About this calculator
Size attic ventilation from the attic floor area using the 1/150 or 1/300 rule, then count ridge vent and soffit vents.
How it’s calculated
Net free area (sq in) = attic floor area (sq ft) × 144 ÷ 150 (or ÷ 300). Split it evenly between intake (soffit) and exhaust (ridge).
Worked example
With these inputs:
- Attic length
- 40 ft
- Attic width
- 28 ft
- Ventilation ratio
- 1/300 (building code, slopes 1 in 6 or steeper)
- Ridge vent NFA (sq in per ft)
- 18
- Soffit vent NFA (sq in each)
- 56
the result is:
- Total net free area
- 538 sq in
- Attic floor area
- 1,120 sq ft
- Ridge vent needed
- 14.9 ft
- Soffit vents needed
- 5 vents
Tips from the trade
- Baffles or rafter trays keep insulation from blocking the eaves.
- Don’t mix ridge vents with gable or powered vents on the same attic. They short-circuit each other.
Results are estimates for planning and pricing. Check quantities against your drawings, the manufacturer’s data and the building code adopted in your province before ordering.