HVAC CFM Calculator
CFM can be figured two ways in the field: from the room's volume and the air changes per hour it needs, or from the system's tonnage at roughly 400 CFM per ton. This calculator does both so you can sanity-check a register, a room, or a whole system.
Method 1 — Room Volume And Air Changes
Room volume
1,792 ft³
Required airflow
179 CFM
Airflow per square foot
0.8 CFM/ft²
Method 2 — System Tonnage
400 standard, 350 humid climates, 450 dry climates.
Total system airflow
1,200 CFM
This room's share
14.9%
Of total system airflow
How To Use Both Numbers
The volume method tells you what the room needs. The tonnage method tells you what the equipment can deliver. When you add up every room's requirement and it exceeds the system's total airflow, either the equipment is undersized or the air change targets are too aggressive for the design.
In the field, the fastest use of this tool is a spot check: measure the room, pick an air change target, and compare the result to what you actually read at the register with a flow hood or anemometer. A register delivering half its target is a duct problem long before it is an equipment problem.
Rule-of-thumb airflow figures. Actual design requires a room-by-room load calculation and a duct design that accounts for static pressure, fittings, and equipment fan curves.
Frequently Asked Questions
- What is the formula for CFM?
- CFM equals room volume in cubic feet multiplied by air changes per hour, divided by 60. Room volume is length times width times ceiling height. The division by 60 converts changes per hour into cubic feet per minute.
- How many CFM per ton of cooling?
- Roughly 400 CFM per ton is the standard design figure for residential comfort cooling. Humid climates are often designed closer to 350 CFM per ton for better moisture removal, and dry climates sometimes run 450.
- How many air changes per hour does a room need?
- It depends on use. Bedrooms and living spaces commonly design around 4 to 6 air changes per hour, kitchens 7 to 8, bathrooms 8 or more, and commercial spaces vary widely by occupancy and code.
- Does this replace a duct design?
- No. CFM tells you the airflow target; a Manual D duct design determines the duct sizes, static pressure, and fitting losses needed to actually deliver it. Use this to check a target, not to size a system.
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