Metric Thread Calculator

Look up ISO metric thread parameters for M1–M48 coarse pitch series. Dimensions per ISO 261 and ISO 965-1.

Formula Reference

Click to expand ISO thread formulas and tolerance calculations

Basic Dimensions (ISO metric, 60° thread)

  • Basic pitch diameter: d₂ = d − 0.649519 × P
  • Basic minor diameter (external): d₃ = d − 1.226869 × P
  • Basic minor diameter (internal): D₁ = d − 1.082532 × P

ISO 965-1 Tolerance Fundamentals

Tolerance grade numbers indicate precision: lower = tighter.

  • Fundamental deviation es (external) / EI (internal): defined by ISO 965-1 tables for each pitch. Negative for external (g), zero for internal (H).
  • Tolerance on pitch diameter (external): Td₂ = k × (180 × P⁰·⁸² − 3.15 × P⁰·²) μm, where k = 0.8×grade for grades 4–7.
  • Tolerance on pitch diameter (internal): TD₂ = k × (180 × P⁰·⁸² − 3.15 × P⁰·²) μm, where k = 0.8×grade.
  • Tolerance on major diameter (external): Td = (180 × P⁰·⁸² − 3.15 × P⁰·²) × k', where k' = 0.8×grade.
  • Tolerance on minor diameter (internal): TD₁ = (230 × P⁰·⁷) × k, where k = 0.8×grade.
  • Tolerance on minor diameter (external): Td₃ = 1.25 × Td₂ + 0.063 × √P (approximate).

Resulting Limits

  • External major: d_max = d + es, d_min = d + es − Td
  • External pitch diameter: d₂_max = d₂ + es, d₂_min = d₂ + es − Td₂
  • External minor diameter: d₃_max = d₃ + es, (no minimum specified)
  • Internal major: D_min = D (nominal, no tolerance band defined)
  • Internal pitch diameter: D₂_min = D₂, D₂_max = D₂ + TD₂
  • Internal minor: D₁_min = D₁, D₁_max = D₁ + TD₁

FAQ

What does the tolerance class notation mean (e.g., 6H/6g)?
The number is the tolerance grade (lower = tighter). The letter indicates the fundamental deviation: H for internal threads (zero deviation, nut-side) and g for external threads (negative deviation, bolt-side). A "6H/6g" pair is the most common general-purpose choice.
What is the difference between d (lowercase) and D (uppercase) diameters?
Lowercase d refers to external threads (bolts/studs). Uppercase D refers to internal threads (nuts/tapped holes). Each has its own tolerance zone.
Why is d_min (external major) sometimes below the nominal diameter?
The fundamental deviation for class g is negative, which shifts the entire tolerance band downward. This provides clearance (backlash) so external and internal threads always assemble freely. Class H has zero deviation, so the internal thread starts exactly at nominal.
Which tolerance class should I use?
6H/6g is the default for most applications. Use 4H/4g for precision fits (e.g., measuring gauges). Use 7H/7g6g for rough or quick-production threads where easy assembly is more important than tightness.
Are these values suitable for manufacturing?
These values are calculated from ISO 965-1 formulas and are intended for reference only. Always consult the official ISO 261 and ISO 965 standards or your manufacturer's specifications for production use. Some approximations are used for minor diameter tolerances.
What's the difference between coarse and fine metric threads?
Coarse threads (default M-series) have a larger pitch — e.g., M10×1.5. They're stronger against stripping, easier to assemble, and less prone to cross-threading. Fine threads (e.g., M10×1.25) have more threads per mm, providing higher tensile stress area, better locking resistance, and finer adjustment capability. Coarse is standard for most applications; fine is used in aerospace, instruments, and precision mechanisms.
What does the 6H/6g tolerance class mean?
The ISO 965 tolerance class combines a grade number and a position letter. '6' is the tolerance grade (medium/standard). 'H' (uppercase) for internal threads means zero fundamental deviation. 'g' (lowercase) for external threads provides a small clearance (allowance) to accommodate plating. So 6H/6g is the most common fit: standard tolerance with a small gap that allows free assembly even with thin coatings.
How do I measure thread pitch?
Use a thread pitch gauge (comb gauge) — a set of thin blades with teeth matching standard pitches. Hold the gauge against the threads until you find a perfect match. Alternatively, measure the distance across 10 thread crests with calipers and divide by 10. For metric threads, pitch is in millimeters. For imperial threads, threads per inch (TPI) is the reciprocal of pitch.
Which thread size should I use for my application?
For general-purpose fastening, common sizes are M3 (electronics), M4-M5 (light machinery), M6-M8 (general mechanical), M10-M12 (automotive/structural), and M16+ (heavy structural). Consider the load, available space, and standardization. Over-specifying wastes weight and cost; under-specifying risks failure. When in doubt, consult engineering references or a mechanical engineer for critical applications.

How to Use the Metric Thread Calculator

  1. Select a thread size — choose from M1 through M48 coarse thread series (M-series per ISO 261).
  2. View the thread parameters — major diameter, pitch diameter, minor diameter, pitch, and tolerance class are displayed instantly.
  3. Check tolerance classes — external threads (g, h) and internal threads (G, H) per ISO 965 for proper fit selection.
  4. Compare thread sizes side by side to understand how parameters scale with diameter and pitch.
  5. Reference the full data table for all M-series threads from M1 to M48 for quick engineering lookups.

Understanding ISO Metric Threads

ISO metric threads, designated by the "M" prefix followed by nominal diameter (e.g., M10), are the world's most widely used screw thread standard, defined by ISO 261. The coarse pitch series is the default for general applications — M10 coarse has a 1.5 mm pitch (distance between adjacent thread crests). Fine pitch variants (e.g., M10×1.25) are used where higher tensile stress area, better loosening resistance, or finer adjustment is needed. The thread profile follows ISO 681 with a 60° flank angle and is symmetrical.

The three critical diameters define thread geometry. The major diameter is the largest diameter of the thread (the nominal size, e.g., 10.000 mm for M10). The pitch diameter is the theoretical diameter where the thread thickness equals the space between threads — this is where mating threads actually contact and is used for fit calculations. The minor diameter is the smallest diameter at the root of the thread. For an M10×1.5 thread: major = 10.000 mm, pitch diameter = 9.026 mm, minor diameter = 8.160 mm (external) / 8.376 mm (internal).

Tolerance classes per ISO 965 ensure proper fit between mating threads. External (bolt) threads use lowercase letters: 6g is the most common general-purpose class, 6h provides a tighter fit with zero allowance, and 4g6g is for precision applications. Internal (nut) threads use uppercase: 6H is standard, creating a sliding fit with 6g external threads. The tolerance position (e.g., 'g' vs 'h') controls the fundamental deviation (allowance), while the grade number (4, 6, 8) controls the tolerance width. For corrosive environments or plating thickness, class 6g is preferred as it provides a clearance that accommodates coating without interference.

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