Ridge Vent Calculator
Check whether your ridge length provides enough exhaust ventilation.
What you'll need to order
- shingle-over ridge vent
- ridge cap shingles
- soffit vents
- baffles
Prices vary by region and haul distance. Get quotes from at least two local suppliers.
How this calculator works
NFA provided = ridge length × NFA per linear foot
Compare the provided figure against the required one. If the ridge is too short to supply enough exhaust, add gable or box vents rather than reducing soffit intake.
Switch the site to metric — the control is in the footer — and your inputs, your results and the tables below follow. The formula, the constants, the guidance text and the sources stay in US units on purpose: that is how the standards they come from are written, and restating them would detach each figure from the document you are meant to be able to check it against.
Where these numbers come from
- ICC International Residential Code 2021, R806.2 — the net free ventilating area a ridge vent counts toward, and the upper-portion balance requirement
Standards are revised on a cycle. The edition above is the one this figure was checked against — confirm it against the edition your jurisdiction has adopted.
Worked example
With ridge length 40 ft, attic floor area 1500 sq ft, vent nfa per linear foot 18 , this calculator returns 720 .
Ridge vent exhaust for roofs of common size
Click through any row to load those measurements. The address bar keeps them, which makes the result something you can paste into a message.
| Job | Size | Exhaust NFA provided |
|---|---|---|
| Small ranch | 30 ft × 900 sq ft × 18 | 540.0 |
| Typical house | 40 ft × 1500 sq ft × 18 | 720.0 |
| Long ridge | 60 ft × 2000 sq ft × 18 | 1,080 |
| Short ridge, large attic This is the case that comes up short |
20 ft × 1800 sq ft × 18 | 360.0 |
| High-capacity vent | 40 ft × 1500 sq ft × 20 | 800.0 |
If the ridge cannot supply enough exhaust, add gable or box vents. Never fix it by cutting soffit intake.
Frequently asked questions
How much ventilation does a ridge vent provide?
Typically 12 to 18 square inches of net free area per linear foot, depending on the product. A 40 ft ridge with an 18 NFA vent delivers 720 sq in of exhaust.
Is my ridge long enough to vent the attic?
Compare provided against required. Exhaust must supply half the total NFA, which at the 1:150 ratio is attic area divided by 150, times 144, divided by 2. A short ridge on a large attic will fall short.
Can I mix ridge vents with gable vents?
It is generally discouraged. Gable vents short-circuit the airflow, letting the ridge vent draw from the gable instead of the soffits, so the lower attic never ventilates. Pick one exhaust strategy.
Does a ridge vent leak in wind-driven rain?
Quality shingle-over vents with an external baffle and internal weather filter perform well. The cheap roll-type without a baffle is where wind-driven rain and snow infiltration problems occur.