Passive Intake Area Calculator

Plan a passive air intake

Enter exhaust airflow, desired intake face velocity, and the percentage of a grille or screen that is open to airflow. The calculator estimates the physical opening needed after accounting for blocked area.

At 500 CFM and 300 feet per minute, 240 square inches of net free area is required. A grille with 60% free area needs about 400 square inches of physical opening.

Why free area matters

Louvers, insect screens, filters, and light traps reduce usable opening area. Product dimensions alone do not describe airflow. Use manufacturer free-area data when available and expect resistance to rise as filters become dirty.

Check room pressure

An undersized intake can increase negative pressure, reduce fan airflow, pull unfiltered air through gaps, and increase noise. Multiple openings may be used if their combined net free area meets the target.

Installation checks

• Keep intakes clear of obstructions.
• Prevent rain, pests, and unwanted light entry.
• Measure actual airflow or pressure where possible.
• Follow building and fire requirements.

This is an opening-area estimate. Real performance also depends on the entire ventilation path and fan curve.