CFM Calculator — Cubic Feet per Minute Airflow
CFM (Cubic Feet per Minute) measures airflow volume in HVAC and ventilation systems. Proper CFM sizing ensures adequate air circulation, temperature control, and indoor air quality for residential and commercial spaces.
Why This Construction Metric Matters
Why: Correct CFM sizing prevents under-ventilation (stale air, moisture buildup) and over-sizing (wasted energy, noise). HVAC professionals use CFM to size fans, ducts, and equipment per ASHRAE standards.
How: CFM is calculated from room volume × air changes per hour ÷ 60, or from duct area × velocity. Duct sizing uses velocity limits (600–900 fpm residential) to balance capacity and noise.
- ●Match ACH to room type: bathroom 8, kitchen 15, bedroom 4 per ASHRAE
- ●Keep duct velocity 600–900 fpm for quiet residential operation
- ●HP = (CFM × Static Pressure) ÷ (6356 × Efficiency) for fan sizing
- ●Round ducts are more efficient than rectangular at same cross-section
Sample Projects - Click to Load
Bathroom Exhaust
Standard 50 CFM bathroom fan
Kitchen Range Hood
400 CFM kitchen exhaust
HVAC Supply Duct
8" round duct at 700 fpm
Server Room Cooling
High ACH data center
Workshop Ventilation
10 ACH workshop exhaust
Spray Booth Exhaust
100 ACH paint booth
🏠 Room Dimensions
Bathroom
Standard bathroom exhaust
Recommended: 8 ACH
⚠️Planning estimates only. Verify with a licensed engineer or contractor before construction.
📐 Construction Industry Facts
1 ton AC = 400 CFM; 12,000 BTU/hr cooling capacity
— ASHRAE
Residential duct velocity 600–900 fpm; commercial 1000–1500 fpm
— ACCA Manual D
HP = (CFM × SP) ÷ (6356 × η); typical fan efficiency 0.6–0.8
— Fan law
Room volume × ACH ÷ 60 = CFM; 1 ACH = full room air replaced per hour
— Ventilation
📋 Key Takeaways
- • CFM | Cubic feet per minute; airflow volume
- • Room method | CFM = (Volume × ACH) ÷ 60
- • Area method | CFM = Area × Velocity (fpm)
- • Duct | Diameter = √(4 × CFM ÷ (π × Velocity))
Did You Know?
Bathroom 8 ACH, Kitchen 15 ACH, Bedroom 4 ACH. Higher ACH = more ventilation.
Source: ASHRAE
Residential duct velocity 600–900 fpm; commercial 1000–1500 fpm. Higher = noise.
Source: HVAC
HP = (CFM × Static Pressure) ÷ (6356 × Efficiency). Typical efficiency 0.6–0.8.
Source: Fan law
Room volume = L × W × H. 1 ACH = full room air replaced once per hour.
Source: Ventilation
Round duct area = π × (D/2)². Velocity = CFM ÷ Area.
Source: Duct
Static pressure " WC. 0.1–0.5" typical residential; 0.5–1" commercial.
Source: Pressure
What is CFM (Cubic Feet per Minute)?
CFM measures airflow volume in HVAC and ventilation. It indicates cubic feet of air passing per minute. Proper CFM sizing ensures adequate circulation, temperature control, and air quality. Used for fans, ducts, and equipment selection.
🔬 Key Concepts
ACH
Air changes per hour. Room volume × ACH ÷ 60 = CFM.
Velocity
Feet per minute (fpm). CFM = Area × Velocity.
Static Pressure
Resistance in " WC. Affects fan power.
Duct Size
Larger duct = lower velocity, less noise.
How to Calculate CFM
Room Volume
Area & Velocity
Duct Diameter
Fan HP
When to Use a CFM Calculator
🏠 Residential
- • Bathroom exhaust
- • Kitchen hood
- • Whole-house
🏢 Commercial
- • Office HVAC
- • Restaurant
- • Retail
🏭 Industrial
- • Exhaust
- • Make-up air
- • Process
CFM Formulas Reference
ACH Reference
| Room | ACH | Description |
|---|---|---|
| 🚿 Bathroom | 8 | Standard bathroom exhaust |
| 🍳 Kitchen | 15 | Kitchen range hood and exhaust |
| 🛏️ Bedroom | 4 | Residential bedroom |
| 🛋️ Living Room | 6 | Residential living area |
| 💼 Office | 6 | Commercial office space |
| 🖥️ Server Room | 20 | Data center cooling |
| 🔧 Workshop | 10 | Workshop ventilation |
| 🚗 Garage | 6 | Residential garage |
| 🎨 Spray Booth | 100 | Paint spray booth exhaust |
| 📦 Warehouse | 4 | Large warehouse space |
| 🍽️ Restaurant | 20 | Commercial kitchen |
| 💪 Gym/Fitness | 8 | Fitness facility |
Duct Size Capacity
| Duct | Max CFM | Velocity |
|---|---|---|
| 4" round | 50 | 600 fpm |
| 5" round | 80 | 600 fpm |
| 6" round | 120 | 600 fpm |
| 7" round | 160 | 600 fpm |
| 8" round | 220 | 700 fpm |
| 10" round | 350 | 700 fpm |
| 12" round | 500 | 800 fpm |
| 14" round | 700 | 800 fpm |
| 16" round | 900 | 900 fpm |
| 18" round | 1150 | 900 fpm |
Important Considerations
⚠️ Common Mistakes
- • Undersized ducts cause noise and pressure drop
- • Ignoring static pressure in fan selection
- • Wrong ACH for room type
✓ Best Practices
- • Size ducts for 600–900 fpm residential
- • Match ACH to ASHRAE recommendations
- • Account for duct losses in long runs
⚠️ Disclaimer: CFM estimates are for planning. Consult ASHRAE, local codes, and HVAC professionals for final design. Actual performance depends on duct layout and equipment.