Convert FPM To CFM: Air Velocity To Air Flow Calculator + Chart

Air velocity (FPM) and air flow (CFM) are closely connected. In many cases, you will need to convert FPM to CFM; basically, you are converting the speed of air per minute to the volume of air per minute. We are going to help you out with the FPM to CFM conversion step-by-step.

First off, we need to distinguish between two kinds of ducts that air flow through:

  1. Round or circular ducts. We have many different round duct diameters; from 2-inch to 20+ inch circular ducts. We will derive CFM to CFM formula for round ducts. Further on, you will also find an FPM To CFM Calculator (1st Calc) (just insert FPM and diameter, get CFM automatically) and accompanying FPM To CFM Chart for circular ducts with 4-inch, 6-inch, 8-inch, and 10-inch diameters.
  2. Rectangular ducts. We can have square ducts (4×4, 6×6, 8×8, 10×10, etc.) and rectangular ducts (4×6, 6×10, 8×12, for example). Here we will need both the width and height of the duct to adequately convert FPM to CFM. To make things easier, you will find the FPM To CFM Calculator (2nd Calc) for rectangular ducts (you insert FPM, duct width, and duct height, and the calculator will determine CFM). To supplement the calculator, we have also calculated FPM to CFM for 4×10, 6×12, 8×12, and 12×12 ducts from 1 FPM to 5000 FPM.
fpm to cfm for round and rectangular ducts
To adequately and precisely convert FPM to CFM, we need to distinguish between round and rectangular ducts.

Here are the quantities we are converting:

  • FPM or Feet Per Minute is a unit of air velocity. 1 FPM means that the air will move for 1 feet in 1 minute (that’s a very low air velocity).
  • CFM or Cubic Foot Per Minute is a unit of air flow. It determines the volume of air that is being moved. 1 CFM means that 1 cubic foot of air has been moved in 1 minute.

To convert FPM to CFM, we will use this general equation:

Air Flow (CFM) = Air Velocity (FPM) × Area (Square Feet)

We understand what CFM and FPM mean. We only have to figure out the ‘Area’ in the equation. This is the area through which air is moving. Based on how we calculate this area, we have to distinguish between round ducts and rectangular ducts. Here are the equations for both of these areas:

CFM (Round Ducts) = FPM × π × (Duct Diameter In Feet / 2)2

CFM (Rectangular Ducts) = FPM × Duct Width In Feet × Duct Height In Feet

Notice that, for manual FPM to CFM conversion, we need to convert all the units into feet (duct sizes are usually expressed in inches; 1 ft = 12 inches).

Here are two quick examples:

  1. How many CFM can a round duct with a 6-inch diameter have at 500 FPM carry? Just insert this in the equation (6 inches = 0.5 ft) like this: CFM (6″ Round Duct) = 500 FPM × 3.14 × (0.5 Ft / 2)2 = 98.15 CFM. In this round duct, 500 FPM is equal to 98.15 CFM.
  2. How many CFM can a rectangular duct with a 4-inch width and 12-inch height carry at 700 FPM? The same thing, just use the rectangular CFM equation like this: CFM (4×12 Duct) = 700 FPM × 0.33 Ft × 1 Ft = 233.33 CFM. In this rectangular duct, 700 FPM is equal to 233.33 CFM.

These are just two examples. To avoid doing these calculations by hand, we have prepared 2 calculators (1st calculator for round ducts, 2nd calculator for rectangular ducts) that do all this math automatically. Below each calculator, you will also find FPM to CFM calculated conversion charts to help you out.

Here is the 1st calculator for circular or round ducts:

FPM To CFM Calculator For Circular Ducts (1st Calc)

Just insert FPM and the diameter of the round duct in inches, and the calculator will automatically output CFMs:

 

Here is how this calculator works:

Let’s say we have a 10-inch diameter round duct and want to know how many CFM of air flow it will produce if we move air within it with 400 FPM air velocity. Just slide the 1st slider to ’10’, the 2nd slider to ‘400’, and we get the result: 400 FPM in a 10-inch diameter round duct is 218.10 CFM.

Here is the full calculated table from 1 FPM to 5000 FPM converted to CFM for 4-inch, 6-inch, 8-inch, and 10-inch circular ducts:

FPM To CFM Chart For Circular Ducts (4-Inch, 6-Inch, 8-Inch, 10-Inch Round Ducts)

Feet Per Minute (FPM): CFM In 4-Inch Round Duct: CFM In 6-Inch Round Duct: CFM In 8-Inch Round Duct: CFM In 10-Inch Round Duct:
1 FPM 0.087 CFM 0.196 CFM 0.349 CFM 0.545 CFM
5 FPM 0.44 CFM 0.98 CFM 1.75 CFM 2.23 CFM
10 FPM 0.87 CFM 1.96 CFM 3.49 CFM 5.45 CFM
20 FPM 1.74 CFM 3.93 CFM 6.98 CFM 10.91 CFM
30 FPM 2.62 CFM 5.89 CFM 10.47 CFM 16.36 CFM
40 FPM 3.49 CFM 7.85 CFM 13.96 CFM 21.81 CFM
50 FPM 4.36 CFM 9.81 CFM 17.45 CFM 27.26 CFM
60 FPM 5.23 CFM 11.78 CFM 20.94 CFM 32.71 CFM
70 FPM 6.11 CFM 13.74 CFM 24.43 CFM 38.17 CFM
80 FPM 6.98 CFM 15.70 CFM 27.92 CFM 43.62 CFM
90 FPM 7.85 CFM 17.67 CFM 31.41 CFM 49.07 CFM
100 FPM 8.72 CFM 19.63 CFM 34.90 CFM 54.53 CFM
200 FPM 17.45 CFM 39.26 CFM 69.79 CFM 109.05 CFM
300 FPM 26.17 CFM 58.89 CFM 104.69 CFM 163.57 CFM
400 FPM 34.90 CFM 78.52 CFM 139.58 CFM 218.10 CFM
500 FPM 43.62 CFM 98.15 CFM 174.48 CFM 272.63 CFM
600 FPM 52.34 CFM 117.77 CFM 209.38 CFM 327.15 CFM
700 FPM 61.07 CFM 137.40 CFM 244.27 CFM 381.68 CFM
800 FPM 69.79 CFM 157.03 CFM 279.17 CFM 436.20 CFM
900 FPM 78.52 CFM 176.66 CFM 314.06 CFM 490.73 CFM
1000 FPM 87.24 CFM 196.29 CFM 348.96 CFM 545.25 CFM
2000 FPM 174.48 CFM 392.58 CFM 697.92 CFM 1090.50 CFM
5000 FPM 436.20 CFM 981.45 CFM 1744.80 CFM 2726.25 CFM

Using this chart, you can quite easily determine how many CFM can these 4, 6, 8, and 10-inch diameter round ducts carry. For example, air blasting through 6-inch round duct at 1000 FPM speed will produce an air flow equal to 196.29 CFM.

Let’s look at a similar calculator + chart for rectangular ducts:

FPM To CFM Calculator For Rectangular Ducts (2nd Calc)

In this 2nd calculator for rectangular ducts, you insert FPM and duct width as well as height. The calculator will determine the air flow (CFM) automatically:

 

Again, let’s look at an example: Let’s say we have a 6×10 duct with an air speed of 500 FPM. How many CFM will this air velocity in this rectangular duct produce? Slide the 1st slider to ‘500’, the 2nd slider to ‘6’, and the 3rd slider to ’10’.

You get the result: 500 FPM in a 6×10 duct is equal to 208.33 CFM.

Here is the promised FPM to CFM calculated tables for 4×10, 6×12, 8×12, and 12×12 ducts:

FPM To CFM Chart For Rectangular Ducts

Feet Per Minute (FPM): CFM In 4×10 Duct: CFM In 6×12 Duct: CFM In 8×12 Duct: CFM In 12×12 Duct:
1 FPM 0.278 CFM 0.500 CFM 0.667 CFM 1.000 CFM
5 FPM 1.39 CFM 2.50 CFM 3.33 CFM 5.00 CFM
10 FPM 2.78 CFM 5.00 CFM 6.67 CFM 10.00 CFM
20 FPM 5.56 CFM 10.00 CFM 13.33 CFM 20.00 CFM
30 FPM 8.33 CFM 15.00 CFM 20.00 CFM 30.00 CFM
40 FPM 11.11 CFM 20.00 CFM 26.67 CFM 40.00 CFM
50 FPM 13.89 CFM 25.00 CFM 33.33 CFM 50.00 CFM
60 FPM 16.67 CFM 30.00 CFM 40.00 CFM 60.00 CFM
70 FPM 19.44 CFM 35.00 CFM 46.67 CFM 70.00 CFM
80 FPM 22.22 CFM 40.00 CFM 53.33 CFM 80.00 CFM
90 FPM 25.00 CFM 45.00 CFM 60.00 CFM 90.00 CFM
100 FPM 27.78 CFM 50.00 CFM 66.67 CFM 100.00 CFM
200 FPM 55.56 CFM 100.00 CFM 133.33 CFM 200.00 CFM
300 FPM 83.33 CFM 150.00 CFM 200.00 CFM 300.00 CFM
400 FPM 111.11 CFM 200.00 CFM 266.67 CFM 400.00 CFM
500 FPM 138.89 CFM 250.00 CFM 333.33 CFM 500.00 CFM
600 FPM 166.67 CFM 300.00 CFM 400.00 CFM 600.00 CFM
700 FPM 194.44 CFM 350.00 CFM 466.67 CFM 700.00 CFM
800 FPM 222.22 CFM 400.00 CFM 533.33 CFM 800.00 CFM
900 FPM 250.00 CFM 450.00 CFM 600.00 CFM 900.00 CFM
1000 FPM 277.78 CFM 500.00 CFM 666.67 CFM 1000.00 CFM
2000 FPM 555.56 CFM 1000.00 CFM 1333.34 CFM 2000.00 CFM
5000 FPM 1388.90 CFM 2500.00 CFM 3333.33 CFM 5000.00 CFM

If you have rectangular ducts with these dimensions, you can now just check the FPM to CFM conversion here. Example: How many CFM will 800 FPM in 8×12 duct produce? Simple. Check the table: The result is 533.33 CFM.

With these two calculators and accompanying charts, you are now fully equipped to convert FPM to CFM for all kinds of ducts. If you need any help at all, you can use the comments below, give us some numbers, and we will help you out with the conversion.

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