Calculate the dehumidifier airflow (CFM) a room needs from its dimensions or volume at 3 air changes per hour, or the room volume an airflow can serve.
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Dehumidifier Airflow (ACH) Formula
This calculator uses 3 air changes per hour (ACH) as the default target. It calculates either the airflow rate needed for a room or the room volume that a given airflow rate can handle.
With the calculator default of 3 ACH, the formulas become:
- CFM = airflow rate in cubic feet per minute
- V = total room or space volume in cubic feet
- ACH = air changes per hour
- 60 = minutes per hour
Choose Airflow to enter room dimensions or volume, or Room volume to enter airflow. Results use 3 ACH unless you enter another rate. Inputs are converted with exact foot/metre factors and then displayed in the selected result unit. This does not establish water-removal capacity or a maximum suitable room size.
Illustrative Air-Circulation Rates
The rates below illustrate airflow calculations only. They are not dehumidifier sizing recommendations; choose the circulation target for the actual design and assess moisture-removal capacity separately.
| Condition | Illustrative ACH | Notes |
|---|---|---|
| Example 1 | 1 to 2 ACH | At 1 to 2 ACH, Q = V/60 to V/30 per minute. |
| Calculator assumption | 3 ACH | This is the value used by the calculator. |
| Example 2 | 3 to 5 ACH | At 3 to 5 ACH, Q = V/20 to V/12 per minute. |
| Example 3 | 5+ ACH | At 5 ACH, Q = V/12 per minute; higher targets require proportionally more airflow. |
Quick Volume and Airflow Reference at 3 ACH
| Space Volume | Required Airflow at 3 ACH |
|---|---|
| 500 ft³ | 25 CFM |
| 1,000 ft³ | 50 CFM |
| 2,000 ft³ | 100 CFM |
| 3,000 ft³ | 150 CFM |
| 5,000 ft³ | 250 CFM |
Example Calculations
Example 1: Find the airflow rate
You have a room with a total volume of 2,400 ft³. Using 3 ACH:
The required airflow rate is 120 CFM.
Example 2: Find the supported volume
You have a dehumidifier airflow rate of 80 CFM. Using 3 ACH:
80 CFM corresponds to 1,600 ft³ at the assumed 3 ACH; moisture-removal suitability requires a separate assessment.
FAQ
What does ACH mean for a dehumidifier?
ACH means air changes per hour. For dehumidification, it describes how many times the air volume of the space passes through the dehumidifier each hour. A target of 3 ACH means the airflow should move three times the room volume in one hour.
How do I find the total volume of a room?
For a rectangular room, multiply length by width by height. For example, a 20 ft by 15 ft room with an 8 ft ceiling has a volume of 2,400 ft³. For irregular spaces, split the area into smaller rectangular sections, calculate each section, then add them together.
Is airflow the same as dehumidifier capacity?
No. Airflow is how much air the unit moves, usually in CFM or m³/min. Dehumidifier capacity is how much water the unit can remove, often listed in pints per day or liters per day. Both matter. A unit can have enough water-removal capacity but still perform poorly if air does not circulate well through the space.
