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.