Attic Ventilation Requirements Calculator

Based on the 1/150 or 1/300 Rule
(see Building Codes for more info)

(This calculator is for the exhaust half only. If an intake ventilator is needed, just double the amount of vents. For example, if 2 AV-14 are needed and inlet vents are needed, then an additional 2 PV-14 vents would be required)

To determine how many vents of a given diameter (inches)
are required to properly ventilate an attic space:

Enter the length of
the building in feet
Enter the width of
the building in feet
Choose the desired vent




vents required - 1/300 rule



vents required - 1/150 rule

Enter the total area of attic
in square feet



Choose the desired vent




vents required - 1/300 rule



vents required - 1/150 rule

OR

Application: One vent for exhaust per:

Vent
Diameter
Sq. Feet
of Floor
1/150
Sq. Feet
of Floor
1/300
Net Free
Vent Area
in Sq. Inches
Net Free
Vent Area
in Sq. Feet
4" Diam
26
52
12.5
.09
6" Diam
50
100
28
.19
8" Diam
100
200
50
.35
10" Diam
163
327
78.5
.55
12" Diam
225
450
113
.78
14" Diam
300
600
144
1.00
16" Diam
418
837
200
1.39
18" Diam
525
1,050
254
1.76
20" Diam
654
1,308
314
2.18
24" Diam
941
1,883
452
3.14
30" Diam
1,470
2,941
706
4.90
36" Diam
2,118
4,237
1,017
7.06
42" Diam
2,883
5,766
1,384
9.61
48" Diam
3,766
7,533
1,808
12.56
54" Diam
4,769
9,538
2,289
15.89
60" Diam
5,886
11,775
2,826
19.62
72" Diam
8,475
16,950
4,069
28.25

Excerpted from IBC-2003 & 2006 Section 1203.2 Attic Ventilation

The net free ventilating area shall not be loss than 1/150 of the area of the space ventilated, with 50 percent of the required ventilating area provided by ventilators located in the upper portion of the space to be ventilated at least 3 feet (914 mm) above eave or cornice vents with the balance of the required ventilation provided by eave or cornice vents.

Exception: The minimum required net free ventilating area sball be 1/300 of the area of the space ventilated, provided a vapor retarder having a transmission rate not exceeding 1 perm in accordance with ASTM E 96 is installed on the warm side of the attic insulation and provided 50 percent of the required ventilating area provided by ventilators located in the upper portion of the space to be ventilated at least 3 feet (914mm) above eave or cornice vents, with the balance of the required ventilation provided by eave or cornice vents.