CFM Calculator

1. Enter room dimensions. Measure the length, width, and ceiling height. For irregular rooms, approximate the total square footage.

2. Select the room type. Each room type has a recommended air changes per hour (ACH) rate based on ASHRAE standards. Kitchens and bathrooms need more ventilation than bedrooms.

3. Enter occupant count. This is used for the ASHRAE outdoor air check — each person needs a minimum of 15 CFM of outdoor air.

4. Read the results. The required CFM tells you how much airflow the room needs. The register count helps you plan supply duct layout.

CFM (cubic feet per minute) measures how much air moves through a space. The calculation is straightforward:

ACH (air changes per hour) varies by room type. A kitchen at 7.5 ACH means the entire room volume of air is replaced 7.5 times per hour. The division by 60 converts from hourly to per-minute flow.

The ASHRAE check ensures the CFM meets minimum outdoor air requirements: at least 15 CFM per occupant. If the calculated CFM falls below this, the room needs supplemental ventilation.

Duct design and layout. Before sizing ducts, you need to know the CFM each room requires. This calculator gives you room-by-room CFM to feed into duct sizing.

Register selection. Knowing the CFM tells you how many registers a room needs and what size. Undersized registers create noise and restrict airflow.

Troubleshooting comfort complaints. Customer says a room is always hot? Check if the CFM delivered matches what's needed. A bedroom getting 50 CFM when it needs 120 will never be comfortable.