Why an inventory is a different question from a requirement
The usual attic ventilation calculation runs forwards: take the attic floor area, divide by a ratio, split the result between top and bottom, and turn each half into feet of vent. That is the design question, and the attic ventilation calculator answers it. This page runs the same arithmetic backwards. You already have a roof with things on it, and what you want to know is what those things add up to, whether the two halves are anywhere near each other, and what ratio the house is actually at.
Running it backwards changes what you need to measure. Forwards, you need the attic area and a ratio. Backwards, you need a walk around the house with a notepad: how many feet of soffit are actually vented panel rather than solid, how long the ridge vent run is, how many gable louvers there are, and the net free area printed on the data sheet for each of them. The attic area becomes optional, useful only for expressing the total as a ratio at the end.
The number on the line is not the size of the hole
Net free area is what is left of an opening after the screen, the louver, the baffle and the filter have taken their share. A 16 by 8 inch soffit vent has 128 square inches of outline and a common published figure for one is around 26 square inches, a fifth of the outline. A ridge vent that looks like a continuous slot might publish 12 square inches per foot or 20, and there is no way to tell from the shelf which one you are holding.
So the fourth number on each line has to come from the manufacturer data sheet, not from a tape measure. If the vent is old and unidentifiable, you have a genuine gap in the inventory, and the honest thing to do is to say so rather than to guess. A guess that could be off by a factor of two makes the whole total meaningless in exactly the way that matters.
Reading the balance
The output names the smaller side and the difference. What that means depends on which side it is.
| Which side is smaller | What the air does |
|---|---|
| Intake | The exhaust makes up the difference from somewhere else. The nearest available opening is the ceiling plane, so the attic pulls house air and house humidity upward. |
| Exhaust | The surplus intake sits still. The attic runs slightly positive, and nothing gets pulled out of the house. |
| Neither, they match | The intended pattern: in at the eaves, out at the top, across the whole attic. |
This asymmetry is why the two errors are not equally bad, and why adding exhaust is the change most likely to make an attic worse. A homeowner with condensation on the sheathing adds a ridge vent, the intake does not change, and the attic now has a mechanism for drawing moist air out of the kitchen and bathrooms and depositing it on cold plywood. The intake side is the one to fix first, and it is also the cheaper one.
What the totals cannot tell you
Three things sit outside this arithmetic entirely. Blocked intake is the first: soffit vents with insulation packed against them from the inside deliver nothing, and the label still says nine square inches per foot. Look up into a bay from a soffit panel you have taken down, or from the attic at the eave, before believing any intake figure. The eave side of that is covered by the attic baffle and eave insulation calculator.
The second is air sealing. Ventilation removes moisture that has already reached the attic. Sealing the ceiling stops it arriving, and it is the higher-value job by a wide margin. The third is that this page says nothing about whether your attic assembly is meant to be vented at all — an unvented conditioned attic with insulation at the roof deck is a different building, and counting vents on one is the wrong question. If the ceiling insulation is being redone at the same time, the depth and bag count are in the attic insulation top-up calculator, and the intake run available at the eave comes off the soffit and fascia calculator.
Questions people ask
Where do I find the net free area of a vent that is already installed?
By identifying the product and looking up its data sheet, which is often possible for ridge vents and soffit panel because the profile is distinctive and the manufacturer is stamped somewhere. For an anonymous louvered gable vent or a box vent from thirty years ago there is no reliable answer, and the range across similar-looking products is wide enough that a guess does not give you a usable total. If a large part of your exhaust is unidentifiable, treat the audit as covering only the part you can document and say so, rather than filling in a number that makes the total look settled.
Does the second field have to be exactly "in" or "out"?
Those are the two it always accepts, but a handful of ordinary words map to them as well: intake, soffit, low and eave all count as intake, and exhaust, ridge, gable and high all count as exhaust. Anything else is ignored and the line is reported as skipped, which is deliberate — a line the page cannot classify should not quietly land on one side. Case does not matter.
My quantity is in feet for some lines and pieces for others. Is that a problem?
No, because the third and fourth numbers multiply together and nothing else looks at them. Thirty-six feet of ridge vent at 18 square inches per foot and four box vents at 26 square inches each are both just a quantity times a per-unit figure. What you must not do is mix them within one line: a line for continuous panel needs feet and a per-foot rating, and a line for individual units needs a count and a per-unit rating.
The implied ratio came out at 1 to 250. Is that good?
This page will not answer that, and the reason is not caution for its own sake. The ratio that applies to a given attic depends on the code your jurisdiction has adopted, on amendments to it, on whether conditions attached to a reduced ratio are met in your assembly, and sometimes on the climate zone. Those are decisions for the building department with authority over the work. What the number does tell you is where you sit relative to the figure you entered, and how far the two sides are from each other, which is arithmetic and does not depend on anyone adopting anything.
Should I count a powered attic fan as exhaust?
Not on this page, because a fan is not a net free area and cannot be added to one. A fan moves a fixed airflow regardless of what the passive openings are doing, and if the intake cannot feed it, it makes up the difference through the ceiling exactly the way an oversized passive exhaust does, only harder. If you have one, audit the passive openings here and treat the fan as a separate question about whether the intake can supply its rated airflow.