CFM or Cubic Feet per Minute is a unit for airflow we use in HVAC calculation. Most commonly, we need to calculate CFM for a room for fans, air purifiers, air conditioners, and so on.
Example of a question LearnMetrics’s received: We have a 300 square foot standard bedroom. How much CFM should a fan for such a room have if we’re looking to completely change all air 2 times per hour (every 30 minutes)?
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CFM Calculation: Airflow has to be strong enough to change the complete volume of 300 sq ft room (with 8 sq ft ceiling height) 2 times per hour. Volume of a room = 300 sq ft x 8 ft = 2,400 ft3. To change it 2 times per hour (ACH = 2), we need to deliver 4,800 ft3 per hour. CFM is a ‘ft3 per minute’ unit. That’s why we need to divide the total volume by 60; hence 4,800/60 = 80 CFM.
Answer: You need an 80 CFM airflow (for 300 sq ft standard room and 2 ACH).
Here’s a neat CFM calculator that calculates CFM based on room area, ceiling height, and the number of air changes per hour (ACH).
Below the calculator, we will demonstrate how the cubic feet per minute calculator works by solving one example using the calculator and CFM formula. You will also find a CFM chart, with airflow in CFM calculated for areas between 100 sq ft and 3,000 sq ft (useful for ductwork as well) further on.
CFM Airflow Calculator
How To Calculate CFM For A Room? (Solved Example)
Let’s say we have a big 1,000 sq ft room with standard 8 ft high ceiling. We want to calculate the CFM of a fan that will exchange all the air in such a room every 15 minutes (ACH = 4).
We can use calculate fan CFM in two ways:
- Use the CFM formula. This is how to calculate CFM of the room.
- Use the room CFM calculator above.
This comes quite useful when you are trying to figure out which HVAC units to buy. Here is an example of how to select the best ceiling fans based on CFMs.
To demonstrate how to use the CFM calculator to calculate fan airflow, we’ll start by using the calculator. Here are the results:
The result is clear. For a 1,000 sq ft room with an 8 ft ceiling and 4 ACH, you need a fan capable of delivering 533 CFM airflow.
Let’s use the CFM calculation formula to see if we get the same number (this is the very formula used in the calculator):
CFM = (Area x Height x ACH) / 60
If we input the figures from our example, we get:
CFM = (1,000 sq ft * 8 ft * 4) / 60 min = 533 ft3/min = 533 CFM
In short, we get the same number.
You can check this list to get an idea of how much CFM the best air purifiers can produce (for reference). It is also interesting to see how the CADR rating for dust-removing air purifiers depends on airflow.
You can freely use the CFM calculator to calculate airflow for any room, and for any ACH. To help you out, we have created a CFM chart where we calculated CFM for the most common room sizes:
CFM Chart For Common Room Sizes
In all these calculations, we predispose 8 ft ceiling height and use 2 ACH. If you want to use other ACH values, you can use the CFM calculator above. For ACH calculation based on CFM, you are free to use the ACH calculaton here.
How Many CFM Per Square Foot
One of the most common questions is how many CFM airflow do we need per sq ft. Obviously, that depends on the ceiling height and ACH. If we predispose 8 ft ceiling height, we can calculate CFM per sq ft for different values of ACH:
- 0.13 CFM per square foot at ACH = 1.
- 0.27 CFM per square foot at ACH = 2.
- 0.40 CFM per square foot at ACH = 3.
- 0.53 CFM per square foot at ACH = 4.
- 0.67 CFM per square foot at ACH = 5.
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ACH is very important for air purifiers, for example. Allergy-prone people will most benefit from the best H13 HEPA air purifiers for allergies; the coverage area of those units needs to be calculated for 5 ACH. On the other hand, even the best air purifiers for mold will require at most 4 ACH. In practice, the coverage area of these units is calculated at 2 ACH.
Another example are bathroom exhaust fans. When sizing a bathroom fan, you need to take into account 8 ACH. You can check how many CFM for the bathroom exhaust fan you need here (both the calculation and application of 8 ACH).
If anything is unclear here, you can pose the question in the comments and we’ll help you out.
Duct Diameter CFM Chart
For everybody who needs CFM calculation for ductwork, you will also need the duct diameter to achieve that airflow.
Example: If we need a 300 CFM airflow, we’ll need a 10-inch flex duct diameter.
To correctly size your ductwork, you can reference the CFM sizing chart here:
Using this duct CFM chart, you can properly estimate how big ducts you need to deliver the airflow needed.
Other Airflow Units Like L/Min Or Cubic Meters Per Hour
CFM is an imperial unit, commonly used in the US. If you are using other units, like l/min or m3/h, you can use these unit-to-unit relations to translate other units in CFM.
1 CFM = 1.699 m3/h 1 CFM = 28.317 l/minute
If you have any problems using the calculator, you can use the comments to give us some numbers and we’ll try our best to help you out.
Note: What is CFM? CFM is cubic feet per minute; it is a unit that measures airflow.
Source: https://gardencourte.com
Categories: Kitchens
