At-CostRoofing

Roof planning calculator

Attic ventilation ratio worksheet

Estimate net-free ventilation area, then verify product ratings and code locally.

Enter your planning inputs

Results update as you type. They stay in this browser and are not submitted to At-Cost.

Live worksheet
Ventilation area inputs

Measure the floor or ceiling area of the vented attic zone, not the sloped roof surface.

This form compares the two common ratios. It cannot decide which one applies to the building.

50% creates an even planning split between low intake and high exhaust.

Total net-free area

720 sq in

5 square feet of unobstructed opening

Intake target

360 sq in

Exhaust target

360 sq in

Use rated net-free area, not the vent's outside dimensions.

See the calculation

  1. 1. Apply the ratio: 1,500 ft² ÷ 300 = 5 ft² of total net-free area.
  2. 2. Convert to square inches: 5 × 144 = 720 in².
  3. 3. Split intake and exhaust: 720 × 50% = 360 in² intake; the remaining 360 in² is exhaust.
Convert the targets into product quantities

Enter the manufacturer's net-free-area rating for one vent, one linear foot, or another listed unit. Leave an input at zero if you do not have the product sheet.

Intake listed units

Add product NFA

Use the rating from the exact intake product.

Exhaust listed units

Add product NFA

Use the rating from the exact exhaust product.

This is a planning worksheet, not a measurement report, code decision, inspection, insurance interpretation, warranty, or construction quote.

The three-step formula

First divide the vented attic-zone area by the ratio denominator to get square feet of net-free opening. Multiply that answer by 144 to show square inches. Finally, multiply the square-inch total by the chosen intake percentage; exhaust receives the remainder. The live breakdown substitutes your entries into all three operations.

Why the ratio is a design input

The 2021 International Residential Code presents 1:150 as the base minimum and a 1:300 exception when its stated conditions are met. A jurisdiction may adopt a different edition or amendments, and an existing assembly may need additional review. The selector lets you inspect the arithmetic; it does not grant the exception.

From NFA to listed product units

Vent screens and louvers reduce the clear opening, so use the manufacturer's net-free-area rating for the exact product. Enter the rating for one listed unit on each side. The worksheet divides the intake or exhaust target by that rating and rounds upward so a fraction is not treated as a complete vent unit.

Where calculation stops and design begins

Matching totals do not prove a working air path. Separated attic spaces, blocked soffits, missing baffles, ceiling leakage, moisture sources, powered fans, other mechanical exhaust, short ridges, and existing vents can change the layout or make another assembly approach appropriate.

Record the priceable ventilation scope

Identify the attic zone, selected ratio and authority, intake and exhaust locations, product names, published ratings, quantities, blocking or baffle work, roof openings and patches, electrical or control work, air sealing, access, and closeout photographs. Recalculate if any product or zone changes.