OneTripDIY

Attic Ventilation Calculator

Enter the attic floor area and this gives the ventilation the code asks for, split half low and half high, and turns it into a number of ridge feet and soffit vents.

By The OneTripDIY Team · Updated

Measure the attic floor

The area of ceiling below the attic. Split-level or an L-shape? Add a section for each part.

sq in per ft

Most shingle-over ridge vent gives 12–20 sq in per linear foot. Check the box.

sq in free area

A common 4 × 16 in rectangular vent is about 28 sq in of free area.

Codes allow 1/300 when the venting is split roughly half low and half high.

Ventilation needed — sq in of free area
Intake, at the soffits — sq in
Exhaust, at the ridge — sq in
Ridge vent — linear ft
Soffit vents — pieces
In square feet — sq ft

Estimates only. Check against your supplier's stated coverage before you buy.

Take this list to the store

We'll email the quantities above, what you typed in, and a link that reopens this calculator already filled in — so you can adjust it on your phone in the aisle.

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How to work it out

  1. Measure the attic floor

    The ceiling area below the attic, not the roof surface. An L-shaped house gets a section for each rectangle.

  2. Split it half and half

    Codes allow the smaller 1/300 ratio when intake and exhaust are roughly balanced, with the intake low at the eaves and the exhaust high at the ridge. Unbalanced venting falls back to 1/150 — twice as much.

  3. Read the net free area, not the size

    A vent's opening is mostly screen and louver. The number that counts is the net free area printed on the box, which for a 4 × 16 in soffit vent is about 28 sq in, not 64.

  4. Keep the path open

    Baffles at every rafter bay stop insulation blocking the intake. Without them, adding attic insulation can shut the ventilation down completely.

Ventilation by attic size

Under the 1/300 rule with balanced venting.

Attic floor areaTotal free areaIntakeExhaustRidge vent at 18 sq in/ft
1,000 sq ft480 sq in24024013 ft
1,200 sq ft576 sq in28828816 ft
1,500 sq ft720 sq in36036020 ft
2,000 sq ft960 sq in48048027 ft
2,500 sq ft1,200 sq in60060033 ft

Typical net free area

Always use the figure on the product; these are for planning.

VentNet free area
Shingle-over ridge vent12–20 sq in per linear ft
Soffit vent, 4 × 16 inabout 28 sq in
Continuous vented soffit panelabout 9 sq in per sq ft
Gable louver, 12 × 18 inabout 80 sq in
Roof louver (box vent), 12 × 12 inabout 50–60 sq in
Turbine, 12 inabout 110 sq in

The 1/300 rule

Codes ask for 1 square foot of net free ventilation for every 150 square feet of attic floor, reduced to 1 in 300 when the venting is balanced — at least 40% and no more than 50% of it high, and the rest low at the eaves.

A 1,500 sq ft attic therefore needs 720 square inches of free area: 360 at the soffits and 360 at the ridge.

Intake is the half people forget

Ridge vent is easy to add and looks like progress. Without matching intake, though, it pulls air from the easiest opening it can find — often a bathroom fan duct or the gaps around light fittings, which drags heated, humid house air into the attic.

If you can only fix one half, fix the intake.

Do not mix exhaust types

A ridge vent with gable louvers left open short-circuits: air enters the nearest gable and leaves at the ridge without ever crossing the attic. Pick one exhaust type. If you fit ridge vent, block the gable vents.

What good ventilation buys you

In winter it keeps the roof deck cold, which is what stops ice dams forming at the eaves. In summer it moves the heat that would otherwise bake the shingles from beneath and drive up the cooling bill.

It also carries out the moisture that leaks up from the house, which is what rots sheathing from the inside.

How this is worked out

No black box — here is the arithmetic the calculator runs, so you can check it against your own numbers.

  1. free area = attic area ÷ 300 × 144 The code ratio, converted from square feet to square inches.
  2. split half low, half high Balanced venting is what allows the 1/300 ratio instead of 1/150.
  3. ridge feet = exhaust ÷ free area per foot Ridge vent is rated per linear foot.
  4. soffit vents = intake ÷ free area per vent Each vent contributes its own rated area.

Common questions

How much attic ventilation do I need?

1 sq ft of net free area per 300 sq ft of attic floor when intake and exhaust are balanced. A 1,500 sq ft attic needs about 720 sq in — 360 low and 360 high.

What is net free area?

The actual open area of a vent after the screen and louvers are deducted. A 4 × 16 in soffit vent measures 64 sq in but gives only about 28.

How many feet of ridge vent do I need?

Divide the exhaust requirement by the vent's rating. At 18 sq in per foot, a 1,500 sq ft attic needs about 20 ft of ridge vent.

Can I have too much attic ventilation?

Too much exhaust without matching intake is the real problem, since it pulls air from the house. More intake than exhaust is harmless.

Should I close gable vents if I have a ridge vent?

Yes. Leaving them open lets the ridge vent draw from the gables instead of the soffits, so the far corners of the attic never get any airflow.

Does attic ventilation stop ice dams?

It helps, by keeping the deck cold. But the first fix is air sealing and insulation at the ceiling — ventilation cannot remove heat that should never have got up there.

Do I need ventilation with spray foam?

No. A sealed, insulated roof deck is a different assembly — the attic is inside the building envelope and is not vented at all.

Where these figures come from

Every figure below is traceable. How we check the numbers explains how they are verified, and the coverage reference collects them all in one place.

  • International Residential Code, attic ventilation — the 1/150 and 1/300 ratios and the balanced-venting condition
  • Asphalt Roofing Manufacturers Association technical bulletins — intake and exhaust balance, and why mixing exhaust types fails
  • Vent manufacturer product data — published net free area per vent and per linear foot

Think a number here is wrong? Tell us — we check every report, correct the page, and date the change. Last checked September 27, 2026.