Vented soffit and ventilating soffit are not the same thing
Two aluminium soffit panels can look identical from the ground and differ by a factor of three in how much air they pass. Air Vent publishes perforated aluminium soffit at 14 square inches of net free area per square foot, and lanced aluminium at 4 to 7. Perforated panels are punched with round holes; lanced panels are slit and pushed open, and the slits close up far more than they appear to.
This calculator uses the low end of that published range, 4, for the same reason the thinset pages on this site count from the low end of a coverage range: the conservative figure is the one that does not leave you short. But the honest answer is that a 4-to-7 range is nearly two to one, so if you are relying on lanced soffit to carry your intake, find the number for the panel you are actually buying.
It is the most common reason an attic with apparently continuous vented soffit still runs hot. The second most common is imbalance, which the page also flags — see below.
1/150 is the rule; 1/300 is a conditional exception
Most ventilation advice leads with the 1/300 ratio, which has it backwards. IRC R806.2 states plainly that "the minimum net free ventilating area shall be 1/150 of the area of the vented space." The 1/300 figure is an exception with two conditions attached, and both must be met: a Class I or II vapour retarder on the warm-in-winter side of the ceiling in Climate Zones 6, 7 and 8, and between 40 and 50 percent of the required vent area located in the upper portion of the attic, within 3 feet of the ridge.
Note the second condition is a band with a ceiling, not just a floor. You cannot qualify by putting 60 percent of your vent area at the ridge — more exhaust is not better here. That is the code's way of saying the same thing the manufacturers say about balance, and it is why an even intake/exhaust split is the most exhaust the exception allows.
The other place this arithmetic goes wrong is units. The code states a ratio of areas in square feet; every vent product on the market is published in square inches. The factor of 144 between them is a step this calculator does in one place and shows in the working, because dropping it is an error of two orders of magnitude that still produces a plausible-looking number.
What we found auditing the manufacturer's own tables
Air Vent's Principles of Attic Ventilation carries two appendix tables: attic area against minimum ridge length, and ridge length against the intake needed to balance it. Both are reproduced widely. We recomputed every cell.
Appendix A reconciles exactly — every row is the attic area divided by the ratio, converted to square inches, halved for the intake/exhaust split, and divided by the 18 square inches per foot their ridge vents deliver — with one exception. The row printed as 2,600 square feet does not reconcile at either ratio. The value that does, exactly, is 2,700, which also restores the even 300 square foot step the rest of the table follows. It is a typo, and our test asserts both halves of that finding so the claim cannot rot.
Appendix B is more interesting. Its vent counts are rounded to nearest, not rounded up — we checked all 27 cells, and round-to-nearest matches every one while rounding up differs on ten. The consequence is that ten of those 27 published combinations give you slightly less intake net free area than the ridge is exhausting, up to 6.7 percent short in the worst case. That quietly contradicts the booklet's own rule a few pages earlier, that intake "should be equal to or greater than" exhaust. This calculator rounds vent counts up, always, and a test asserts our counts never land below the published figures.
The 7/16 rule hiding in the vent table
Air Vent lists three undereave vents: 16×8 inch at 56 square inches of net free area, 16×6 at 42, and 16×4 at 28. Divide each by its gross opening and you get 0.4375 every time — exactly 7/16. The whole published range sits on one line, so the louvres and screen cost a consistent nine sixteenths of the hole.
That is a property of these particular published figures rather than a law of vents, and a different manufacturer's louvre will have a different ratio. But it is a useful sanity check on any vent you are quoted: if someone claims net free area much above 45 percent of the gross opening for a screened louvre, ask how it was measured. The relationship is exact enough that a test pins it, the same way this site pins Quikrete's 0.0075 cubic feet per pound.
One more relationship worth knowing when you are balancing a system: continuous ridge vent runs 18 square inches per linear foot and continuous soffit strip runs 9 — exactly half. So a balanced install needs two feet of soffit strip for every foot of ridge, which is where Air Vent's own balancing table comes from. Once the soffit is sized, the roofing shingles calculator handles the field above it, and the attic insulation calculator covers the floor below — just keep the insulation out of the soffit path, because a blocked vent has a net free area of zero no matter what the panel is rated at.