Calculator
Roof Vent Calculator
Most building codes require 1 sq ft of net free vent area for every 300 sq ft of attic — split 50/50 between intake and exhaust. Enter attic size for the balanced target.
Total NFA
720 sq in
Intake
360 sq in
Exhaust
360 sq in
NFA = Net Free Area. Check the label on each vent — decorative screens reduce actual NFA.
Need a full estimate?
For an installed price by material, state, and complexity, use the main Roof Cost Calculator.
Get full estimateFrequently asked
- What is the 1:300 ventilation rule?
- Codes generally require 1 sq ft of net free ventilation area for every 300 sq ft of attic floor when intake and exhaust are balanced. It becomes 1:150 without balanced venting.
- Should intake and exhaust be equal?
- Yes. Balanced intake at the soffit and exhaust at the ridge prevents pressure imbalances that pull conditioned air into the attic.
Related roofing calculators
Free tools that pair with this one — jump between them to plan every part of your project.