How to Use the Supply/Exhaust Airflow Balance Calculator
Indoor air quality and room pressure depend on the balance between supply air (coming in) and exhaust air (going out). More supply than exhaust pushes a room positive, keeping outside air from infiltrating; more exhaust than supply pulls it negative, which helps odors and contaminants exit but can pull air in from adjacent spaces. Enter your supply and exhaust airflow to get the difference (CFM) and the supply-to-exhaust ratio, plus a quick status read.
As a practical guideline, a difference within about ±10% of the exhaust airflow is generally treated as balanced; outside that range the room is classified as running positive or negative. Keep in mind that restrooms, kitchens, and contaminant-handling spaces are often intentionally designed to run negative to contain odors or hazardous air, so this general threshold doesn't apply uniformly to every room type.
Running too negative can pull odors or contaminated air back in from adjacent spaces and make doors hard to close, while running too positive can make doors hard to open and increase energy loss through gaps and cracks. Target pressure requirements vary by application and code, so any adjustment to your supply/exhaust system should be reviewed with an HVAC engineer.
Frequently Asked Questions
As a practical rule of thumb, a difference within ±10% of the exhaust airflow is generally considered balanced. Spaces like restrooms or kitchens that are intentionally designed to run negative are an exception to this rule.
Excess negative pressure can pull odors or contaminated air back in from adjacent spaces and make doors hard to close. Excess positive pressure can make doors hard to open and increase energy loss through gaps and cracks.