CFM calculator
Three ways to get an airflow figure: from the system's tonnage, from air changes per hour for a room, or from the ASHRAE 62.2 whole-house ventilation rate.
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Result
Airflow split between rooms
| Room | Area, ft² | Share | Airflow, cfm |
|---|
System airflow, from tonnage
Cooling equipment is designed around a nominal 400 cfm per ton, with 350 to 450 the range in normal use. At 400 cfm per ton and typical return air of 80 °F at 50% relative humidity, a coil does roughly 70% sensible cooling and 30% latent, on about a 20 °F temperature drop.
Moving less air per ton makes the coil colder, which removes more moisture but risks drafts and a frozen coil. Moving more air per ton does the opposite: better sensible cooling, less dehumidification, and larger ducts to carry it. In humid climates technicians often aim nearer 350; in dry climates nearer 450.
Room airflow, from air changes
Air changes per hour asks how many times the air in a room is replaced each hour. It is the usual method for ventilation and exhaust, rather than for sizing cooling supply.
A 300 ft² room with an 8 ft ceiling holds 2,400 ft³. At 2 air changes an hour that is 80 cfm.
Whole-house ventilation, ASHRAE 62.2
ASHRAE Standard 62.2 sets the mechanical ventilation rate for a dwelling:
Occupancy is assumed from the bedroom count rather than asked for: two people in the main bedroom and one in each other bedroom, which works out as bedrooms plus one. The standard also sets local exhaust separately, listed in the table above.
Once you have the airflow, the duct size calculator turns it into duct sizes, and the static pressure calculator tells you whether the system can actually move it.
Questions
How many CFM per ton of air conditioning?
400 cfm per ton is the nominal design figure, with 350 to 450 the normal range. A 3 ton system is therefore about 1,200 cfm.
How do I calculate CFM for a room?
Multiply the floor area by the ceiling height to get the volume, multiply by the air changes per hour you want, then divide by 60. A 300 ft² room with 8 ft ceilings at 2 ACH needs 80 cfm.
What is a good CFM per square foot?
For residential cooling supply, roughly 0.5 to 1 cfm per square foot falls out of the 400 cfm per ton figure once a system is sized. Ventilation is a different calculation: ASHRAE 62.2 uses 0.03 cfm per square foot plus 7.5 cfm per occupant.
Why is my temperature drop not 20 degrees?
Temperature drop and airflow move together. A drop much above 20 °F usually means too little airflow, from a dirty filter, a restricted return or a slow blower. A drop well below it usually means too much airflow or a refrigerant problem.
How much ventilation does ASHRAE 62.2 require?
0.03 cfm per square foot of floor area plus 7.5 cfm per occupant, with occupancy counted as the number of bedrooms plus one. A 2,000 ft² home with 3 bedrooms needs 90 cfm continuously.
More HVAC calculators
Duct size calculator
Round and rectangular duct sizes from airflow and friction rate, with a CFM chart for every size.
Static pressure calculator
Total external static pressure from your readings, plus the friction rate for duct sizing.
AC tonnage calculator
Cooling capacity estimate by floor area and climate zone, with an honest check against real load calculations.