Calculators
Attic ventilation and insulation calculators
Net free area by the 1:150 and 1:300 rules, ridge and soffit vents, box vents, attic fans, insulation depth and baffles.
An attic needs to breathe and it needs a blanket, and the two are sized together. The building code asks for one square foot of vent opening for every 150 square feet of attic floor, or one for every 300 if the openings are balanced between the eaves and the ridge or a vapor retarder sits under the insulation. The opening that counts is net free area, the clear hole in a vent after its screen and louvres, which is always smaller than the vent itself.
Insulation is sized by the climate zone you live in and by what is already there. The calculators here turn the attic floor area into square inches of intake and exhaust, then into feet of ridge vent and numbers of soffit vents and box vents, and turn a target R-value into inches of material and bags to buy, with baffles to keep the soffit vents clear of it.
7 calculators
Attic Ventilation Calculator
Net free vent area by the 1:150 or 1:300 rule, split between soffit intake and ridge or roof exhaust.
Ridge Vent Calculator
Feet of ridge vent, pieces and nails for the exhaust an attic needs, with the shortfall if the ridge is too short.
Soffit Vent Calculator
Rectangular, round or strip soffit vents, or vented soffit panel, to deliver an attic's intake net free area.
Roof Box Vent Calculator
How many box (low-profile) roof vents an attic needs for its exhaust and how to space them below the ridge.
Attic Insulation Calculator
Inches and bags of blown fiberglass, cellulose or batts to reach the Energy Star R-value for your climate zone.
Attic Fan Sizing Calculator
Powered attic fan CFM from attic volume and air changes or the 0.7 CFM rule, with the intake the fan needs.
Rafter Baffles Calculator
Rafter vent baffles for every vented bay, and whether one reaches past the insulation at your depth and pitch.
Guides for this topic
Attic insulation R-values by climate zone, and how much to add
Most attics already have some insulation. Measure it, convert it to R-value, and the zone map tells you how much more to add.
Attic ventilation: the 1:150 rule and balancing intake and exhaust
An attic needs air moving from the eaves to the ridge. The code gives the amount; the balance between intake and exhaust decides whether it works.
Questions people ask
- How much ventilation does a 1,500 square foot attic need?
- Under the 1:150 rule, 1,500 ÷ 150 = 10 square feet of net free area, which is 1,440 square inches, split roughly half intake at the soffits and half exhaust at or near the ridge. If the vents are balanced that way, the code allows the 1:300 rule instead: 5 square feet, or 720 square inches.
- How many feet of ridge vent do I need?
- Divide the exhaust net free area by the vent's rating per foot, which is commonly 12 to 20 square inches per linear foot. For 720 square inches of exhaust and an 18 square inch vent, 720 ÷ 18 = 40 feet. If the ridge is shorter than that, the balance comes from box vents or a second ridge.
- How many soffit vents does a house need?
- Divide the intake net free area by the rating of each vent. A 16 by 8 inch aluminum undereave vent typically provides about 56 square inches, so 720 square inches of intake takes 13 of them; continuous soffit strip at about 9 square inches per foot takes 80 feet. Vented vinyl soffit panels often give far less than their size suggests, so check the rating.
- What R-value does my attic need?
- For an uninsulated attic, Energy Star recommends R-30 to R-49 in climate zone 1, R-49 to R-60 in zones 2 and 3, and R-60 in zones 4 to 8. At about 2.5 per inch for blown fiberglass, R-49 is 20 inches of material; at 3.5 per inch for cellulose it is 14 inches.
- How many bags of blown-in insulation do I need?
- Read the coverage chart printed on the bag: it gives the number of bags per 1,000 square feet for each R-value, because a bag's coverage depends on the depth. As a rough scale, a 1,000 square foot attic going from nothing to R-49 takes around 35 to 45 bags of blown fiberglass or 55 to 70 bags of cellulose. The attic insulation calculator uses your product's figures.
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