Roof measurement
Attic ventilation and ridge vent calculator
Estimate attic net free ventilation area and ridge vent quantity. A vented attic uses a selected locally applicable ratio and specified ridge-vent product.
Try the calculation
Calculate a selected ventilation-area target and a product-specific ridge-vent takeoff. This does not determine local code compliance.
Adjustable example. Change these illustrative inputs to match your task, then recalculate. Reset restores the example and its result.
Understand the result
How this calculation works
Attic ventilation planning converts attic area and selected vent assumptions into NFVA targets.
Steps
- Enter attic area, ratio, exhaust share, ridge vent NFVA and roll length.
- Use the selected ratio and share as assumptions for the scenario.
The form starts with the worked example below. Change its values and selections to match your task; Reset restores the example.
Formula
total NFVA = area ÷ ratio × 144; exhaust = total NFVA × exhaust share percent ÷ 100; ridge LF = ceiling(exhaust ÷ NFVA per LF); rolls = ceiling(ridge LF ÷ roll length).
Variables and units
- Attic floor area (sq ft)
- Required. Use a positive value. Starting example: 1200. Replace it for your task.
- Ventilation ratio (selected option)
- Options: 1 sq ft per 150 sq ft attic area; 1 sq ft per 300 sq ft (conditional exception). Initial selection: 1 sq ft per 300 sq ft (conditional exception). Use 1:300 only when the adopted code and its exception conditions apply.
- Upper/exhaust share (selected option)
- Options: 40%; 45%; 50%. Initial selection: 50%. For the 1:300 exception, the 2021 IRC specifies 40–50% in the upper portion. This is a selected planning split for 1:150.
- Ridge vent NFVA (sq in/LF)
- Required. Use a positive value. Starting example: 12.5. Replace it for your task. GAF Cobra RidgeRunner example; use the selected product rating.
- Vent roll length (ft)
- Required. Use a positive value. Starting example: 30. Replace it for your task. GAF Cobra RidgeRunner example; use the selected package length.
What the result means
It is a target arithmetic check, not a complete intake/exhaust layout or code approval.
Worked example
Inputs: Attic floor area = 1200; Ventilation ratio = 1 sq ft per 300 sq ft (conditional exception); Upper/exhaust share = 50%; Ridge vent NFVA = 12.5; Vent roll length = 30.
Calculation: 1,200 sq ft at 1:300 gives 576 sq in total NFVA; at 50% exhaust and 12.5 sq in/LF, 23.04 LF rounds up to 24 LF and one 30 ft roll.
Result: 24 linear ft ridge vent.
Compare the 1:150 planning ratio
Inputs: Attic floor area = 1200; Ventilation ratio = 1 sq ft per 150 sq ft attic area; Upper/exhaust share = 50%; Ridge vent NFVA = 12.5; Vent roll length = 30.
Calculation: At 1,200 sq ft, 1:150 yields 1,152 sq in total NFVA. A 50% exhaust target is 576 sq in. At 12.5 sq in/LF, 46.08 LF rounds to 47 LF; 30 ft rolls round to two.
Result: 47 linear ft ridge vent.
Frequently asked questions
Does it verify balanced intake?
No, it reports the selected split only.
Why is 1:300 conditional?
The source condition is retained as a selected planning band, not a universal rule.
Does roll length prove installation coverage?
No, placement and product instructions remain separate.