ft
Enter room length.
ft
Enter room width.
ft
Enter ceiling height.
ACH
Enter ACH value.
Times room air is fully replaced per hour
Required CFM
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📋 Calculation Breakdown
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Sources & Methodology
CFM formulas and ACH recommendations verified against ASHRAE Standard 62.1 (Ventilation for Acceptable Indoor Air Quality) and HVI (Home Ventilating Institute) residential ventilation standards.
ASHRAE Standard 62.1 — Ventilation for Acceptable Indoor Air Quality
Industry-standard ACH requirements by space type, minimum outdoor air rates, and ventilation effectiveness for both residential and commercial applications
HVI — Home Ventilating Institute Guidelines
Residential fan CFM requirements including 1 CFM per sq ft bathroom standard and kitchen range hood exhaust recommendations used in this calculator
Methodology: Room mode: Volume = L x W x H (cu ft); CFM = Volume x ACH / 60. Duct mode (round): Area = pi x (diameter/24)^2 (sq ft); CFM = Velocity x Area. Duct mode (rect): Area = (width x height) / 144 (sq ft); CFM = Velocity x Area. ACH mode: ACH = CFM x 60 / Volume. m3/h = CFM x 1.699. All ACH recommendations from ASHRAE 62.1.
⏱ Last reviewed: April 2026
How to Calculate CFM for Any Room or Duct
CFM (Cubic Feet per Minute) is the standard measurement of airflow in HVAC, ventilation, and fan applications. This CFM calculator is one of the most practical tools for homeowners, HVAC technicians, and building engineers. Whether you are sizing a bathroom exhaust fan, a kitchen range hood, a grow tent ventilation system, or calculating duct airflow, the formula is straightforward once you know your room volume and required air changes per hour.
The CFM Formulas
Room CFM = Volume (cu ft) x ACH / 60
Volume = Length x Width x Height (feet)
Example: 12 x 15 x 9 ft room = 1,620 cu ft at 6 ACH: CFM = 1,620 x 6 / 60 = 162 CFM required
Example: 12 x 15 x 9 ft room = 1,620 cu ft at 6 ACH: CFM = 1,620 x 6 / 60 = 162 CFM required
Duct CFM = Velocity (FPM) x Duct Area (sq ft)
Round duct area = pi x (diameter in feet / 2)^2
Example: 8-inch round duct (0.667 ft) at 600 FPM: Area = 0.349 sq ft; CFM = 0.349 x 600 = 209 CFM
Example: 8-inch round duct (0.667 ft) at 600 FPM: Area = 0.349 sq ft; CFM = 0.349 x 600 = 209 CFM
Recommended ACH by Room Type (ASHRAE 62.1)
| Room Type | ACH Range | Example Room | Required CFM |
|---|---|---|---|
| Bedroom / Home Office | 4-6 ACH | 12x12x9 ft (1,296 cu ft) | 86-130 CFM |
| Living Room | 4-6 ACH | 16x18x9 ft (2,592 cu ft) | 173-259 CFM |
| Bathroom | 6-8 ACH | 8x6x8 ft (384 cu ft) | 38-51 CFM |
| Kitchen | 7-12 ACH | 12x14x9 ft (1,512 cu ft) | 176-302 CFM |
| Server Room / IT | 20-30 ACH | 10x10x9 ft (900 cu ft) | 300-450 CFM |
| Grow Tent | 30-60 ACH | 4x4x7 ft (112 cu ft) | 56-112 CFM |
Common Round Duct CFM at Standard Velocities
| Duct Diameter | Area (sq ft) | 500 FPM | 700 FPM | 1000 FPM |
|---|---|---|---|---|
| 4 inch | 0.087 sq ft | 44 CFM | 61 CFM | 87 CFM |
| 6 inch | 0.196 sq ft | 98 CFM | 137 CFM | 196 CFM |
| 8 inch | 0.349 sq ft | 174 CFM | 244 CFM | 349 CFM |
| 10 inch | 0.545 sq ft | 272 CFM | 381 CFM | 545 CFM |
| 12 inch | 0.785 sq ft | 393 CFM | 550 CFM | 785 CFM |
HVAC System CFM by Tonnage
Standard HVAC design uses 400 CFM per ton of cooling. A 2-ton system needs 800 CFM through the air handler. A 3-ton unit needs 1,200 CFM. A 4-ton unit needs 1,600 CFM. Insufficient airflow reduces system efficiency, causes comfort issues, and can freeze evaporator coils. Always verify your blower fan delivers the rated CFM for your system.
💡 Pro Tip: Always add 20-25% to your calculated CFM when sizing fans for systems with filters, carbon filters, or long duct runs. Static pressure from filters and ductwork significantly reduces actual delivered airflow. For grow tents with carbon filters, size your fan at 125-150% of the calculated minimum CFM to compensate for filter resistance.
Frequently Asked Questions
CFM stands for Cubic Feet per Minute, the standard unit for measuring airflow volume. It describes how many cubic feet of air a fan, duct, or HVAC system moves per minute. Higher CFM means more airflow. HVAC systems, exhaust fans, and air purifiers are all rated in CFM.
CFM = Room Volume x ACH / 60. Volume = Length x Width x Height in feet. ACH depends on room type: bedrooms 4-6, kitchens 7-12, bathrooms 6-8. Example: 12x15x9 ft room (1,620 cu ft) at 6 ACH: CFM = 1,620 x 6 / 60 = 162 CFM.
ACH is the number of times the total air volume of a room is replaced in one hour. ACH = CFM x 60 / Room Volume. Standard guidelines: bedrooms 4-6, living rooms 4-6, kitchens 7-12, bathrooms 6-8, server rooms 20-30, grow tents 30-60 ACH.
Room CFM = Volume (cu ft) x ACH / 60. Duct CFM = Air Velocity (FPM) x Duct Area (sq ft). For round ducts, area = pi x (diameter in feet / 2)^2. For rectangular ducts, area = width x height in feet.
Duct CFM = Velocity (FPM) x Cross-sectional Area (sq ft). For an 8-inch round duct (0.667 ft diameter): Area = pi x 0.333^2 = 0.349 sq ft. At 600 FPM: CFM = 0.349 x 600 = 209 CFM. Always convert duct dimensions from inches to feet first.
HVI recommends 1 CFM per square foot of floor area, minimum 50 CFM. For an 80 sq ft bathroom: 80 CFM minimum. Standard fan sizes: 50, 80, 110, 130 CFM. Always choose the next size up from your calculated minimum to ensure adequate ventilation.
Standard HVAC design uses 400 CFM per ton. A 2-ton system needs 800 CFM. A 3-ton unit needs 1,200 CFM. Insufficient airflow reduces efficiency and can freeze evaporator coils. Always verify your blower delivers the rated CFM for your system.
CFM is volume flow rate (total air moved per minute). FPM is velocity (how fast air moves). They relate via duct area: CFM = FPM x Area. A 1 sq ft duct at 500 FPM moves 500 CFM. A 2 sq ft duct at 500 FPM moves 1,000 CFM.
Multiply CFM by 1.699 to get cubic meters per hour (m3/h). Example: 200 CFM x 1.699 = 339.8 m3/h. To convert m3/h to CFM, divide by 1.699. Many European HVAC products are rated in m3/h while US products use CFM.
At least 100 CFM per 10,000 BTU of stove output, or 15 ACH for the kitchen volume. For a 12x14x9 kitchen (1,512 cu ft) at 15 ACH: CFM = 378. For a 60,000 BTU stove: 600 CFM. Use whichever is higher.
Static pressure (inches of water column, in. WC) is the resistance a fan must overcome in ducts, filters, and registers. As static pressure increases, delivered CFM decreases. Always select a fan rated for both the required CFM and the expected static pressure in your system.
Grow tents need 30-60 ACH. CFM = Volume x ACH / 60. A 4x4x7 ft tent (112 cu ft) at 60 ACH: 112 x 60 / 60 = 112 CFM minimum. Add 25% for carbon filter resistance: 140 CFM. Use a variable speed fan for fine-tuning.
Bedrooms typically need 4-6 ACH. For a 12x12x9 ft bedroom (1,296 cu ft) at 5 ACH: CFM = 1,296 x 5 / 60 = 108 CFM. ASHRAE 62.2 recommends minimum 0.35 ACH for the whole house plus 7.5 CFM per person as supplemental outdoor air.
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