Attic Ventilation Calculator
Work out the net free area your attic needs under the 1:300 or 1:150 rule, split it between intake and exhaust, and convert it into soffit, ridge, box or gable vents.
Vent counts from your product NFA figures
Net free area is printed on the packaging or the manufacturer's data sheet — it is the open area left after the mesh and louvres, not the size of the hole. Replace the defaults below with your own product's figure.
| Vent type | Role | NFA each (sq in) | Needed on its own |
|---|---|---|---|
| Soffit / eave vents | Intake (low) | 7 vents | |
| Ridge vent | Exhaust (high) | 20.0 ft | |
| Box / roof louvre vents | Exhaust (high) | 8 vents | |
| Gable-end vents | Exhaust (high) | 5 vents |
Each exhaust row shows what that product would need on its own to cover the whole exhaust requirement. Pick one exhaust type — mixing ridge vent with gable or box vents lets the roof short-circuit and pull air back in through the higher opening.
Check what you actually plan to install
Required net free area = attic floor area × 144÷ 300 (or ÷ 150), split between low intake and high exhaust. The 1:300 ratio is the usual code allowance where a vapour retarder is fitted on the warm side or where the venting is balanced between eave and ridge; 1:150 is the fallback. This is a sizing estimate, not a code ruling — your local building department, and the vent manufacturer's own instructions, take precedence. Never block the soffits with insulation: fit baffles so the air path stays open.
What is the Attic Ventilation Calculator?
The ByteTools Attic Ventilation Calculator turns your attic floor area into a net free area requirement using either the 1:300 rule, which applies where there is a vapour retarder or balanced venting, or the 1:150 fallback.
- Required net free area from the 1:300 or 1:150 rule, in square inches or square centimetres
- Intake and exhaust split with an adjustable intake share
- Per-product NFA fields so counts match the vents you are actually buying
- Ridge vent reported in linear feet or metres, other types as counts
- Balance check that warns when intake falls short of exhaust
- All calculations run locally in your browser — nothing is uploaded
How to use the Attic Ventilation Calculator
- 1
Choose square feet or square metres and enter the attic floor area — the ceiling area below, not the sloped roof.
- 2
Pick the 1:300 or 1:150 ventilation ratio and set the intake share of the total, normally 50%.
- 3
Replace the default net free area figures in the table with the numbers printed on the vents you intend to buy.
- 4
Read how many of each vent type would cover the requirement on its own, and pick a single exhaust type.
- 5
Enter the soffit, ridge, box and gable vents you actually plan to install to check the totals and the intake-to-exhaust balance.
About the Attic Ventilation Calculator
The ByteTools Attic Ventilation Calculator turns your attic floor area into a net free area requirement using either the 1:300 rule, which applies where there is a vapour retarder or balanced venting, or the 1:150 fallback. It splits the total between low intake and high exhaust — 50/50 by default, adjustable where a manufacturer specifies otherwise.
Net free area is then converted into real products. Enter the NFA printed on your soffit vents, ridge vent, box vents or gable vents and the calculator shows how many of each you would need on its own, so you can pick one exhaust type rather than mixing them. A second panel checks what you actually plan to install and flags an intake shortfall.
Every calculation happens in your browser and nothing is uploaded. This is a sizing estimate, not a code ruling: your local building department and the vent manufacturer's own instructions take precedence, and soffits blocked by insulation vent nothing at all.
Frequently asked questions
How much attic ventilation do I need?
Under the 1:300 rule you need one square foot of net free area for every 300 square feet of attic floor, which is attic area × 144 ÷ 300 in square inches. A 1,500 square foot attic therefore needs 720 square inches, split 360 for intake and 360 for exhaust.
What is the difference between the 1:150 and 1:300 rule?
1:150 is the base requirement. It relaxes to 1:300 where a vapour retarder is installed on the warm side of the ceiling, or where the venting is balanced with at least 40% and no more than 50% of it in the upper portion of the attic. Local code decides which applies.
What is net free area on a vent?
Net free area is the actual open area left once the louvres, screens and insect mesh are subtracted — always less than the size of the hole you cut. It is printed on the packaging or the data sheet, and it is what you should enter here rather than the vent's physical dimensions.
Can I mix ridge vents with gable vents?
It is generally discouraged. Air takes the shortest path, so a ridge vent and an open gable vent can short-circuit each other, with the gable feeding the ridge instead of drawing fresh air up from the soffits. Pick one exhaust type and make sure the intake is generous.
What happens if intake is less than exhaust?
The exhaust vents pull their make-up air from wherever they can — often back down through the ridge itself or up through leaks in the ceiling below, dragging conditioned air and moisture into the attic. Under-venting is bad; exhaust-heavy venting is usually worse.
How many soffit vents do I need?
Divide your intake requirement by the net free area of one vent. With 360 square inches of intake needed and typical 56 square inch soffit vents, that is seven vents — but check that insulation baffles keep the path from the soffit into the attic actually open.
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