Free tool
Attic ventilation calculator
How much net free area the attic needs, how much you already have, and how far short it is. No sign-up, no email, nothing stored.
The attic
A measured attic floor area always wins. Otherwise we derive it from the roof.
What is already up there
Count openings — nobody knows their net free area in square inches. Both types are derated 50% for the insect screen.
Total required
1,288 in²
2,683 sq ft of attic ÷ 300 × 144 — attic area derived from roof area at 6/12
- Intake required
- 644 in²
- Exhaust required
- 644 in²
What you have
- 20 bird blocks
- 31 in²
- 4 rectangular vents (64.0 in² each)
- 256 in²
- Existing intake
- 287 in²
The shortfall
- Intake short by
- 357 in²
- Exhaust short by
- 644 in²
A recommendation for a licensed contractor to confirm, not an engineering certification. Under-venting voids most shingle warranties; over-exhausting drives moisture into the house. Runs entirely in your browser — nothing you type is sent anywhere.
How attic ventilation is calculated
Required net free area = attic floor area ÷ 300 × 144, in square inches, split evenly between intake and exhaust. A 3,000 square foot attic needs 1,440 in² — 720 in² low at the eaves and 720 in² high at the ridge. That is the whole formula. What makes a figure trustworthy is the three things underneath it.
1. Attic floor area is not roof area
Squares × 100 is the sloped surface of the roof. The attic footprint is its horizontal projection, which is smaller — by about 12% at a 6/12 pitch and 25% at 9/12. Sizing ventilation from the sloped area over-vents every pitched roof. Divide roof area by √(144 + pitch²) ÷ 12 to get the footprint, or measure the attic directly and skip the correction.
2. Count the intake you have — do not look for a spec
Nobody standing in an attic knows their net free area in square inches, but anyone can count holes. A 2-inch round bird block is about 3.14 in² of gross opening; a rectangular soffit vent is width × height. Halve both — a screened louvre delivers roughly half its gross opening. Twenty bird blocks plus four 16 × 8 inch vents is about 287 in², which against a 720 in² requirement leaves you 433 in² short.
3. Exhaust must never exceed intake
This is the one that does damage. An attic with more exhaust than intake cannot pull enough air through the soffits, so it takes make-up air from the house below — through ceiling penetrations, light fittings and the loft hatch — carrying household humidity straight into the attic. The result is the condensation, mould and sheathing rot the ventilation was meant to prevent. When a roof is short on both sides, the intake is fixed first. Adding ridge vent to a roof with blocked soffits makes it worse, not better.
Why this matters commercially
Under-venting voids most shingle manufacturers’ warranties, so a roof sold without checking it carries a liability the contractor keeps. It is also the easiest upsell in the trade to justify, because the arithmetic is checkable by the homeowner in front of you — which is precisely why it should be arithmetic rather than a rule of thumb.
Questions we get asked
- How much attic ventilation do I need?
- The common balanced-ventilation requirement is 1 square foot of net free area for every 300 square feet of attic floor, split evenly between intake at the eaves and exhaust at the ridge. In square inches that is attic area ÷ 300 × 144. A 3,000 square foot attic needs 1,440 in² total — 720 in² of intake and 720 in² of exhaust. Where a vapour retarder or balanced venting cannot be shown, 1/150 applies instead.
- Is attic ventilation calculated from roof area or attic floor area?
- Attic FLOOR area, and confusing the two is the most common mistake. Squares × 100 gives the sloped roof surface, which is larger than the attic footprint beneath it — about 12% larger at a 6/12 pitch and 25% at 9/12. Sizing from the sloped area over-vents every pitched roof. The horizontal projection is roof area ÷ √(144 + pitch²) ÷ 12.
- How do I work out the net free area of vents I already have?
- Count the openings rather than looking for a specification. A 2-inch round bird-block hole is π × 1² ≈ 3.14 in² of gross opening, and a rectangular soffit vent is width × height. Then halve both: a louvre with an insect screen delivers roughly half its gross opening as net free area. Twenty bird blocks and four 16 × 8 inch vents come to about 287 in².
- Why must exhaust never exceed intake?
- An attic with more exhaust than intake cannot draw enough air through the soffits, so it pulls make-up air from the conditioned space below — through ceiling penetrations, light fittings and the loft hatch. That carries household humidity into the attic and causes exactly the condensation, mould and sheathing damage the ventilation was installed to prevent. If a roof is short on both, the intake gets fixed first.
- Can I mix ridge vents with box or turbine vents?
- Not on the same plane. A ridge vent alongside box or turbine vents short-circuits the airflow — the ridge pulls from the nearest opening rather than from the soffits, leaving most of the attic unventilated. Both GAF and Owens Corning advise against it. If a roof already has one exhaust style, extend that style rather than adding a second.
- Does this calculator store what I enter?
- No. It runs entirely in your browser — there is no network request and nothing is saved. You can put a real customer's roof into it without anything leaving your device.
This is built into ACE Platforms
The same calculation runs against the roof measurements already on the job, turns the shortfall into a count of the vents you actually stock, prices it as an optional line on the estimate, and produces a branded report for the homeowner — with the quantities and none of your costs on it.
See it against a real roof.
Bring a job you are quoting. We will run the measurements through it and you can check the ventilation figure yourself.