Airflow CFM Calculator

Calculate required airflow in cubic feet per minute for HVAC system design and ventilation

Calculation Method

Room Ventilation (ACH) Duct Airflow Cooling / Heating Load

Room Length (ft)

Room Width (ft)

Ceiling Height (ft)

Room Type (determines ACH requirement)

What is CFM?

CFM stands for Cubic Feet per Minute and measures the volume of air flowing through a system. It is the standard unit for HVAC airflow in North America. Proper CFM ensures adequate ventilation, comfortable temperatures, and healthy indoor air quality.

Key Formulas

Room Ventilation (ACH method):

CFM = (Room Volume × ACH) ÷ 60

Where ACH = Air Changes per Hour

Duct Airflow:

CFM = Duct Area (sq ft) × Velocity (FPM)

From Heating/Cooling Load:

CFM = BTU/hr ÷ (1.08 × Delta-T)

Where 1.08 = air density constant (0.075 lb/ft³ × 0.24 BTU/lb°F × 60 min/hr)

Recommended Air Changes per Hour (ACH)

ACH indicates how many times the complete volume of air in a room is replaced per hour. Higher ACH is needed for spaces with more pollutants, moisture, or occupants.

Room Type	ACH Range	Notes
Bedroom	4 - 6	Low pollutant load, comfort
Living Room	6 - 8	General occupancy
Office	6 - 8	Equipment heat loads
Bathroom	8 - 12	Moisture removal
Kitchen (residential)	15 - 20	Cooking fumes, heat, moisture
Classroom	10 - 12	High occupancy, CO₂ control
Conference Room	8 - 12	Variable occupancy
Restaurant	12 - 15	Food odors, occupancy
Gym / Exercise Room	15 - 20	High metabolic load
Workshop / Garage	10 - 15	Fumes, dust, chemicals
Server Room	15 - 20	Equipment heat dissipation
Hospital Room	10 - 12	Infection control
Laboratory	15 - 25	Chemical safety
Clean Room	20 - 60	Particulate control, class dependent

Duct Size vs. CFM Reference

Duct size determines maximum airflow capacity at a given velocity. These values assume typical residential duct velocity of 700 FPM for supply ducts.

Round Duct (in)	Area (sq in)	CFM @ 600 FPM	CFM @ 700 FPM	CFM @ 900 FPM
4"	12.6	52	61	79
5"	19.6	82	95	123
6"	28.3	118	137	177
7"	38.5	160	187	241
8"	50.3	209	244	314
10"	78.5	327	382	491
12"	113.1	471	550	707
14"	153.9	641	748	962
16"	201.1	837	977	1,257
18"	254.5	1,060	1,238	1,591
20"	314.2	1,309	1,527	1,963

Common CFM Applications

Exhaust Fans

• Bathroom fan: 50-110 CFM (1 CFM per sq ft)
• Kitchen range hood: 100-600 CFM
• Attic fan: 1 CFM per sq ft of attic
• Whole-house fan: 3-6 CFM per sq ft
• Dryer vent: 100-200 CFM

HVAC Systems

• Central AC: 400 CFM per ton of cooling
• Residential furnace: 350-400 CFM per ton
• Heat pump: 400-450 CFM per ton
• Fresh air intake: 15-20 CFM per person
• ASHRAE 62.2 minimum: 7.5 CFM/person + 3 CFM/100 sq ft

Air Velocity Guidelines

• Residential supply duct: 600-900 FPM
• Main trunk duct: 700-1,000 FPM
• Branch duct: 600-700 FPM
• Grille/register face: 300-500 FPM
• Commercial ductwork: 1,000-2,000 FPM

Noise Considerations

• Below 500 FPM: Very quiet
• 500-700 FPM: Quiet (residential ideal)
• 700-1,000 FPM: Moderate (some noise)
• 1,000-1,500 FPM: Noticeable noise
• Above 1,500 FPM: Loud (commercial only)

References

The airflow calculations and ACH recommendations are based on industry standards:

Related Calculators

BTU Requirement Calculator Water Flow Rate Calculator Pipe Volume Calculator Electrical Load Calculator

Note: This calculator provides estimates for HVAC planning and ventilation design. Actual duct system design must account for static pressure losses, friction rates, fitting equivalent lengths, and equipment specifications. For complex installations, consult an HVAC engineer and reference ACCA Manual D for residential duct design or SMACNA standards for commercial applications.

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