1,500 sq ft attic, 1:150, 50/50 split
- NFVA = 1,500 ÷ 150 = 10 sq ft
- NFVA sq in = 10 × 144 = 1,440 sq in
- Intake = 720 sq in; Exhaust = 720 sq in
construction calculator
Attic Ventilation Calculator
Estimate required net free vent area (NFVA) using 1:150 or 1:300 rules, split between intake and exhaust.
Results
- Required NFVA (sq ft)
- 10.00
- Required NFVA (sq in)
- 1,440
- Intake NFVA (sq in)
- 720
- Exhaust NFVA (sq in)
- 720
How to use this calculator
- Enter attic floor area.
- Select 1:150 or 1:300 ratio.
- Set the intake percentage (default 50%).
- Review total NFVA and intake/exhaust split in square inches.
Inputs explained
- Ventilation ratio
- 1:150 is standard; 1:300 may apply with vapor barrier or balanced venting.
- Intake %
- Portion of NFVA assigned to intake (soffit). The rest goes to exhaust (ridge/gable).
How it works
NFVA (sq ft) = Attic area ÷ Vent ratio (1:150 or 1:300). NFVA (sq in) = NFVA × 144.
Intake/exhaust split distributes NFVA between soffit/roof vents (e.g., 50/50).
Formula
NFVA (sq ft) = Area ÷ Ratio NFVA (sq in) = NFVA × 144 Intake = NFVA × Intake % Exhaust = NFVA − Intake
When to use it
- Planning soffit/ridge vent requirements for new roofs.
- Checking if existing attic vents meet guideline NFVA.
- Balancing intake vs exhaust for better airflow.
Tips & cautions
- Use manufacturer NFVA ratings (per vent) to translate required sq in into vent counts.
- Intake-heavy splits (e.g., 60/40) can help ensure positive airflow from soffits to ridge.
- Air sealing/insulation complement ventilation; address those alongside venting.
- Guideline calculation; verify local code requirements.
- Does not account for blocked/ineffective vents; inspect actual NFVA per vent product.
- High wind/snow regions may have additional requirements.
Worked examples
1,200 sq ft attic, 1:300, 60/40 split
- NFVA = 1,200 ÷ 300 = 4 sq ft
- NFVA sq in = 576 sq in
- Intake 60% ≈ 346 sq in; Exhaust 40% ≈ 230 sq in
Deep dive
Estimate attic ventilation NFVA using 1:150 or 1:300 rules and split intake vs exhaust.
Enter attic area and vent ratio to see required sq in for soffit and ridge/gable vents.
FAQs
- When can I use 1:300 instead of 1:150?
- Often when a vapor barrier is present and vents are balanced. Check your local code for conditions.
- How do I convert NFVA to vent counts?
- Divide required intake/exhaust sq in by the NFVA per vent from the manufacturer.
- Should intake be greater than exhaust?
- A slight intake bias (e.g., 55–60%) can help, but 50/50 is common. Avoid too little intake.
- Does this account for blocked soffits?
- No. Ensure soffits are clear and baffles are installed if insulation could block airflow.
- Is this code compliant?
- It follows common guidelines. Always verify with local building codes and inspector requirements.
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Guideline calculation only. Verify local code, manufacturer NFVA, and on-site conditions before finalizing ventilation plans.