Vent & Drain Hole Calculator
Quick check of vent and drain hole sizing for a hollow steel section before hot-dip galvanizing — so air vents, zinc drains and the section isn't rejected (or worse, blows in the kettle).
Advanced — wall, open-area rule, minimum hole
- Enclosed area 8,464 mm²
- Required open area 2,539 mm²
- Open area per hole 1,270 mm²
- Governing rule 30% area
How vent & drain holes are sized
A hollow section dipped in molten zinc must let air escape (vent) and zinc flow out (drain). If it can't, the inside won't coat, trapped zinc adds dead weight — and trapped moisture or air can cause a violent blow-out in the kettle. The common galvanizing rule sizes the holes from the section's enclosed cross-sectional area:
Holes go at diagonally opposite ends — one high to vent air, one low to drain zinc as the piece is withdrawn at an angle. Bigger or more numerous holes drain faster and cut dead weight.
Worked example
A 100 × 100 mm RHS, 4 mm wall, 2 holes, 30% rule:
- Enclosed area = (100−8) × (100−8) = 8,464 mm²
- Required open area = 30% × 8,464 = 2,539 mm²
- Per hole (2 holes) = 1,270 mm²
- Hole Ø = √(4 × 1,270 ÷ π) ≈ Ø 40 mm
Use more holes to bring each diameter down. For real fabricated assemblies — frames, trusses, nodes — mark and size every hole and export a driller-ready sheet with the full Vent & Drain Hole Planner.
The full Hole Planner adds
- Mark vent/drain holes on photos of the real steel
- Size from section volume, not just a single tube
- Numbered hole schedule (vent / drain / both)
- Driller-ready PDF for the workshop
Questions
How big should vent and drain holes be?
A common rule: total hole area ≥ 25–30% of the enclosed cross-sectional area, with a practical minimum of about Ø 10–12 mm. Place them at diagonally opposite ends.
Why are they needed?
Sealed hollow sections trap air (which prevents coating) and molten zinc (dead weight, and an explosion hazard if moisture is sealed in). Venting and draining is a safety and quality requirement.
Round vs rectangular — does it matter?
The rule is the same — it's based on the enclosed cross-sectional area. This tool computes that area for round (CHS) or rectangular (SHS/RHS) sections from your dimensions.
Is this calculator free?
Yes — no sign-up. The members Hole Planner marks holes on real photos and exports a driller-ready PDF.