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:
- 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.
- 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.
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
Example: What is the CFM of air moving through an 8 × 10 inch duct if the air velocity is 800 FPM? Let’s put all of this in the equation like this:
CFM (8×10 Duct) = 800 FPM × 0.66 Ft × 0.83 Ft = 438 CFM
In short, an 8×10 inch duct at 700 FPM air velocity has a 438 CFM air flow.
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:
- 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.
- 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.