Ridge Vent Calculator
Attic vent NFA with soffit/ridge split.
What it does
Attic vents work as balanced pairs, not holes. The 1:150 rule gives a 2,000 sq ft attic 13.33 sq ft of net free area split half low at soffits and half high at ridges, so air washes the whole deck. All-high or all-low layouts stall.
The tool above runs the rule with unit conversion attached. Enter attic area, pick 1:150 or the 1:300 vapor-barrier case, and it prints total NFA, soffit/ridge halves, and square inches. Defaults give 13.33 with 6.67 halves and 1,920 sq in.
Roofing and energy authorities agree. NRCA guidelines set 1 sq ft per 150 sq ft balanced low/high, and DOE Building America works 1,260 sq ft cases at both ratios with 40-50% high placement and 3 ft of rise. IRC text behind both demands the same split.
Product math needs square inches. Vent catalogs rate in sq in of NFA per foot or unit, so divide the printed halves by product ratings for counts. Keep intakes clear of insulation with baffles; buried soffits vent nothing.
How to use
- Enter the attic floor area.
- Pick 1:150 or 1:300.
- Read total NFA first.
- Read soffit/ridge halves.
- Convert to sq in for product math.
- Keep ridge 3 ft above intake.
Formula
- attic
- Vented attic floor area in sq ft.
- code
- 1:150 standard or 1:300 with barrier.
NRCA 1:150 balanced guidance with DOE/BASC worked cases (1,260 sq ft gives 4.2/8.4) and IRC placement rules.
Worked example
Example: 2,000 sq ft attic
Defaults: 2,000 sq ft at 1:150.
- Input: 2,000 sq ft attic at 1:150.
- Total: 2,000 / 150 = 13.33 sq ft NFA.
- Halves: 6.67 soffit + 6.67 ridge.
- Square inches: 13.33 x 144 = 1,920.
- Tool shows 13.33 sq ft NFA, halves, 1,920 sq in.
Reference table
| Attic | Total | Each half |
|---|---|---|
| 1,000 | 6.67 | 3.33 |
| 1,500 | 10.00 | 5.00 |
| 2,000 | 13.33 | 6.67 |
| 2,500 | 16.67 | 8.33 |
| 3,000 | 20.00 | 10.00 |
Limits
- Area math only. Product ratings separate.
- 1:300 case needs qualifying barrier.
- Wind and snow zones add rules.
- Balanced halves assumed.
Common mistakes
- Venting only high or only low.
- Burying soffits in insulation.
- Skipping the 1:300 qualifier check.
- Mixing gross and net free area.
- Ignoring the 3 ft rise rule.
Try it yourself
Quick checks. Answers sit tucked under each question.
1,500 sq ft at 1:150?
1,500 / 150 = 10 sq ft. Halves 5 + 5. Inches 1,440.
2,000 sq ft at 1:300?
2,000 / 300 = 6.67 sq ft. Halves 3.33 + 3.33. Inches 960.
Questions
How much vent area?
Attic over 150 (or 300 with vapor barrier). Here 13.33 sq ft total.
How is intake/exhaust split?
Half each: 6.67 soffit low plus 6.67 ridge high here.
How many square inches?
Multiply by 144: 1,920 sq in here, 960 per half.
When does 1:300 apply?
1:300 with vapor retarder in cold zones and 40-50% high placement.
How high above intake?
Yes. Ridge runs 3 ft or more above soffit intake for stack effect.
Must halves balance?
Yes. Balanced halves beat all-high or all-low layouts for airflow.
Sources
- DOE Building America: Attic Passive Ventilation (worked 1:150/1:300)
- NRCA Roofing Guidelines (1:150 balanced rule)