Ridge Vent Calculator
Reviewed by the My Roof Pitch editorial team · Updated · Free, no sign-up, works offline after first load
A balanced attic needs 1 square foot of net free vent area for every 150 square feet of attic floor. Half of that should be low intake and half high exhaust. A 2,000 sq ft attic needs 13.3 sq ft NFVA total, so 6.7 sq ft of ridge vent and 6.7 sq ft of soffit vent.
Your measurements
Enter the values you know — results update live.
The plan area of the attic floor or ceiling below.
1:150 is standard; 1:300 is allowed when half is high and half is low.
Typical ridge vents provide 9 to 18 sq in of NFVA per linear foot.
Analysis
Live results from your measurements.
Show the working
- Total NFVA:
2,000 ÷ 150= 13.3 sq ft - Ridge NFVA:
13.3 ÷ 2= 6.7 sq ft - Intake NFVA:
13.3 ÷ 2= 6.7 sq ft - Ridge length:
(6.7 × 144) ÷ 12= 80 ft
What this calculator does
Attic ventilation works by letting cooler air enter low at the soffit and warmer, moist air exit high at the ridge. The total opening is measured in net free vent area, or NFVA — the actual open area after subtracting louvers, screens and insect mesh. The IRC requires 1 square foot of NFVA for every 150 square feet of attic floor, or 1:300 when the ventilation is balanced between intake and exhaust and a vapor retarder is present. Ridge vent is the most common exhaust path because it is hidden at the peak and works with wind from any direction.
How to use it
- Measure the attic floor area.
- Choose the ventilation ratio: 1:150 standard or 1:300 for balanced systems with a vapor retarder.
- Divide the attic area by the ratio to get total NFVA.
- Split the total in half: half for ridge exhaust, half for soffit intake.
- Divide the ridge NFVA by the vent's rated NFVA per linear foot to get ridge vent length.
The formula
Total NFVA
total NFVA = attic area / ventilation ratio
The IRC sets the ratio at 1:150 or 1:300 depending on conditions.
Ridge vent length
ridge length = (ridge NFVA × 144) / NFVA per linear foot
Converts square feet to square inches, then divides by the vent rating.
Worked example
A 1,800 sq ft attic with a ridge vent rated at 12 sq in NFVA per linear foot, using the 1:150 ratio.
- Total NFVA: 1,800 ÷ 150 = 12 sq ft
- Ridge NFVA: 12 ÷ 2 = 6 sq ft
- Intake NFVA: 6 sq ft
- Ridge length: (6 × 144) ÷ 12 = 72 ft
Install 72 feet of ridge vent and match it with 6 sq ft of soffit intake vent area.
Reference chart
| Attic area | Total NFVA | Ridge NFVA | Ridge length |
|---|---|---|---|
| 1500 | 10 | 5 | 60 |
| 2000 | 13.3 | 6.7 | 80 |
| 2500 | 16.7 | 8.3 | 100 |
| 3000 | 20 | 10 | 120 |
Vent type and typical NFVA
| Vent type | Typical NFVA | Best use |
|---|---|---|
| Ridge vent | 9–18 sq in/ft | High exhaust, hidden profile |
| Soffit vent strip | 9–12 sq in/ft | Continuous intake |
| Box vent | 50–75 sq in each | Spot exhaust near ridge |
| Gable vent | Varies | Cross-ventilation, not balanced |
Common mistakes
- Installing ridge vent without matching intake vents. Exhaust cannot exceed intake by much without pulling conditioned air from the house.
- Using gross vent area instead of the manufacturer's net free area rating.
- Blocking soffit vents with insulation.
- Mixing exhaust types without accounting for their combined NFVA.
- Ignoring the 1:150 vs 1:300 code distinction.
Assumptions and limits
- Ventilation is balanced: half high exhaust and half low intake.
- The ridge vent NFVA rating comes from the manufacturer.
- No powered attic ventilation is present.
- Local code may amend the 1:150 or 1:300 ratios.
Frequently asked questions
How much ridge vent do I need?
Divide the attic floor area by 150 to get total NFVA, then take half of that for ridge exhaust. Convert to linear feet using the vent's rated NFVA per foot. A 2,000 sq ft attic needs about 80 feet of ridge vent at 12 sq in/ft.
What is net free vent area?
NFVA is the actual open area of a vent after subtracting louvers, mesh and framing. It is always less than the gross vent opening.
Can I have too much ridge vent?
Only if it exceeds your intake venting. Exhaust should be roughly balanced with intake. Too much exhaust without intake can pull conditioned air through ceiling penetrations.
Do I need a ridge vent if I have gable vents?
Gable vents provide cross-ventilation but do not create the stack effect of low intake and high exhaust. For best performance, add soffit intake and either ridge vent or box vents near the ridge.
What is the 1:150 vs 1:300 ventilation rule?
The 1:150 rule requires 1 sq ft of NFVA for every 150 sq ft of attic floor. The 1:300 rule is allowed when at least 40% of ventilation is in the upper portion of the attic and a vapor barrier is present.
Can I mix ridge vent with box vents?
You can, but avoid short-circuiting where exhaust vents compete with each other. Keep intake low and exhaust high, and make sure the total NFVA meets code.
Last updated 2026-08-05. Results are estimates for planning and are not a substitute for a licensed engineer or local code review.