CFM Calculator (Air Changes per Hour)

A CFM calculator returns the airflow a fan needs, in cubic feet per minute, for a target number of air changes per hour (ACH). It multiplies the room's volume by the target ACH, then divides by 60. A 10 ft × 10 ft room with an 8 ft ceiling needs 80 CFM at 6 ACH.

Last reviewed September 2026Method & sources Report an error

Airflow (CFM)

80 CFM

Breakdown
Airflow (CFM)	80 CFM
Air Changes per Hour	6.00
Room Volume	800 cu ft

How to Calculate CFM

Room volume (length × width × height) times the target number of air changes per hour gives the total air volume that needs to move through the room every hour. Dividing by 60 converts that hourly figure into cubic feet per minute (CFM), the unit fans and ventilation equipment are rated in. The same relationship runs the other direction too. Given a fan's actual rated CFM, multiplying by 60 and dividing by the room's volume gives the air changes per hour that fan actually delivers in that specific room. That helps you check whether a fan you already have, or are considering, is adequate before you buy based on CFM alone.

Example: Finding CFM for a 10 ft × 10 ft room with an 8 ft ceiling at a target

Finding CFM for a 10 ft × 10 ft room with an 8 ft ceiling at a target of 6 air changes per hour: 1. Room volume: 10 × 10 × 8 = 800 cu ft 2. Air volume needed per hour: 800 × 6 = 4,800 cu ft/hr 3. CFM: 4,800 ÷ 60 = 80 CFM Running it the other way, checking a 100 CFM fan in that same 800 cu ft room: 1. Air volume moved per hour: 100 × 60 = 6,000 cu ft/hr 2. ACH achieved: 6,000 ÷ 800 = 7.5 air changes per hour

CFM: Compare Scenarios

Scenario	Airflow (CFM)	Air Changes per Hour	Room Volume
Small bathroom (5×7 ft, 7 ft ceiling, ACH 8)	32.7 CFM	8.00	245 cu ft
Medium bedroom (10×10 ft, 8 ft ceiling, ACH 6)	80 CFM	6.00	800 cu ft
Large great room (20×15 ft, 9 ft ceiling, ACH 4)	180 CFM	4.00	2,700 cu ft

What Is CFM?

A CFM calculator converts a room's size and a target ventilation rate into the airflow rating a fan or exhaust vent needs to hit that target. The ventilation rate is air changes per hour (ACH), meaning how many times the room's full air volume is replaced each hour. Run in reverse, the calculator converts a specific fan's known CFM rating into the ACH it actually achieves in a given room. Different spaces call for very different ACH targets. A bathroom exhaust fan needs much more frequent air changes than a general living space.

CFM: Pros and Cons

Pros

Applies the standard, universal CFM formula directly, with no hidden assumptions about room shape or fan type
Air changes per hour is a direct, adjustable input, since the right target varies enormously by room type and use
Reports room volume alongside the CFM result, useful context for choosing a properly-sized fan

Cons

Doesn't recommend an ACH target for you. That depends heavily on room type (bathroom, kitchen, whole-house ventilation), so check your local code or a ventilation guideline for your specific application
Doesn't account for duct losses, filter resistance, or other factors that reduce a fan's rated CFM in actual installed conditions. A fan's rated CFM is usually higher than what it delivers once ducted
Assumes a simple rectangular room. An irregular room needs its volume estimated separately

CFM: What a Pro Checks

Size a fan somewhat above your calculated CFM need. A fan's rated CFM is measured under ideal, unrestricted conditions, and actual delivered airflow usually drops once it's installed with ductwork, elbows, or a filter.
Don't just buy the biggest fan available. An oversized fan for the space can be noisier, use more energy, and in some cases pull conditioned air out faster than needed.
Check your local building code for the right ACH target before picking a CFM. The appropriate target varies enormously by room type, and a bathroom or kitchen typically needs a much higher rate than a general living space.

Frequently Asked Questions

How do I calculate CFM for a room?

Multiply the room's volume (length × width × height) by your target air changes per hour, then divide by 60. A 10 ft × 10 ft room with an 8 ft ceiling targeting 6 air changes per hour needs 80 CFM.

What air changes per hour should a bathroom fan have?

Bathroom exhaust fans are commonly sized for a fairly high ACH target, often well above what a general living space needs, since they need to quickly clear moisture and odors. Check your local building code and the fan manufacturer's sizing guidance for your specific bathroom size.

Why does a fan's rated CFM not match what I actually get?

A fan's rated CFM is typically measured under ideal, unrestricted conditions. Once it's installed with ductwork, elbows, a filter, or a long duct run, actual delivered airflow is usually lower than the rating. That's why it's common to size a fan somewhat above your calculated CFM need.

What does air changes per hour (ACH) mean?

ACH is how many times a room's entire air volume is replaced with new air in one hour. A higher ACH means faster, more frequent air turnover. That matters in a space that needs to clear moisture, odors, or contaminants quickly, like a bathroom or kitchen.

Is a higher CFM fan always better?

Not necessarily. An oversized fan for the space can be noisier, use more energy, and in some cases pull conditioned air out faster than needed, reducing comfort and efficiency. Sizing to your actual room volume and appropriate ACH target gives a better result than buying the biggest fan available.

How do I find the ACH a specific fan actually delivers?

Switch this calculator to "ACH from known CFM," enter the fan's rated CFM and your room's dimensions. It runs the same formula in reverse (CFM × 60 ÷ room volume) to tell you the air changes per hour that fan achieves in that specific room.

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