads. Imperial threads use threads per inch (TPI) instead. The two are inversely related: Pitch (mm) = 25.4 / TPI. Thread pitch determines how far a fastener advances per revolution and affects the clamping force and ease of assembly.

Key Formulas

  • TPI = 25.4 / Pitch(mm) — convert pitch to threads per inch
  • Pitch(mm) = 25.4 / TPI — convert TPI to pitch
  • Minor ∅ = Major ∅ − 1.0825 × Pitch — 60° thread (ISO/Unified)
  • Pitch ∅ = Major ∅ − 0.6495 × Pitch — effective diameter
  • Tap Drill = Major ∅ − Pitch — approximate for 75% thread engagement
  • Tap Drill = Major ∅ − (Thread% / 76.98) × Pitch — adjustable engagement

Thread Standards

  • ISO Metric (M): The most widely used thread standard worldwide. Designated by M followed by diameter and pitch (e.g., M10×1.5). Uses a 60° thread angle.
  • UNC (Unified National Coarse): Standard coarse thread series used primarily in the US. Fewer threads per inch for faster assembly.
  • UNF (Unified National Fine): Fine thread series with more TPI. Stronger in tension and better for thin-wall applications.
  • BSP (British Standard Pipe): Used for pipe fittings. Available in parallel (BSPP) and tapered (BSPT) forms.

Coarse vs. Fine Threads

Property Coarse Thread Fine Thread
PitchLargerSmaller
Assembly speedFasterSlower
Vibration resistanceLowerHigher
Tensile strengthLower (smaller minor ∅)Higher (larger minor ∅)
Stripping resistanceHigher in soft materialsLower in soft materials
Typical useGeneral purposePrecision, automotive, aerospace

Thread Engagement

Thread engagement percentage determines how deep the threads cut into the tapped hole. 75% engagement is standard and provides a good balance between strength and ease of tapping. Going beyond 75% adds minimal strength (a 100% thread is only ~5% stronger than 75%) but significantly increases tapping difficulty and tap breakage risk. For hard materials, 50–60% engagement is often preferred.

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