How to use the attic ventilation calculator
- Enter the attic floor area in square feet, which is the area of the ceiling below the attic.
- Enter the ratio of attic floor area to each square foot of vent area. The 150 shown is an example only, because your local code, your roof maker and your attic design decide the real ratio.
- Set the share that is exhaust. The rest is intake.
- Open More options to enter the net free area printed on the vents you plan to use, then read the vents of that size you need.
Formula
total net free area in square feet = attic floor area / ratio
square inches = square feet x 144
exhaust area = total x exhaust share
intake area = total - exhaust area
vents = area / net free area of one vent, rounded up
Net free area is the open area of a vent after screens and louvers, which is smaller than the size of the opening.
A 1,200 square foot attic
At a ratio of 1 to 150 the attic needs 1,200 / 150 = 8 square feet, which is 1,152 square inches. Split 50 and 50, that is 576 square inches of exhaust and 576 of intake. With exhaust vents of 60 square inches that is 9.6, so 10 vents, and with intake vents of 9 square inches it is 64 vents.
For 1,850 square feet at a ratio of 1 to 300 the total is 6.17 square feet, or 888 square inches. A 60 and 40 split gives 532.8 square inches of exhaust and 355.2 of intake, which is 9 exhaust vents of 60 square inches and 40 intake vents of 9 square inches.
Net free area is not the size of the hole
A vent label lists the net free area, which is the open space air can actually pass through. Screens, louvers and baffles take up part of the opening. Always plan with the net free area printed on the product, not the dimensions of the vent.
Continuous soffit and ridge vents list net free area per linear foot. Multiply by their length to get the total, then enter that as the area of the vent type you use.
Why the ratio is your choice
Ventilation requirements differ between places and between roof designs, and some call for a different ratio when there is a vapor barrier or when vents are balanced between high and low. This page does not state a requirement. Look up the rule for your area or ask the roofer or building department, then enter that ratio.
Balanced intake and exhaust matters because exhaust that is larger than intake can pull air from the living space. A simple even split is only a starting point.
Related roof tools
See the rest of the roof with the Roofing Calculator, the Roof Pitch Calculator and the Rafter Length Calculator. The Insulation Calculator covers the attic floor.
Assumptions and limits
- The ratio is an input you choose. It is not a code requirement from this page.
- The attic floor area is the area of the ceiling below, with no adjustment for slopes.
- Exhaust and intake are shares of the same total net free area.
- Vent net free areas are examples until you enter the values printed on your products.
Frequently asked questions
How much attic ventilation do I need?
Divide the attic floor area by the ratio for your area and roof design. A 1,200 square foot attic at a ratio of 1 to 150 needs 8 square feet, or 1,152 square inches, of net free vent area.
What is net free vent area?
It is the open area that air can pass through after screens and louvers. It is smaller than the vent opening, and it is printed on the vent or its packaging.
Should intake and exhaust be equal?
Many designs aim for a balance, and the page starts with an even split. Your plan or building rules may ask for something different, so you can change the share.
Why does the page not tell me the ratio to use?
The right ratio depends on your local rules, your roof and your attic. The page asks you to enter it, so it never states a requirement that may not apply to you.