FPS To CFM Calculator For Flexible Round, Rectangular Ducts (+Charts)

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):

  1. 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.
  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 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.
fps to cfm for round and rectangular ducts
To adequately and precisely convert FPS to CFM, we need to distinguish between round and rectangular ducts.

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):

fps to cfm conversion

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):

fps to cfm for rectangular ducts or square ducts

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

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