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Calculation Machine

Roof Ventilation Calculator

Net free area for intake and exhaust vents.

Reviewed by James Whitfield · How we check our maths

Your values

ⓘ Live results

Results
Total net free area
5 sq ft (720 sq in)
Intake (soffit) area
360 sq in
Exhaust (ridge) area
360 sq in
Soffit vents needed
14 at 26 sq in each
Ridge vent length
20 ft at 18 sq in per ft

Quick answer

Attics need 1 square foot of net free vent area per 150 square feet of attic floor, or per 300 with a vapour barrier and balanced venting.

The formula

NFA = attic area / ratio, split 50/50 intake and exhaust

Worked example

Worked example

Using the values the calculator loads with.

Inputs

  • Attic floor area1500 sq ft
  • Code rule1:300 (balanced with barrier)

Result

  • Total net free area5 sq ft (720 sq in)
  • Intake (soffit) area360 sq in
  • Exhaust (ridge) area360 sq in
  • Soffit vents needed14 at 26 sq in each
  • Ridge vent length20 ft at 18 sq in per ft

How to use the Roof Ventilation Calculator

  1. 1Enter the attic floor area in square feet.
  2. 2Choose 1:300 if you have a vapour barrier and balanced vents, otherwise 1:150.
  3. 3Split the area roughly evenly between low intake and high exhaust.

Frequently asked questions

Why split intake and exhaust?

Air must enter low and leave high; exhaust-only venting pulls conditioned air out of the house instead.

Does vent net free area equal vent size?

No — louvres and screens block much of the opening, so use the manufacturer's NFA rating.

About this calculator