Air velocity (FPS) and airflow (CFM) are closely connected. In many cases, you will need to convert FPS to CFM; basically, you are converting the speed of air in Feet Per Second (FPS or ft/s) to airflow in Cubic Feet Per Minute (CFM or ft3/min). We are going to help you out with the FPS to CFM conversion step-by-step.
First of all, we need to distinguish between two kinds of ducts that airflow through (the area through which air travels is the key parameter here):
- Round or circular ducts. We have many different round duct diameters; from 2-inch to 20+ inch flexible circular ducts. We will derive the FPS to CFM formula for round ducts. Further on, you will also find an FPS To CFM Calculator (1st Calc) (just insert FPS and diameter, and get CFM automatically) and accompanying FPS To CFM Chart for circular ducts with 4-inch, 6-inch, 8-inch, 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 convert FPS to CFM adequately. To make things easier, you will find the FPS To CFM Calculator (2nd Calc) for rectangular ducts (you insert FPS, duct width, and duct height, and the calculator will determine CFM). To supplement the calculator, we have also calculated FPS to CFM for 4×10, 6×12, 8×12, and 12×12 ducts from 1 FPS to 500 FPS.
Here is the 1st calculator for flexible round calculators (with formula and conversion chart), followed by the 2nd calculator for rectangular ducts:
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 airflow it will produce if we move air within it with 50 FPS air velocity. Just slide the 1st FPS slider to ’50’, the 2nd duct diameter slider to ’10’, and we get this result (screenshot):
We can immediately see that 50 FPS in an 8-inch circular duct generates 1636.25 CFM of airflow.
Let’s have a look at the FPS to CFM formula for round ducts, and the 1st conversion chart:
FPS To CFM Formula For Round Ducts
Here are the quantities we are converting:
- FPS or Feet Per Second (ft/s) is a unit of air velocity. 1 FPS means that the air will move for 1 foot in 1 second (that’s a fairly high air velocity).
- CFM or Cubic Foot Per Minute ft3/min) 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 FPS to CFM, we will use this general equation:
Air Flow (CFM) = Air Velocity (FPS) × Area (Square Feet)
We understand what CFM and FPS 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 must distinguish between round and rectangular ducts.
On top of that, we have to convert seconds to minutes (FPS is in seconds, CFM is in minutes) as well as convert feet to inches (since duct dimensions are always expressed in inches, not feet). Here is the FPS to CFM formula for round ducts (d is the round duct diameter):
CFM (Round Ducts) = FPS × π × d2 × 60 / (4 × 144)
Example: What is the CFM of air moving through a 12-inch round duct if the air velocity is 20 FPS? Let’s put all of this in the equation like this:
CFM (12-Inch Round Duct, 20 FPS) = 20 FPS × π × (12 Inches)2 × 60 / (4 × 144) = 942.48 CFM
At 20 FPS air velocity, a 12-inch round duct will produce 942.28 CFM. You can use the 1st calculator above to confirm this result.
This is just 1 example for round ducts. We have calculated CFMs for 4-inch, 6-inch, 8-inch, and 10-inch round ducts at FPS ranging from 1 FPS to 500 FPS (bunch of calculations, honestly), and summarized the results in this 1st conversion chart:
FPS To CFM Chart For Circular Ducts (4-Inch, 6-Inch, 8-Inch, 10-Inch Round Ducts)
| Feet Per Second (FPS): | 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 FPS | 5.24 CFM | 11.78 CFM | 20.94 CFM | 32.72 CFM |
| 2 FPS | 10.47 CFM | 23.56 CFM | 41.89 CFM | 65.45 CFM |
| 3 FPS | 15.71 CFM | 35.34 CFM | 62.83 CFM | 98.17 CFM |
| 4 FPS | 20.94 CFM | 47.12 CFM | 83.78 CFM | 130.90 CFM |
| 5 FPS | 26.18 CFM | 58.90 CFM | 104.72 CFM | 163.62 CFM |
| 10 FPS | 52.36 CFM | 117.81 CFM | 209.44 CFM | 327.25 CFM |
| 15 FPS | 78.54 CFM | 176.71 CFM | 314.16 CFM | 490.87 CFM |
| 20 FPS | 104.72 CFM | 235.62 CFM | 418.88 CFM | 654.50 CFM |
| 30 FPS | 157.08 CFM | 353.43 CFM | 628.32 CFM | 981.75 CFM |
| 40 FPS | 209.44 CFM | 471.24 CFM | 837.76 CFM | 1309.00 CFM |
| 50 FPS | 261.80 CFM | 589.05 CFM | 1047.20 CFM | 1636.25 CFM |
| 60 FPS | 314.16 CFM | 706.86 CFM | 1256.64 CFM | 1963.50 CFM |
| 70 FPS | 366.52 CFM | 824.67 CFM | 1466.08 CFM | 2290.74 CFM |
| 80 FPS | 418.88 CFM | 942.48 CFM | 1675.52 CFM | 2617.99 CFM |
| 90 FPS | 471.24 CFM | 1060.29 CFM | 1884.96 CFM | 2945.24 CFM |
| 100 FPS | 523.60 CFM | 1178.10 CFM | 2094.40 CFM | 3272.49 CFM |
| 150 FPS | 785.40 CFM | 1767.15 CFM | 3141.59 CFM | 4908.74 CFM |
| 200 FPS | 1047.20 CFM | 2356.19 CFM | 4188.79 CFM | 6544.98 CFM |
| 250 FPS | 1309.00 CFM | 2945.24 CFM | 5235.99 CFM | 8181.23 CFM |
| 300 FPS | 1570.80 CFM | 3534.29 CFM | 6283.19 CFM | 9817.48 CFM |
| 350 FPS | 1832.60 CFM | 4123.34 CFM | 7330.38 CFM | 11453.72 CFM |
| 400 FPS | 2094.40 CFM | 4712.39 CFM | 8377.58 CFM | 13089.97 CFM |
| 450 FPS | 2356.19 CFM | 5301.44 CFM | 9424.78 CFM | 14726.22 CFM |
| 500 FPS | 2617.99 CFM | 5890.49 CFM | 10471.98 CFM | 16362.46 CFM |
Using this chart, you can quite easily determine how many CFM can these 4, 6, 8, and 10-inch diameter round ducts carry. Here are some examples:
- 1 FPS is equal to 5.24 CFM if you have a 4-inch round duct. This CFM airflow increases with duct size; thus, 1 FPS is equal to 11.78 CFM, 20.94 CFM, and 32.72 CFM for 6-inch, 8-inch, and 10-inch round ducts, respectively.
- 50 FPS is equivalent to 261.80 CFM, 589.05 CFM, 1047.20 CFM, and 1636.25 CFM for 4-inch, 6-inch, 8-inch, and 10-inch circular ducts, respectively.
- At 100 FPS, we get some serious airflow. 100 FPS is equal to 523.60 CFM, 1178.10 CFM, 2094.40 CFM, and 3272.49 CFM for 4-inch, 6-inch, 8-inch, and 10-inch round ducts, respectively.
Alright, these were round ducts. Let’s look in a similar manner at FPS to CFM conversion for rectangular ducts (including the 2nd calculator + explanation + 2nd conversion chart):
FPS To CFM Calculator For Rectangular Ducts (2nd Calc)
Again, let’s look at an example: Let’s say we have an 8×12 duct with an air speed of 100 FPS. How many CFM will this air velocity in this rectangular duct produce? Slide the 1st FPS slider to ‘100’, the 2nd duct width slider to ‘8’, the 3rd duct height slider to ’12’, and we get this result (screenshot):
We can see that 100 FPS for an 8×12 rectangular duct produces exactly 4000 CFM of airflow.
Let’s have a look at a fairly simple formula, and the 2nd conversion chart:
FPS To CFM Formula For Rectangular Ducts
Since we have a rectangular duct, we can quite easily calculate its cross-section or area. We use the basic formula for rectangle area (A = a × B or A = Duct Width × Duct Height). On top of that, we do the same seconds-to-minutes and inches-to-feet conversion we did before. Here is the resulting FPS to CFM formula for rectangular ducts:
CFM (Rectangular Ducts) = FPS × Duct Width (Inches) × Duct Height (Inches) × 60 / 144
Compared to the round ducts, this rectangular duct equation is a bit simpler. Let’s solve this example to illustrate how to use this formula:
How many CFM will a 10×12 rectangular duct produce at 50 FPS? Let’s insert these numbers into the formula like this (with result):
CFM (10×12 Duct, 50 FPS) = 50 FPS × 10 Inches × 12 Inches × 60 / 144 = 2500 CFM
We can see that 50 FPS on a 10×12 duct will produce 2500 CFM of airflow. You can use the 2nd calculator above to confirm this result.
Here are the promised FPS to CFM calculated tables for 4×10, 6×12, 8×12, and 12×12 ducts (again, we did a lot of calculations to create this):
FPS To CFM Chart For Rectangular Ducts
| Feet Per Second (FPS): | CFM In 4×10 Duct: | CFM In 6×12 Duct: | CFM In 8×12 Duct: | CFM In 12×12 Duct: |
| 1 FPS | 16.67 CFM | 30.00 CFM | 40.00 CFM | 60.00 CFM |
| 2 FPS | 33.33 CFM | 60.00 CFM | 80.00 CFM | 120.00 CFM |
| 3 FPS | 50.00 CFM | 90.00 CFM | 120.00 CFM | 180.00 CFM |
| 4 FPS | 66.67 CFM | 120.00 CFM | 160.00 CFM | 240.00 CFM |
| 5 FPS | 83.33 CFM | 150.00 CFM | 200.00 CFM | 300.00 CFM |
| 10 FPS | 166.67 CFM | 300.00 CFM | 400.00 CFM | 600.00 CFM |
| 15 FPS | 250.00 CFM | 450.00 CFM | 600.00 CFM | 900.00 CFM |
| 20 FPS | 333.33 CFM | 600.00 CFM | 800.00 CFM | 1200.00 CFM |
| 30 FPS | 500.00 CFM | 900.00 CFM | 1200.00 CFM | 1800.00 CFM |
| 40 FPS | 666.67 CFM | 1200.00 CFM | 1600.00 CFM | 2400.00 CFM |
| 50 FPS | 833.33 CFM | 1500.00 CFM | 2000.00 CFM | 3000.00 CFM |
| 60 FPS | 1000.00 CFM | 1800.00 CFM | 2400.00 CFM | 3600.00 CFM |
| 70 FPS | 1166.67 CFM | 2100.00 CFM | 2800.00 CFM | 4200.00 CFM |
| 80 FPS | 1333.33 CFM | 2400.00 CFM | 3200.00 CFM | 4800.00 CFM |
| 90 FPS | 1500.00 CFM | 2700.00 CFM | 3600.00 CFM | 5400.00 CFM |
| 100 FPS | 1666.67 CFM | 3000.00 CFM | 4000.00 CFM | 6000.00 CFM |
| 150 FPS | 2500.00 CFM | 4500.00 CFM | 6000.00 CFM | 9000.00 CFM |
| 200 FPS | 3333.33 CFM | 6000.00 CFM | 8000.00 CFM | 12000.00 CFM |
| 250 FPS | 4166.67 CFM | 7500.00 CFM | 10000.00 CFM | 15000.00 CFM |
| 300 FPS | 5000.00 CFM | 9000.00 CFM | 12000.00 CFM | 18000.00 CFM |
| 350 FPS | 5833.33 CFM | 10500.00 CFM | 14000.00 CFM | 21000.00 CFM |
| 400 FPS | 6666.67 CFM | 12000.00 CFM | 16000.00 CFM | 24000.00 CFM |
| 450 FPS | 7500.00 CFM | 13500.00 CFM | 18000.00 CFM | 27000.00 CFM |
| 500 FPS | 8333.33 CFM | 15000.00 CFM | 20000.00 CFM | 30000.00 CFM |
If you have rectangular ducts with these dimensions, you can now just check the FPS to CFM conversion here. Example: How many CFM will 80 FPS in 12×12 duct produce? Simple. Check the table: The result is 4800 CFM.
You can check these following similar calculators for air velocity and airflow:
With these two calculators and accompanying charts, you are now fully equipped to convert FPS 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