The ridge on the tape is not the run that vents
Three things take length out of a ridge before any vent goes on it. The slot stops short of each end, because the last stretch is where the cap has to land on solid deck and where the vent needs something to close against. Anything crossing the ridge — a chimney, a dormer, a change in ridge height — breaks one run into two, and each new run brings two more ends with it. And on a hip roof, a large part of what people call the ridge is hip, which may or may not be getting vent at all.
With a single run on a forty foot ridge and a foot held back at each end, you lose two feet and vent thirty-eight. Split that ridge into three runs by a chimney and a dormer and you lose six feet instead of two, and the exhaust falls by ten percent without the ridge getting any shorter. That is why the run count is a field here rather than an assumption.
The slot and the vent are two different openings
The slot is what comes out of the sheathing: a strip on each side of the ridge board, with the board itself left uncut between them. At one inch per side that is 24 square inches of hole per linear foot, and the vent sitting over it might publish 18 square inches per foot of net free area. The vent is the restriction, and that is the intended arrangement — the slot has to be at least as open as the vent or the published figure is fiction.
Both dimensions come from the installation instructions for the specific product, and this page does not supply either. What it does is put them side by side, so that if you have entered a vent rated at more per foot than the slot you are cutting, you can see that the two numbers cannot both be right.
Sticks and rolls count differently
Rigid vent comes in sticks, typically four feet, and they join with connectors. Thirty-eight feet takes ten sticks, and ten sticks in one run have nine joints between them, so nine connectors — one fewer than the stick count for each unbroken run. Split the same length into two runs and the connector count drops by one, because one of the joints becomes a pair of ends instead.
Rolled vent has no connectors at all. It comes off a roll, gets cut where you want it to stop, and its fastening is usually specified per foot rather than per piece. Two twenty foot rolls cover the same thirty-eight feet, with no joints to count and a different nail figure. The two products are not interchangeable in an estimate even though they do the same job on the roof.
End plugs are two per run regardless of which you use, and they are the piece most often left off an order because the ridge looks like one continuous thing from the ground.
What the run is worth, and what it is not
Multiply the vented run by the net free area per foot and you have the exhaust this ridge contributes. That figure is only half a ventilation design: it means nothing until it sits next to whatever intake exists at the eaves, which is what the attic vent net free area audit is for. A long ridge vent over blocked soffits is not ventilation, it is a mechanism for pulling house air through the ceiling, and it is a common outcome because ridge vents are easy to add and soffit intake is not.
If you are sizing rather than counting, the forward calculation from attic area to required area is on the attic ventilation calculator. The intake run available at the eave comes off the soffit and fascia calculator, and whether that intake is actually open where it meets the insulation is the subject of the attic baffle and eave insulation calculator. The cap shingles that go over the vent are counted by the foot on the roofing squares calculator, and they run the full ridge and hip whether or not there is vent underneath.
Questions people ask
How far back from the end should the slot stop?
The installation instructions for the vent say, and they are the only source worth using because the figure differs between products and because it interacts with how the end closure works. What is consistent is that there is always a cutback and it is always at both ends of every run, which is why the calculation multiplies by two per run rather than two per roof. If you are estimating before you have chosen a product, run the numbers at a couple of values and see how much it moves the exhaust figure.
Can I vent the hips as well as the ridge?
Some products are made for it and some are not, and there are roofs where it is the only way to get meaningful exhaust because the horizontal ridge is short. Whether it is permitted and how the detail is built at the top where hips meet the ridge is a product question. The reason the field exists is that on many hip roofs the hips are a larger length than the ridge, so including or excluding them changes the answer by more than any other input on the page.
Why does the connector count come out one less than the stick count?
Because connectors sit between sticks, not at the ends of a run. Ten sticks laid end to end have nine gaps between them. When there are several runs, each run has one fewer joint than it has sticks, so the total is the stick count minus the number of runs. The ends of each run get a plug or closure instead, which is a different part, and there are two of those per run.
Does the ridge board thickness change the ventilation?
Not directly, because the air goes through the slots either side of it, not through the board. What it changes is the total width you are cutting out of the deck, which matters when you are marking the roof and when you are checking that the vent you bought covers the whole cut. A ridge beam rather than a ridge board is a different situation entirely: it is wider, it is structural in a way a ridge board is not, and the venting detail over one is a design question rather than a cut width.
Is a longer ridge vent always better?
Only up to the point where the intake can feed it, and after that it is worse rather than neutral. Exhaust in excess of intake makes up the difference through whatever opening is nearest, and on most houses that is the ceiling plane below, so the attic starts drawing warm humid air out of the house and depositing it on cold sheathing. The order of operations that helps is intake first, exhaust second — which is the opposite of the order in which the two are easy to install.