Ridge Vent Length Calculator
Ridge vent run to balance attic intake.
Enter your values
Enter the attic floor area in sq ft.
Choose one of: 1:300 (with vapour barrier), 1:150.
Enter the vent nfa per foot in sq in/ft.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- 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
At a glance
| What it does | Ridge vent run to balance attic intake. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Attic floor area, Vent ratio, Vent NFA per foot |
| Main output | Total NFA required |
| Formula | feet = required NFA (sq in) / NFA per foot |
| Cost | Free — no sign-up, no download |
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.
Inputs explained
- Attic floor area(sq ft)
- Enter the attic floor area in sq ft.
- Vent ratio
- Choose one of: 1:300 (with vapour barrier), 1:150.
- Vent NFA per foot(sq in/ft)
- Enter the vent nfa per foot in sq in/ft.
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.
What do I need to enter into the Ridge Vent Length Calculator?
Just 3 values: attic floor area, vent ratio, vent nfa per foot. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Ridge Vent Length Calculator work out the answer?
Ridge vent length is the required exhaust net free area divided by the vent's NFA per linear foot. It applies the formula feet = required NFA (sq in) / NFA per foot and shows the working so you can check each step by hand.
Is the Ridge Vent Length Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.