Attic Ventilation Calculator

What it calculates and who it’s for

What it estimates and who it is for

This calculator works out how much attic ventilation the code calls for, as net free area (NFA) in square inches, splits it between intake vents low on the roof and exhaust vents high on the roof, and turns each into feet of continuous vent or a number of individual vents using the NFA printed on the products you plan to buy. It’s for homeowners and roofers planning soffit and ridge vents during a re-roof or an insulation project. It’s a quantity estimate, not a diagnosis of moisture or mold problems.

How to use

Step by step

Enter the attic floor area in square feet (usually the ceiling area under the attic).
Pick the ratio: 1/150, or 1/300 only if your attic meets the exception’s conditions, or your local code’s ratio.
Set the exhaust share, the part of the vent area at the top of the roof.
Choose your intake and exhaust vents and enter the NFA from their labels.
Read the required area and the vent to buy.

The formula

Every variable and unit

Net free area required (sq ft) = Attic floor area ÷ Ratio (150, 300, or your local figure)
Net free area required (sq in) = sq ft × 144
Exhaust area (sq in) = Required × Exhaust share % ÷ 100; intake area = the rest
Feet of continuous vent, or number of vents = Area ÷ NFA per foot (or per vent), rounded up

Net free area is the open area air can actually pass through after screens and louvers, which is why it comes from the product label. The defaults are Air Vent’s published figures: 9 sq in per foot for its continuous soffit vent and 18 sq in per foot for ShingleVent II ridge vent.

Worked example

Real numbers, worked by hand

A 1,200 sq ft attic at 1/150, half the area at the ridge, continuous soffit vent at 9 sq in per foot, ridge vent at 18 sq in per foot:

Required: 1,200 ÷ 150 = 8 sq ft × 144 = 1,152 sq in
Split 50/50: 576 sq in intake and 576 sq in exhaust
Soffit vent: 576 ÷ 9 = 64 ft
Ridge vent: 576 ÷ 18 = 32 ft

If the attic qualifies for 1/300, everything halves: 576 sq in, 32 ft of soffit vent, and 16 ft of ridge vent.

How to read your results

Rounding and buying tips

Net free area required is the code minimum for the whole attic, intake plus exhaust.
Soffit vent and ridge vent (or vent counts) are rounded up. Ridge vent has to fit on the ridge you have; if it doesn’t, shift some exhaust to other upper vents or check with a roofer.
Area your vents provide shows what the rounded-up vents add up to.

Assumptions and limits

What this estimate does not cover

The attic is a vented attic. Unvented (conditioned) attics follow different rules and aren’t covered.
The 1/300 ratio is allowed only when the exception’s conditions are met: under the 2021 IRC as published on UpCodes, 40 to 50% of the required area must be in upper vents within 3 ft of the ridge, and in climate zones 6, 7, and 8 a Class I or II vapor retarder is needed on the warm-in-winter side of the ceiling. Local codes may word this differently; check yours.
Net free area must come from your product’s label. Screens, louvers, and paint can reduce it.
This doesn’t diagnose moisture, mold, ice dams, or airflow problems, and it doesn’t cover air sealing, baffles, or bath and kitchen fans. Confirm requirements with your local building department and the vent manufacturer.

FAQ

5 questions

How much attic ventilation do I need?

Under the 2021 IRC, 1 sq ft of net free area for every 150 sq ft of attic floor, or 1 per 300 when the exception’s conditions are met. A 1,200 sq ft attic needs 8 sq ft (1,152 sq in) at 1/150.

What is the 1/300 rule for attic ventilation?

It’s an exception that halves the required vent area. In the 2021 IRC it applies when 40 to 50% of the area is in upper vents near the ridge with the rest low on the roof, and, in climate zones 6 through 8, a Class I or II vapor retarder is on the warm side of the ceiling.

How much ridge vent do I need?

Divide the exhaust area in square inches by the ridge vent’s NFA per foot. With 576 sq in of exhaust and an 18 sq in per foot ridge vent, that’s 32 ft.

Should intake and exhaust be balanced?

The 1/300 exception requires 40 to 50% of the area in upper (exhaust) vents. For 1/150 the code doesn’t set a split; an even split is a simple starting point.

What does net free area mean?

The open area air can flow through, after screens and louvers are taken into account. It’s printed on vent packaging, per vent or per linear foot.

Vent for common attic sizes

Reference

All rows split the area 50/50, with continuous soffit vent at 9 sq in per foot and ridge vent at 18 sq in per foot.

Attic floor (sq ft)	1/150: NFA (sq in)	1/150: soffit (ft)	1/150: ridge (ft)	1/300: NFA (sq in)	1/300: soffit (ft)	1/300: ridge (ft)
800	768	43	22	384	22	11
1000	960	54	27	480	27	14
1200	1152	64	32	576	32	16
1500	1440	80	40	720	40	20
2000	1920	107	54	960	54	27
2500	2400	134	67	1200	67	34

Your results

Required area

Split