Attic Ventilation Calculator
Size intake and exhaust vents to the 1:150 or 1:300 code ratio.
What you'll need to order
- ridge vent
- soffit vents
- baffles
- gable vents
Prices vary by region and haul distance. Get quotes from at least two local suppliers.
How this calculator works
NFA (sq in) = attic area ÷ ratio × 144
Net free area is the actual open area of a vent, not its overall size — louvres and insect screen cut it by half or more. Manufacturers publish NFA per unit or per linear foot; always use that figure, never the vent dimensions.
Switch the site to metric — the control is in the footer — and your inputs, your results and the tables below follow. The formula, the constants, the guidance text and the sources stay in US units on purpose: that is how the standards they come from are written, and restating them would detach each figure from the document you are meant to be able to check it against.
Where these numbers come from
- ICC International Residential Code 2021, R806.2 — minimum net free ventilating area — 1/150 of the vented space, or 1/300 where a vapour retarder and balanced upper venting are provided
Standards are revised on a cycle. The edition above is the one this figure was checked against — confirm it against the edition your jurisdiction has adopted.
Worked example
With attic floor area 1500 sq ft, ventilation ratio (1 in ...) 150 , this calculator returns 1440 .
Net free area needed for common attic sizes
Click through any row to load those measurements. The address bar keeps them, which makes the result something you can paste into a message.
| Job | Size | Net free area required |
|---|---|---|
| Small ranch attic | 900 sq ft × 150 | 864.0 |
| Typical two-car-garage house | 1500 sq ft × 150 | 1,440 |
| Same attic, balanced venting with a vapour retarder | 1500 sq ft × 300 | 720.0 |
| Large two-storey attic | 2400 sq ft × 150 | 2,304 |
| Detached garage | 600 sq ft × 150 | 576.0 |
The 1/300 ratio is an exception, not the default — it needs a vapour retarder and 40–50% of the area up high.
Frequently asked questions
How much attic ventilation do I need?
One square foot of net free area per 150 sq ft of attic floor, split evenly between intake and exhaust. A 1,500 sq ft attic needs 1,440 sq in of NFA — 720 in the soffits and 720 at the ridge.
When can I use the 1:300 ratio?
Only when a vapour barrier is installed on the warm side of the ceiling, or when venting is balanced with 40 to 50% of the NFA in the upper half. Otherwise code requires 1:150.
What is net free area?
The genuine open area a vent provides once louvres and insect screening are accounted for. A 16 by 8 inch soffit vent measures 128 sq in but typically delivers only 56 sq in of NFA.
Why does intake need to match exhaust?
Ridge vents pull air out and must draw replacement air from the soffits. If intake is undersized, the ridge vent pulls conditioned air from the house through ceiling gaps instead, which raises energy bills and drives moisture into the attic.