Air Flow Calculator - CFM, Velocity, Duct Sizing and Fan Power
Calculate airflow in CFM, m³/h, L/s and m³/s from duct velocity, size a duct from a target flow rate, work out air changes per hour, or estimate fan power. Five calculation modes, live duct sizing, and velocity warnings based on your application.
Advanced options (air density correction)
Result Summary
Detailed Breakdown
| Parameter | Value | Notes |
|---|
Using Air Flow Calculator for Duct Design, Velocity and Fan Sizing
Pick the calculation you need, enter the values you have, and get every airflow unit, equivalent duct size, and engineering check you need - all in one step.
Recommended Duct Velocities by Application
Choosing the right air velocity is the balance between duct size, noise, and pressure drop. Higher velocity means smaller ducts but more noise and higher fan power. These are the industry-standard ranges.
| Application | Velocity (m/s) | Velocity (fpm) | Noise level |
|---|---|---|---|
| Residential supply / return | 4 - 6 | 800 - 1,200 | Quiet |
| Commercial supply | 5 - 8 | 1,000 - 1,600 | Low |
| Main trunk duct | 6 - 10 | 1,200 - 2,000 | Medium |
| Branch duct | 3 - 6 | 600 - 1,200 | Low |
| Industrial exhaust | 10 - 20 | 2,000 - 4,000 | Loud |
| Laboratory exhaust | 8 - 12 | 1,600 - 2,400 | Medium |
| Cleanroom supply | 0.3 - 0.6 | 60 - 120 | Very quiet |
| Grille face velocity | 2 - 5 | 400 - 1,000 | Application dependent |
| Filter face velocity | 0.5 - 2.5 | 100 - 500 | Very quiet |
| HEPA filter face | 0.3 - 0.6 | 60 - 120 | Very quiet |
Air Changes Per Hour (ACH) Reference
ACH tells you how many times the entire volume of air in a room is replaced in one hour. It is the most common ventilation requirement in building codes. Higher ACH means better air quality but more energy use.
| Room type | Recommended ACH | Why it matters |
|---|---|---|
| Bedroom | 2 - 4 | Comfort, fresh air for sleep |
| Living room | 4 - 6 | Odour removal, comfort |
| Office | 6 - 8 | CO₂ control, productivity |
| Conference room | 8 - 12 | High occupancy, CO₂ control |
| Classroom | 6 - 10 | High occupancy, learning environment |
| Hospital ward | 6 - 8 | Infection control, comfort |
| Operating theatre | 15 - 25 | Sterile field, positive pressure |
| Cleanroom ISO 7 | 30 - 60 | Particle control |
| Laboratory | 6 - 12 | Fume and vapour removal |
| Commercial kitchen | 15 - 60 | Heat, smoke and odour extraction |
| Bathroom | 6 - 10 | Moisture and odour removal |
| Server room | 20 - 30 | Heat removal, equipment cooling |
Round Duct Size Chart
Standard round duct sizes and the airflow they handle at three common velocities. Use this as a quick sanity check on your calculated duct size.
| Duct diameter | Cross-section area | CFM at 3 m/s (600 fpm) | CFM at 5 m/s (1,000 fpm) | CFM at 7 m/s (1,400 fpm) |
|---|---|---|---|---|
| 100 mm (4 in) | 0.008 m² | 50 | 85 | 120 |
| 150 mm (6 in) | 0.018 m² | 110 | 190 | 265 |
| 200 mm (8 in) | 0.031 m² | 200 | 330 | 465 |
| 250 mm (10 in) | 0.049 m² | 310 | 520 | 730 |
| 300 mm (12 in) | 0.071 m² | 450 | 750 | 1,050 |
| 350 mm (14 in) | 0.096 m² | 610 | 1,020 | 1,430 |
| 400 mm (16 in) | 0.126 m² | 800 | 1,330 | 1,860 |
| 500 mm (20 in) | 0.196 m² | 1,250 | 2,080 | 2,900 |
| 600 mm (24 in) | 0.283 m² | 1,790 | 3,000 | 4,200 |
How the Air Flow Calculator Works
Every result on this page uses the standard fluid dynamics equations that HVAC engineers work from. Here is the exact process behind each mode.
- Flow mode takes your air velocity and duct dimensions, converts them to metres and metres per second, then multiplies cross-sectional area by velocity to give volumetric flow.
- Duct sizing mode takes your required flow and target velocity, divides flow by velocity to get required cross-section area, then converts that area to a recommended standard duct size.
- Face velocity mode divides your airflow by grille face area to give the face velocity in metres per second, then compares it to the recommended range for your grille type.
- Air changes mode either multiplies room volume by target ACH to find required airflow, or divides actual airflow by room volume to find the ACH you are achieving.
- Fan power mode uses the air power formula (flow × pressure) and divides by fan and drive efficiency to size the motor.
All modes adjust air density based on your temperature and altitude inputs. Standard sea-level air at 20 °C has a density of 1.204 kg/m³. Higher temperatures and higher altitudes both reduce density, which reduces mass flow for the same volumetric flow.
Air Flow Formulas Used
These are the exact formulas the calculator uses. Bookmark this section if you want to verify the maths yourself.
- Volumetric flow = velocity × cross-section area
- Round duct area = π × (diameter / 2)²
- Rectangular duct area = width × height
- Required duct area = flow / target velocity
- Round duct diameter from area = 2 × √(area / π)
- Equivalent round diameter (rect) = 1.30 × (W × H)^0.625 / (W + H)^0.25
- Air changes per hour = (airflow × 60) / room volume
- Required airflow for ACH = room volume × ACH / 60
- Air density (altitude) = 1.204 × (1 - 2.25577 × 10^-5 × altitude)^4.2559
- Air power = flow (m³/s) × pressure (Pa)
- Shaft power = air power / fan efficiency
- Motor power = shaft power / drive efficiency
Worked example: A 300 mm round duct with air flowing at 5 m/s. Area = π × 0.15² = 0.0707 m². Flow = 0.0707 × 5 = 0.3535 m³/s = 1,273 m³/h = 749 CFM. That is enough to ventilate a 60 m³ office (4 × 5 × 3 m) at 21 ACH, or a 200 m³ home living area at 6.4 ACH.
Frequently Asked Questions About Air Flow and Duct Sizing
What is the difference between CFM and m³/h?
Both measure volumetric airflow. CFM is cubic feet per minute (imperial), and m³/h is cubic metres per hour (metric). One m³/h equals 0.5886 CFM. One CFM equals 1.699 m³/h. To convert CFM to m³/h, multiply by 1.699. To convert m³/h to CFM, multiply by 0.5886. The calculator shows both units together.
How do I calculate CFM from duct size and velocity?
Multiply the duct cross-section area (in square feet) by the air velocity (in feet per minute). For a round duct: area = π × (diameter in feet / 2)². For a rectangular duct: area = width × height in feet. The calculator above does this automatically - just enter your velocity and duct dimensions in Flow mode.
What size duct do I need for 500 CFM?
It depends on your target velocity. At a residential target of 5 m/s (1,000 fpm), you need about 0.24 m² of cross-section, which is a 250 mm (10 inch) round duct or a 350 × 200 mm rectangular duct. At commercial velocity (7 m/s), a 200 mm (8 inch) round duct is enough. The Duct Size mode above gives exact recommendations.
How do I calculate air changes per hour (ACH)?
Divide the total airflow supplied to the room by the room volume, then multiply by 60 to convert from minutes to hours. The formula is ACH = (airflow in m³/h) / (room volume in m³). If airflow is given in CFM, use ACH = (CFM × 60) / room volume in cubic feet. The Air Changes mode above handles both directions.
What is face velocity and why does it matter?
Face velocity is the speed of air passing through a grille, diffuser, or filter. It is measured at the face of the grille. Face velocity matters because higher velocities create noise, while too low a velocity fails to distribute air properly. Supply diffusers work best between 2 - 4 m/s; filters between 0.5 - 2.5 m/s; HEPA filters between 0.3 - 0.6 m/s. The Face mode above checks your grille against the correct range.
How do I size a fan for my duct system?
Fan sizing has two parts: airflow and pressure. Airflow comes from your CFM/m³/h requirement. Pressure comes from the total static pressure of the ductwork (friction loss plus fittings plus filters). The Fan Power mode above takes both, applies your fan efficiency and motor efficiency, and gives you the required shaft and motor power in watts and horsepower.
What is the recommended duct velocity for a home?
For residential HVAC, use 4 - 6 m/s (800 - 1,200 fpm) for supply and return ducts. Main trunk ducts can run at up to 6 m/s. Branch ducts (going to individual rooms) should be 3 - 4 m/s to keep noise low. Higher velocities make ducts smaller but noisier.
What is the equivalent diameter of a rectangular duct?
The equivalent diameter is the round duct that would have the same friction loss as a rectangular duct. For a 400 × 200 mm rectangular duct, the equivalent round diameter is about 305 mm. The formula is D = 1.30 × (W × H)^0.625 / (W + H)^0.25. The calculator shows both the actual recommended rectangular size and its equivalent round diameter.
Why does altitude affect airflow?
At higher altitudes, air is less dense. This means the same volumetric flow carries less mass flow. Fans must spin faster (or be larger) to move the same mass of air. The calculator corrects for altitude using the standard atmosphere formula so your fan power estimate is accurate whether you are at sea level or in Denver (1,600 m) or Mexico City (2,240 m).
How does temperature affect air flow?
Hot air is less dense than cold air. At 100 °C, air density is about 0.95 kg/m³ vs 1.204 kg/m³ at 20 °C. This means the same volume of hot air weighs less and carries less heat energy. For HVAC design, always use the actual operating temperature to size fans and ducts correctly. The calculator corrects for temperature automatically.
What is total static pressure and how do I estimate it?
Total static pressure is the sum of all resistances in the duct system: friction in straight duct, losses at elbows and fittings, losses through filters, and losses at grilles. For a typical residential system, total static pressure is 250 - 500 Pa. For commercial systems, 500 - 1,500 Pa. For industrial systems, 1,500 - 3,000+ Pa. Your duct designer or supplier should provide exact figures.
What is the ideal ACH for a bedroom?
Bedrooms need 2 - 4 ACH for comfort and fresh air. The main goal is to remove CO₂ and moisture while maintaining temperature. If you have allergies or the bedroom is occupied by multiple people, aim for 4 ACH. If you are designing a passive house or very low energy building, 0.5 - 1 ACH with heat recovery is standard.
How do I know if my duct velocity is too high?
High velocity shows up as: (1) whistling or rushing noise at grilles, (2) whistling at duct fittings, (3) excess pressure drop that starves downstream rooms, or (4) vibration in flexible ducts. The calculator flags any velocity outside the recommended range for your application with a warning badge.
What is the difference between supply, return and exhaust airflow?
Supply air is conditioned air delivered into a room. Return air is air drawn back to the air handler for reconditioning. Exhaust air is air expelled to the outside. Balanced systems have supply roughly equal to return plus exhaust. Kitchens and bathrooms are usually exhaust-dominated; living areas are supply-dominated.
How do I convert between CFM, m³/h, L/s and m³/s?
1 m³/s = 3,600 m³/h = 1,000 L/s = 35.31 ft³/s = 2,119 CFM. 1 m³/h = 0.2778 L/s = 0.5886 CFM. 1 L/s = 3.6 m³/h = 2.119 CFM. 1 CFM = 1.699 m³/h = 0.4719 L/s = 0.000472 m³/s. The calculator shows all of these units together for every result.
References & Standards
- ASHRAE Handbook - Fundamentals (Chapter 21: Duct Design)
- ASHRAE Standard 62.1 - Ventilation for Acceptable Indoor Air Quality
- ASHRAE Standard 55 - Thermal Environmental Conditions for Human Occupancy
- SMACNA HVAC Duct Construction Standards - Metal and Flexible
- CIBSE Guide B - Heating, Ventilating, Air Conditioning and Refrigeration
- ISO 5801 - Industrial fans: Performance testing using standardized airways
- AMCA 210 - Laboratory Methods of Testing Fans for Certified Aerodynamic Performance Rating
- EN 13779 - Ventilation for non-residential buildings
- ISO 14644-1 - Cleanrooms and associated controlled environments
- NSW / AS 1668 - The use of ventilation and air conditioning in buildings