About

Incorrect airflow sizing leads to moisture buildup, mold growth, occupant discomfort, and compressor failure. A room undersized by 20% in CFM will fail to maintain setpoint temperature under peak load, while oversizing wastes energy proportional to the cube of fan speed per the Fan Affinity Laws. This calculator computes CFM (Cubic Feet per Minute) using three industry-standard methods: volumetric air changes (V × ACH ÷ 60), duct cross-section velocity (A × v), and the sensible heat equation (BTU ÷ 1.08ΔT) per ASHRAE Fundamentals. It also reverse-calculates required duct diameter from target CFM.

Results assume standard air density at sea level (0.075 lb/ft³, approximately 70°F dry bulb). At elevations above 5000 ft, apply a correction factor of roughly 1.20 to compensate for reduced air density. The tool does not account for duct leakage, which typically adds 10 - 30% loss in unsealed systems per SMACNA standards.

Formulas

Room Volume Method - converts volumetric air changes per hour to flow rate in cubic feet per minute:

CFM = L × W × H × ACH60

Where L = room length ft, W = room width ft, H = ceiling height ft, ACH = air changes per hour.

Duct Velocity Method - relates cross-sectional area of a duct to air speed:

CFM = A × v

For a round duct: A = π × D²4 where D is diameter in ft and v = velocity in FPM. For a rectangular duct: A = W × H in ft².

Reverse duct sizing - given target CFM and velocity, solve for required round duct diameter:

D = √4 × CFMπ × v × 12

Result in inches. Multiply by 12 to convert from feet.

Sensible Heat (Cooling Load) Method - derives required airflow from thermal load:

CFM = Q1.08 × ΔT

Where Q = sensible heat load in BTU/hr, 1.08 = 0.24 BTU/(lb⋅°F) × 0.075 lb/ft³ × 60 min/hr, and ΔT = temperature differential between supply and return air in °F.

Reference Data

Space Type	Recommended ACH	Typical CFM/ft²	Notes
Residential Bedroom	4 - 6	0.5 - 1.0	Low occupancy, comfort priority
Residential Living Room	6 - 8	1.0 - 1.5	Moderate activity levels
Residential Kitchen	10 - 15	2.0 - 3.0	Exhaust hood adds dedicated CFM
Residential Bathroom	8 - 12	1.5 - 2.0	Minimum 50 CFM per IRC code
Residential Garage	6 - 8	1.0 - 1.5	CO exhaust ventilation required
Office (General)	6 - 10	1.0 - 1.5	ASHRAE 62.1 outdoor air rates apply
Conference Room	10 - 15	1.5 - 2.5	High occupancy density spikes CO₂
Restaurant Dining	12 - 15	2.0 - 3.0	Separate kitchen exhaust system
Commercial Kitchen	15 - 30	3.0 - 5.0	Type I/II hoods per IMC Chapter 5
Retail Store	8 - 12	1.0 - 2.0	Varies with foot traffic
Hospital Patient Room	6 - 8	1.0 - 1.5	Positive pressure maintained
Hospital Operating Room	20 - 25	4.0 - 6.0	HEPA filtration, positive pressure
Laboratory	8 - 15	1.5 - 3.0	Fume hoods add 100 - 150 FPM face velocity
Server Room / Data Center	15 - 20	3.0 - 5.0	Heat load driven, not occupancy
Warehouse	4 - 6	0.3 - 0.8	Large volume, low occupancy density
School Classroom	6 - 10	1.0 - 2.0	ASHRAE 62.1: 15 CFM/person outdoor air
Gymnasium	8 - 12	1.5 - 2.5	High metabolic rate increases load
Indoor Pool / Natatorium	6 - 8	1.0 - 2.0	Dehumidification is primary concern
Auditorium / Theater	10 - 15	1.5 - 2.5	Peak occupancy ventilation sizing
Clean Room (ISO 7)	30 - 60	5.0 - 10.0	Particle count driven, HEPA ceiling coverage

Frequently Asked Questions

Air density decreases with elevation. At sea level, standard air density is 0.075 lb/ft³. At 5000 ft, density drops to roughly 0.0625 lb/ft³. The sensible heat constant 1.08 must be reduced proportionally (to approximately 0.90), meaning you need more CFM to deliver the same cooling. Multiply your calculated CFM by the ratio of sea-level density to local density.

CFM defines volume flow; static pressure (measured in inches of water gauge, in. w.g.) defines the resistance the fan must overcome. As duct diameter decreases for a given CFM, velocity increases and friction losses rise roughly with the square of velocity. ASHRAE recommends keeping duct velocity below 900 FPM in residential trunk lines to limit noise below 35 NC (Noise Criteria).

The constant 1.08 bundles three values: specific heat of air (0.24 BTU/lb⋅°F), standard air density (0.075 lb/ft³), and the 60 min/hr conversion. It assumes dry air at standard conditions. For humid climates, total (sensible + latent) heat uses 4.5 × CFM × Δh (enthalpy difference). The 1.08 constant is inaccurate above 5000 ft elevation or at temperatures deviating significantly from 70°F.

1 CFM = 1.699 m³/h = 0.4719 L/s. Conversely, 1 m³/h = 0.5886 CFM. European HVAC standards (EN 13779) typically specify ventilation rates in L/s per person, so conversion is essential when comparing international equipment specifications.

Flexible duct has a corrugated inner surface that increases friction loss by 50 - 100% compared to smooth galvanized duct of the same diameter, especially when not pulled taut. A 6in flex duct with bends can deliver 30 - 40% less CFM than rated. ACCA Manual D recommends upsizing flex duct by one diameter increment (e.g., use 8in instead of 6in) to compensate.

ACH (Air Changes per Hour) is dictated by building codes and space function, not personal preference. ASHRAE Standard 62.1 specifies outdoor air ventilation rates per person and per unit floor area. For residential, IRC requires minimum exhaust rates: 50 CFM intermittent or 20 CFM continuous for bathrooms, 100 CFM intermittent for kitchens. Convert these to ACH by: ACH = (CFM × 60) ÷ Volume. The reference table above provides typical ranges per space type.