CFM Calculator
Find the airflow in cubic feet per minute (CFM) a fan or ventilation system needs for a room from its size and air changes per hour (ACH), or find ACH from CFM.
Results
| Item | Value |
|---|
CFM at other air change rates
| ACH | CFM needed |
|---|
Quick Answer
CFM = room volume (cubic feet) × air changes per hour ÷ 60. A 15 × 12 ft room with an 8 ft ceiling has 1,440 cubic feet. To change the air 6 times an hour, you need 144 CFM.
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Table of Contents
- CFM Calculator
- Quick Answer
- Related Calculators
- How This CFM Calculator Works
- How to Use the CFM Calculator
- Core Formulas
- Key Terms
- Worked Example
- Real-Life Examples
- CFM vs ACH
- CFM by Room Size at 6 ACH
- What Affects the Airflow You Need
- Common Mistakes to Avoid
- Frequently Asked Questions
- References & Sources
How This CFM Calculator Works
Enter the room’s length, width and ceiling height to get its volume. Then enter how many times per hour you want the air replaced (ACH), and the calculator gives the airflow in cubic feet per minute. You can also enter a fan’s CFM to see how many air changes per hour it gives the room.
Quick explanation: CFM is how much air moves each minute. ACH is how often the room’s air is fully replaced.
- Bigger rooms: need more CFM for the same ACH.
- Higher ACH: fresher air, but more fan power.
- 60: converts air changes per hour to per minute.
How to Use the CFM Calculator
- Choose whether you want CFM or air changes per hour.
- Enter the room length, width and ceiling height in feet.
- Enter the air changes per hour, or the fan’s CFM.
- Press Calculate.
- Compare the CFM needed at other air change rates in the table.
Core Formulas
- Volume: length × width × height
- CFM: volume × ACH ÷ 60
- ACH: CFM × 60 ÷ volume
- Minutes per air change: 60 ÷ ACH
Key Terms
- CFM: cubic feet per minute, a measure of airflow.
- ACH: air changes per hour, how often a room’s air is replaced.
- Static pressure: resistance to airflow from ducts, filters and grilles.
- Makeup air: outside air that replaces air removed by exhaust fans.
- ASHRAE 62.2: a US standard for ventilation in homes.
Worked Example
A 30 × 20 ft workshop with 10 ft ceilings needs 4 air changes per hour.
- Volume = 30 × 20 × 10 = 6,000 cu ft
- CFM = 6,000 × 4 ÷ 60 = 400 CFM
- The air is replaced every 15 minutes
Real-Life Examples
- A garage workshop: 24 × 24 ft with 9 ft ceilings at 6 ACH needs 518 CFM.
- A grow room: 10 × 10 × 8 ft at 15 ACH needs 200 CFM.
- Checking a fan: a 110 CFM fan in a 10 × 8 × 8 ft room gives 10.3 air changes per hour.
- An office: 20 × 15 × 9 ft at 4 ACH needs 180 CFM.
CFM vs ACH
- CFM: the volume of air a fan moves each minute. It is what fans are rated in.
- ACH: how many times the room’s full volume of air is replaced each hour.
- Link: CFM = volume × ACH ÷ 60, so the same CFM gives fewer air changes in a bigger room.
CFM by Room Size at 6 ACH
| Room | Volume | CFM |
|---|---|---|
| 8 × 8 ft | 512 cu ft | 51 |
| 10 × 10 ft | 800 cu ft | 80 |
| 12 × 12 ft | 1,152 cu ft | 115 |
| 15 × 12 ft | 1,440 cu ft | 144 |
| 20 × 15 ft | 2,400 cu ft | 240 |
| 24 × 24 ft | 4,608 cu ft | 461 |
| 30 × 20 ft | 4,800 cu ft | 480 |
What Affects the Airflow You Need
- Room use: kitchens, bathrooms, workshops and garages need more ventilation.
- Codes: local codes and standards such as ASHRAE 62.2 set minimums for homes.
- Duct losses: long or narrow ducts reduce a fan’s real airflow.
- Moisture and fumes: sources like showers and paint need extra exhaust.
Common Mistakes to Avoid
- Using floor area instead of volume. Include the ceiling height.
- Trusting the box rating. Real airflow drops with ducts and filters.
- Ignoring makeup air. Air pushed out must be replaced from somewhere.
- Mixing units. Enter all dimensions in feet.
- Skipping code checks. Bathrooms and kitchens often have minimum airflow rules.
Frequently Asked Questions
Multiply length × width × height to get cubic feet, multiply by the air changes per hour you want, then divide by 60.
ACH = CFM × 60 ÷ room volume in cubic feet.
Cubic feet per minute.
It depends on the room’s use and local code. Kitchens, bathrooms and workshops usually need more than bedrooms or living rooms.
Ducts, elbows and filters add resistance, which lowers airflow. Check the rating at the expected static pressure.
No. Too much airflow wastes energy and can cause drafts or noise.
Multiply CFM by about 1.699.
Check your local code. Many codes are based on ASHRAE 62.2, which sets minimum bathroom exhaust rates.
Yes. Taller ceilings mean more volume and more CFM for the same ACH.
For whole-house ventilation, codes use other methods based on floor area and bedrooms. Ask an HVAC professional.
Many people aim for several air changes per hour, more when using paints or fuel. Follow product and safety instructions.
Split the room into rectangles, find each volume, and add them up.
References & Sources
| Source | Used For | Link |
|---|---|---|
| Wikipedia, Air changes per hour | The formula linking CFM, volume and ACH. | https://en.wikipedia.org/wiki/Air_changes_per_hour |
| U.S. Department of Energy, Meeting ASHRAE 62.2 in multifamily buildings | Ventilation standards for homes. | https://www.energy.gov/eere/buildings/articles/best-way-meet-ashrae-622-multifamily-buildings |