CFM Calculator

Calculate CFM airflow for ventilation. Select room type for auto ACH, add occupant-based ASHRAE minimum, and get duct size recommendation.

πŸ’¨ CFM Calculator

ROOM

Floor Area120.00 sq ft
Volume960.00 cu ft

VENTILATION

CFM (6 ACH)96.0 CFM
ASHRAE Min (15 CFM/person)30.00 CFM
Recommended CFM96.0 CFM
Airflow (metric)45.3 L/s
CFM per Sq Ft0.80 CFM/ftΒ²

DUCT SIZING

Min Round Duct5" diameter

πŸ’‘ How to Calculate CFM

CFM (cubic feet per minute) measures the volume of air flowing through a space or duct. It's the primary metric for sizing exhaust fans, range hoods, HVAC systems, air purifiers, and ventilation equipment.

The calculator above includes 9 room type presets with auto-set ACH, ASHRAE occupant-based ventilation, L/s metric conversion, and duct diameter recommendation.

Size your furnace with our furnace BTU calculator. For cooling, see our window AC size calculator. Measure room area with our square footage calculator.

CFM Formula

CFM = (Length Γ— Width Γ— Height Γ— ACH) Γ· 60

Where L, W, H are room dimensions in feet, and ACH = air changes per hour.

Step 1: Measure the Room

Measure length, width, and ceiling height in feet. For irregular rooms, break into rectangles and add areas.

Step 2: Calculate Volume

Volume (cu ft) = L Γ— W Γ— H. Example: 12 Γ— 10 Γ— 8 = 960 cu ft.

Step 3: Determine ACH

Air changes per hour depend on room type and use. Higher ACH = more air replacement per hour = better air quality.

Step 4: Calculate CFM

CFM = (960 Γ— 6) Γ· 60 = 96 CFM for a living room at 6 ACH.

Air Changes per Hour by Room Type

Room TypeRecommended ACHCFM for 120 sq ft (8 ft ceiling)
Bedroom464 CFM
Living Room696 CFM
Office / Study696 CFM
Kitchen8128 CFM
Bathroom10160 CFM
Laundry Room10160 CFM
Garage / Workshop12192 CFM
Restaurant Kitchen15–20240–320 CFM
Server Room15–20240–320 CFM

Residential Ventilation Requirements

SpaceMinimum CFMCode Reference
Bathroom (≀100 sq ft)50 CFM continuous or 1 CFM/sq ftIRC M1507
Bathroom (>100 sq ft)1 CFM/sq ft + 50 per fixtureIRC M1507
Kitchen (range hood)100 CFM/ft of rangeIMC Table 403
Kitchen (downdraft)300–600 CFMManufacturer spec
Utility/Laundry50 CFM continuousIRC M1507
Whole-house0.35 ACH or 15 CFM/personASHRAE 62.2
Garage (attached)100 CFM continuousIRC M1505
Crawl space1 CFM/150 sq ftIRC R408

Range Hood Sizing

  • Wall-mounted: 100 CFM per linear foot of range. 30" range = 250 CFM min.
  • Island hoods: 150 CFM per linear foot. 36" island range = 450 CFM min.
  • Professional ranges (>60K BTU): 1 CFM per 100 BTU. 90K BTU range = 900 CFM.
  • Makeup air: Hoods >400 CFM require makeup air per IRC M1503.6.

Bathroom Fan Sizing

  • Small bathroom (≀100 sq ft): 1 CFM per sq ft, minimum 50 CFM.
  • Large bathroom (>100 sq ft): Add 50 CFM per toilet, shower, bathtub, and jetted tub.
  • Sone rating: ≀1.0 sone = very quiet, 1.0–2.0 = quiet, >3.0 = noticeable noise.
12 Γ— 10 Γ— 8 ft living room, 6 ACH, 2 occupants: ACH-based = 96 CFM, ASHRAE min = 30 CFM β†’ recommended 96 CFM. Min duct: 5" round.

CFM Calculator FAQ

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