Ridge Vent Length Calculator
Ridge vent run to balance attic intake.
Enter your values
ⓘ Live results
Results
- Total NFA required
- 768 sq in
- Ridge (exhaust) share
- 384 sq in
- Ridge vent length
- 21.3 ft
- Soffit intake share
- 384 sq in
- Balance rule
- 50% high, 50% low
Quick answer
Ridge vent length is the required exhaust net free area divided by the vent's NFA per linear foot.
feet = required NFA (sq in) / NFA per foot
How to use the Ridge Vent Length Calculator
- 1Measure the attic floor area under the roof.
- 2Use 1:300 when a ceiling vapour retarder is present, otherwise 1:150.
- 3Split the ventilation half at the ridge and half at the soffits.
Worked example
Using the values the calculator loads with:
Inputs
- Attic floor area1600 sq ft
- Vent ratio1:300 (with vapour barrier)
- Vent NFA per foot18 sq in/ft
Results
- Total NFA required768 sq in
- Ridge (exhaust) share384 sq in
- Ridge vent length21.3 ft
- Soffit intake share384 sq in
- Balance rule50% high, 50% low
Frequently asked questions
What if the ridge is short?
Add roof or gable vents to make up the exhaust area, but never mix ridge and powered vents on the same attic.
Too much exhaust?
Yes — exhaust exceeding intake pulls conditioned air up through the ceiling.
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
- Report an error
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.