# Ridge Ventilation Planning Guide

> Translate an entered attic ventilation ratio into exhaust net-free area and ridge length while checking intake balance and product ratings.

- Canonical URL: https://buildmeter.net/guides/ridge-ventilation-planning-guide/
- Category: Energy & Electrical
- Author: Liron Elimeleh
- Publisher: BuildMeter, developed by ELStudios
- Published: July 21, 2026
- Last reviewed: July 21, 2026

## Direct answer

Ridge vent is sized from required exhaust net-free area, not from attic perimeter or visible vent width. Use the project ventilation ratio, reserve a balanced share for low intake vents and divide the exhaust requirement by the product’s published NFA per foot or metre.

## Primary calculator

- [Ridge Vent Calculator](https://buildmeter.net/ridge-vent-calculator/)

## Use net free area, not visible vent dimensions

Vent manufacturers publish net free area (NFA), the open area available for airflow after screens and internal geometry. Total attic ventilation is commonly divided between low intake and high exhaust, but the required ratio and balance must come from the assembly and applicable requirements.

### Ridge-vent calculation sequence

| Step | Calculation |
| --- | --- |
| Total NFA | Attic floor area ÷ entered ventilation ratio |
| Exhaust share | Total NFA × planned exhaust percentage |
| Ridge length | Exhaust NFA ÷ product NFA per length |
| Packages | Required length + cuts ÷ package length |

## Worked example

An 1,800 ft² attic modeled at 1:300 needs 6 ft², or 864 in², of total NFA. At a 50% exhaust share, ridge exhaust needs 432 in². A ridge vent rated at 18 in² per linear foot needs 24 ft of active ridge before end and cut allowance.

## Intake and air sealing matter

Ridge vent cannot compensate for blocked soffits. Baffles and wind dams keep insulation from blocking intake paths, while ceiling air sealing reduces warm moist indoor air entering the attic. Adding exhaust without intake can draw air from the house or from unintended openings.

## Common mistakes

- Counting vent length instead of product NFA.
- Installing ridge vent with no soffit intake.
- Mixing powered exhaust and passive ridge vent without system analysis.
- Cutting the roof slot outside manufacturer dimensions.
- Ventilating an assembly designed to be unvented.

## Sources and professional limits

- [U.S. DOE — Guide to Durable Attics](https://www.energy.gov/sites/prod/files/guide_to_durable_attics.pdf)
- [DOE — Attic baffles](https://bsesc.energy.gov/energy-basics/baffles)
- [DOE — Air barriers in ventilated attics](https://bsesc.energy.gov/energy-basics/air-barriers-ventilated-attics)

## Related calculators

- [Attic Ventilation Calculator](https://buildmeter.net/roof-vent-calculator/)
- [Attic Ventilation Baffle Calculator](https://buildmeter.net/attic-ventilation-baffle-calculator/)
- [Soffit Vent Calculator](https://buildmeter.net/roof-vent-calculator/)

## Related guides

- [Attic Ventilation Baffle Planning Guide](https://buildmeter.net/guides/attic-ventilation-baffle-planning-guide/)
- [Insulation R-Values Explained](https://buildmeter.net/guides/insulation-r-values-explained/)
- [Roof Underlayment Selection and Coverage Guide](https://buildmeter.net/guides/roof-underlayment-selection-and-coverage-guide/)