One-Stop Fasteners Supplier
Blank (Wire) Diameter Calculator
Calculate the wire (blank) diameter needed for cold heading a fastener. Based on volume conservation — the wire blank volume equals the finished part volume. Essential for fastener manufacturing process planning.
Calculator
Quick Reference — Hex Bolt Blank Diameters
| Size | S (mm) | Head H (mm) | Typical Blank ∅ |
|---|---|---|---|
| M5 | 8 | 3.5 | ≈ 7.6 mm |
| M6 | 10 | 4 | ≈ 9.5 mm |
| M8 | 13 | 5.3 | ≈ 12.3 mm |
| M10 | 16 | 6.4 | ≈ 15.2 mm |
| M12 | 18 | 7.5 | ≈ 17.1 mm |
| M14 | 21 | 8.8 | ≈ 19.9 mm |
| M16 | 24 | 10 | ≈ 22.8 mm |
| M18 | 27 | 11.5 | ≈ 25.6 mm |
| M20 | 30 | 12.5 | ≈ 28.5 mm |
| M22 | 34 | 14 | ≈ 32.3 mm |
| M24 | 36 | 15 | ≈ 34.2 mm |
| M27 | 41 | 17 | ≈ 38.9 mm |
| M30 | 46 | 18.7 | ≈ 43.7 mm |
Formula & Principles
Volume Conservation: Vblank = Vfinished part
dw = √(4 × Vtotal / (π × Lblank))
Where:
- Vtotal = Total volume of the finished fastener (head + shank), mm³
- Lblank = Estimated blank length ≈ bolt length + head height × 1.1 (accounts for trim and flash)
- dw = Required wire diameter (mm)
Rule of thumb: For hex bolts, the blank diameter is approximately 95% of the width across flats (S). The actual blank should be rounded up to the nearest standard wire gauge.
Note: This is a theoretical calculation. Actual blank selection should also consider material flow, heading ratio limits (typically max 2.3:1 for single-blow, 4.5:1 for double-blow), and trimming allowance.