Attic Ventilation Calculator & NFA Plan
Compare retained vent openings with rounded additions and the actual extra vent length or slots available.
Will your vent plan fit?
Compare retained intake and exhaust with the NFA your vented attic needs. Enter the actual product ratings and the extra length or vent slots available.
NFA targets met within entered space
Attic floor: 1,500 ft². Planning target: 1,440 in² total · 720 in² per side (1:150).
NFA arithmetic only. This does not approve a code exception, installation layout or airflow performance.
Low intake
Already installed: 0 retained groups
Include compatible, unobstructed intake openings only. You can enter separate groups with different NFA ratings.
High exhaust
Already installed: 0 retained groups
Retain only openings compatible with the selected exhaust strategy. Different exhaust strategies need an installation assessment, not an added NFA total.
Current → proposed
■ Retained ■ Added │ Target · Bars compare NFA, not airflow.
Current openings
Intake short by 720 in². Exhaust short by 720 in².
After fitting additions
Both half-targets met; intake NFA is at least exhaust NFA.
Intake additions
28 vents needed- Fits entered space
- 28 vents
- Cannot fit
- 0 vents
- Product label NFA
- 26 in²
- Additional space
- 100 vents
- Installation increment
- 1 vents
- Retained openings
- None
- Resulting total NFA
- 728 in²
Exhaust additions
40 ft needed- Fits entered space
- 40 ft
- Cannot fit
- 0 ft
- Product label NFA
- 18 in² / ft
- Additional space
- 80 ft
- Installation increment
- 4 ft
- Retained openings
- None
- Resulting total NFA
- 720 in²
Intake addition target: 720 in². Rounded exhaust quantities are included before intake is sized. Only additions that fit complete increments are included in proposed totals. This is an NFA arithmetic check, not code approval or a prediction of airflow.
Try a complete example
These are editable arithmetic examples, not product specifications or local code approvals. Choosing one resets all fields and units.
How the plan is calculated
Total target in in² = attic floor area in ft² × 144 ÷ the confirmed ratio denominator. Each side starts at half this total. Retained NFA is installed quantity × actual rated NFA per vent or per linear foot.
Additional exhaust is rounded up to complete installation increments. It is limited to the extra length or whole slots entered; an incomplete remaining section is not counted. Intake then targets the larger of the half-target and resulting exhaust NFA. This accounts for rounding and retained excess exhaust. It does not remove or cap existing vents.
Blank additional space means unknown. Proposed totals then assume the theoretical addition can fit and are labelled unconfirmed. Zero means no extra space. An already sufficient side needs no space confirmation because no additions are necessary.
Use the label’s NFA after screens and louvers, not the hole’s gross dimensions. Available space must be the usable additional length or verified slots after product setbacks, end closures and existing openings. This calculator cannot verify clear paths through each bay, vent positioning, obstruction, product compatibility or local requirements.
Scope: one connected, vented attic and one compatible passive exhaust strategy. No powered fans, unvented/conditioned assemblies, mixed exhaust systems or airflow simulation. Assess isolated attic compartments separately. Limits: 1,000,000 ft² floor area and 12 retained groups per side. Numbers accept an ungrouped decimal point or comma; no thousands separators.
Sources and related tools
GAF’s balanced ventilation planning guide explains floor-area/NFA arithmetic, low intake and high exhaust, and the need to avoid mixed exhaust strategies. Its 1:300 example is not approval for every assembly or location.
For actual insulation product coverage, use the insulation calculator. Insulation quantity and NFA planning are separate tasks.
