Roof Ventilation Calculator

How the ventilation math works

The calculation has two steps. First comes the code number: IRC section R806.2 (2021 text on UpCodes) requires net free vent area equal to 1/150 of the vented attic, measured at the floor (ceiling) level, not along the roof. Then you divide that area by the rating of each vent.

• Total NFA (sq ft) = attic floor area / 150 (or / 300)
• Total NFA (sq in) = sq ft x 144
• Exhaust = total x exhaust share. Intake = total minus exhaust.
• Vents needed = target sq in / NFA per vent, rounded up. For ridge vent or soffit strip, it's target / NFA per foot, which gives linear feet.
• Power fans skip the ratio: CFM = attic sq ft x 0.7, and intake sq in = fan CFM / 300 x 144.

The worked example is what the tool shows when it loads. A 1,500 sq ft attic at 1/150 needs 1,500 / 150 = 10 sq ft of NFA, and times 144 that's 1,440 sq in. Split 50/50, that's 720 sq in up top and 720 down low. A ridge vent rated 18 sq in a foot needs 720 / 18 = 40 feet of ridge, and a continuous soffit strip at 9 sq in a foot needs 720 / 9 = 80 feet. The house has 50 feet of ridge, so ridge vent alone works.

Start at 1/150 unless you meet both 1/300 conditions

Use 1/150 unless both conditions in the current code are met. First, 40 to 50 percent of the vent area must sit in the upper attic, no more than 3 feet below the ridge (measured straight up and down), with the rest in the bottom third. Second, in climate zones 6, 7 and 8, there has to be a Class I or II vapor retarder on the warm-in-winter side of the ceiling.

You need both, not either. That's the 2021 wording, which is what most places enforce right now. Plenty of roofers use 1/150 no matter what, because it's cheap insurance.

Net free area by vent type

Net free area is the open hole left after you subtract the louvers, screens and baffles, and it is always smaller than the vent looks. These are the tool's defaults, and you can type over any of them with the number from your vent's box:

The ridge figure is in ICC-ES report ESR-2071. The turbine, undereave and edge vent numbers come from an Air Vent University lesson plan that walks through this same shortcut (attic sq ft divided by 2 gives the sq in you need on each side, at 1/150). Lomanco's box vent number comes from the 750-G spec sheet.

Never mix exhaust types over one attic

This mistake shows up on a lot of houses. Someone adds a ridge vent and leaves the old box vents in place, assuming more is better. It isn't. Air takes the easiest path, so the ridge vent pulls air in through a box vent three feet away and ignores the soffits.

Air Vent's own help center is blunt about it: never mix two exhaust types over a common attic. The same goes for gable louvers and a power fan near a ridge vent.

Hip roofs often have too little ridge for a ridge vent

Hip roofs are the usual problem. They have four slopes and a short ridge, maybe 15 or 20 feet of it, while the math may call for 40. The tool warns you when the feet of ridge vent needed is more than the ridge you entered.

Your options are a higher-rated ridge vent or box vents across the whole attic instead. Ridge vent also usually stops a few inches short of each end, so a ridge that only barely fits is already tight.

Power fans are sized by CFM, not the ratio

Fans don't follow the ratio. Air Vent's rule of thumb is attic sq ft times 0.7 for the CFM, and its explanation says this works out to roughly 10 to 12 air changes an hour.

Intake is sized from the fan: CFM divided by 300 gives sq ft of intake (multiply by 144 for sq in), which keeps the incoming air slow enough that it won't drag dust in. With too little intake, a fan will pull conditioned air out of your house through the light fixtures.

Intake matters as much as exhaust

A ridge vent with painted-shut soffits does almost nothing. Exhaust is the part you can see from the street, so that's where the money tends to go. Check that insulation isn't packed into the eaves (baffles fix that).

Our soffit guide covers the panel types, and the roof ventilation guide has prices for each vent type. In snow country, a cold, well-vented deck is also a big part of ice dam prevention.

If you're replacing the roof anyway, that's the cheap time to fix ventilation, since the deck is open. The roof cost calculator prices the whole job, ridge vent included.