ISO metric threads have a 60° profile and are designated by diameter and pitch – M10 × 1.5. Coarse pitches are the default; fine pitches are used for adjustment and thin walls. The tensile stress area is used to calculate bolt strength, and the tap drill size gives about 75% thread engagement.
How to use the Thread Calculator
- Nominal (major) diameter
- Pitch
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the nominal diameter and the pitch (see the coarse pitch table below).
ISO coarse pitches
| Size | Pitch (mm) | Tap drill (mm) | Stress area (mm²) |
|---|---|---|---|
| M3 | 0.5 | 2.5 | 5.0 |
| M4 | 0.7 | 3.3 | 8.8 |
| M5 | 0.8 | 4.2 | 14.2 |
| M6 | 1 | 5 | 20.1 |
| M8 | 1.25 | 6.75 | 36.6 |
| M10 | 1.5 | 8.5 | 58.0 |
| M12 | 1.75 | 10.25 | 84.3 |
| M16 | 2 | 14 | 156.7 |
| M20 | 2.5 | 17.5 | 244.8 |
| M24 | 3 | 21 | 352.5 |
| M30 | 3.5 | 26.5 | 560.6 |
| M36 | 4 | 32 | 816.7 |
Tips for an accurate estimate
- Keep units consistent – the calculator handles conversions, but check every field’s unit before reading a result.
- Use values from standards, data sheets or test pieces whenever you have them; typical values are only a starting point.
- Apply appropriate safety factors and have structural, electrical and pressure work checked by a qualified professional.
- Record the inputs you used so the calculation can be reviewed later.
Common mistakes to avoid
- Confusing diameter and radius, or nominal and actual sizes.
- Using allowable stresses and ultimate strengths interchangeably.
- Treating rule-of-thumb results as a substitute for code tables or an engineered design.
Inputs explained
| Field | What to enter |
|---|---|
| Nominal (major) diameter | Units: mm, in, cm |
| Pitch | Units: mm, in |
What each result means
- Fundamental triangle height H = 0.866025×[Pitch]
- Pitch diameter = [Nominal (major) diameter] – 0.649519×[Pitch]
- Minor diameter (external) = [Nominal (major) diameter] – 1.226869×[Pitch]
- Minor diameter (internal) = [Nominal (major) diameter] – 1.082532×[Pitch]
- Thread depth (external) = 0.613343×[Pitch]
- Tap drill = [Nominal (major) diameter] – [Pitch]
- Tensile stress area = pi÷4×([Nominal (major) diameter] – 0.938194×[Pitch])^2
Formula
H = 0.866025 P; d₂ = d − 0.649519 Pd₃ = d − 1.226869 P (external minor); D₁ = d − 1.082532 P (internal minor)Thread depth h₃ = 0.613343 P; tap drill ≈ d − PStress area Aₛ = π/4 × (d − 0.938194 P)²Worked example
Inputs — Nominal (major) diameter: 10 mm; Pitch: 1.5 mm.
Results — Fundamental triangle height H = 1.299 mm; Pitch diameter = 9.0257 mm; Minor diameter (external) = 8.1597 mm; Minor diameter (internal) = 8.3762 mm; Thread depth (external) = 0.92 mm; Tap drill = 8.5 mm; Tensile stress area = 57.99 mm².
Frequently asked questions
What is the tap drill for M10 × 1.5?
8.5 mm (10 − 1.5).
What is the stress area of an M12 bolt?
84.3 mm² for M12 × 1.75.
What pitch is M8?
1.25 mm coarse; 1.0 mm fine.
What formula does the Thread Calculator use?
H = 0.866025 P; d₂ = d − 0.649519 P.