CFM Calculator: How To Calculate CFM? (CFM Calculation + Charts)

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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)?

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:

  1. Use the CFM formula. This is how to calculate CFM of the room.
  2. 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.

Room Size: CFM (At 2 ACH)
How many CFM for a 100 sq ft room? 27 CFM
How many CFM for a 200 sq ft room? 53 CFM
How many CFM for a 300 sq ft room? 80 CFM
How many CFM for a 400 sq ft room? 107 CFM
How many CFM for a 500 sq ft room? 133 CFM
How many CFM for a 600 sq ft room? 160 CFM
How many CFM for a 700 sq ft room? 187 CFM
How many CFM for a 800 sq ft room? 213 CFM
How many CFM for a 900 sq ft room? 240 CFM
How many CFM for a 1000 sq ft home? 267 CFM
How many CFM for a 1500 sq ft home? 400 CFM
How many CFM for a 2000 sq ft house? 533 CFM
How many CFM for a 2500 sq ft house? 667 CFM
How many CFM for a 3000 sq ft house? 800 CFM

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.

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:

Flex Duct Diameter: CFM (Airflow)
4-inch 20 CFM
5-inch 50 CFM
6-inch 80 CFM
7-inch 120 CFM
8-inch 170 CFM
9-inch 230 CFM
10-inch 300 CFM
12-inch 500 CFM
14-inch 740 CFM
16-inch 1050 CFM
18-inch 1400 CFM
20-inch 1875 CFM

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.

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