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# Attic Ventilation Formula

## 1 square foot (or 144 square inches) of net free area (NFA) ventilation for every 150 square feet of attic space.

### Attic Ventilation Using The 1 in 150 Rule

#### Exhaust Net Free Area

500 square feet of attic space
240 square inches
240 square inches
600 square feet of attic space
288 square inches
288 square inches
700 square feet of attic space
336 square inches
336 square inches
800 square feet of attic space
384 square inches
384 square inches
900 square feet of attic space
432 square inches
432 square inches
1000 square feet of attic space
480 square inches
480 square inches
1100 square feet of attic space
528 square inches
528 square inches
1200 square feet of attic space
576 square inches
576 square inches
1300 square feet of attic space
624 square inches
624 square inches
1400 square feet of attic space
672 square inches
672 square inches
1500 square feet of attic space
720 square inches
720 square inches
1600 square feet of attic space
768 square inches
768 square inches
1700 square feet of attic space
816 square inches
816 square inches
1800 square feet of attic space
864 square inches
864 square inches
1900 square feet of attic space
912 square inches
912 square inches
2000 square feet of attic space
960 square inches
960 square inches

Example of how to calculate net free area:
1200 square foot attic divided by 150 square feet = 8 square feet (1200 / 150 = 8)
8 square feet times 144 square inches = 1152 square inches (8 x 144 = 1152)
1152 square inches divided by 2 = 576 square inches at soffit and at exhaust (1152 / 2 = 576)