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

When working with ducts, we often need to convert CFM to FPM. Basically, we are converting airflow in terms of Cubic Feet Per Minute (CFM or ft3/min) to air velocity in terms of Feet Per Minute (FPM or ft/min). Here, we have everything you need to convert CFM to FPM for both round ducts (1st section) as well as rectangular ducts (2nd section).

Why do we separate round vs rectangular ducts? Well, the CFM to FPM conversion depends on the cross-section of the duct. Since this cross-section is calculated differently for these two duct types (π×d2/4 for round ducts, and a × b for rectangular ducts), we prepared these 2 sections:

  1. CFM To FPM Round Ducts Calculator + Formula + Conversion Chart (1st part). This 1st calculator automatically converts CFM to FPM for round ducts (you insert FPM, and the diameter of the duct, and you get CFMs). We will explain how to use the formula to calculate this as well. On top of this, we converted 1-5000 CFM to FPM, and summarized all the results in the 1st conversion chart.
  2. CFM To FPM Rectangular Ducts Calculator + Formula + Conversion Chart (2nd part). Here, we are dealing with rectangular ducts (4×10, 6×8, 10×10, 12×14, etc.). You insert the duct dimensions, and this 2nd calculator will turn CFMs to FPM. Additionally, we also converted 1-5000 CFM to FPM for these rectangular ducts, and created the 2nd conversion chart.

Useful note: If you have a 12×12 rectangular duct, 1 CFM = 1 FPM applies. This makes the conversion for the 12×12 duct super simple.

Let’s start with the 1st part covering CFM to FPM conversion for round ducts, and then we will proceed with the 2nd part for rectangular ducts:

 

Let’s solve one example with this 1st calculator just to demonstrate how it works:

How many FPM air velocity we need to produce 1000 CFM of airflow using an 8-inch round duct? Simple. Just insert ‘1000’ in the CFM field, slide the duct diameter slider to ‘8’, and we get this result (1st screenshot):

1000 cfm to fpm for 8 inch round duct

We can see that we need a very high 2864.79 FPM air velocity to produce 1000 CFM in an 8-inch round duct. For reference, most residential HVAC systems operate at 500 FPM, and more powerful commercial HVAC systems can operate at 2000 FPM.

Note: If you need this conversion in reverse – converting FPM to CPM – you can check this FPM to CPM calculator for round and rectangular ducts.

Now, before we check the 1st conversion chart, let’s first have a look under the hood on this conversion:

How To Convert CFM To FPM For Round Ducts? (Formula)

The higher airflow we need (CFMS), the higher air velocity (FPM) we have to apply to deliver that airflow through a duct with a certain cross-section. In general, we use this formula to convert any CFM to FPM:

FPM = CFM / A

A is the area through which air is traveling (cross-section of the duct). To convert CFM to FPM, we need to divide CFM by the area (duct cross-section) through which the air travels. We use different formulas to express this area (for round ducts, we use the formula for the area of a circle, and for rectangular ducts, we use a formula for the area of a rectangle).

On top of that, we need to convert inches to feet as well since airflow and air velocity (CFM, FPM) are expressed in feet, while duct dimensions are expressed in inches (Example: 10-inch round duct).

Based on this, we can write the following CFM to FPM formula for round ducts:

FPM (Round Ducts) = CFM × 144 × 4 / (π × d2)

Quick Example: How many FPM we need to produce 500 CFM using a 10-inch round duct? We just insert these numbers in the equation above like this (with the result):

FPM (500 CFM, 6-Inch Round Duct) = 500 CFM × 144 × 4 / (π × 102) = 916.73 FPM

We see that we need 916.73 feet per minute air velocity to produce 500 CFM through a 10-inch flexible round duct.

Now, this is just 1 example. To help you out, we have calculated FPM in 4-inch, 6-inch, 8-inch, and 10-inch ducts with CFMs ranging from 1 to 5000 CFM. Here is the full conversion chart for round ducts:

Round Duct CFM To FPM Conversion Chart

CFM (Cubic Feet Per Minute):  FPM In 4-Inch Round Duct: FPM In 6-Inch Round Duct: FPM In 8-Inch Round Duct: FPM In 10-Inch Round Duct:
1 CFM 11.46 FPM 5.09 FPM 2.86 FPM 1.83 FPM
2 CFM 22.92 FPM 10.19 FPM 5.73 FPM 3.67 FPM
3 CFM 34.38 FPM 15.28 FPM 8.59 FPM 5.50 FPM
4 CFM 45.84 FPM 20.37 FPM 11.46 FPM 7.33 FPM
5 CFM 57.30 FPM 25.46 FPM 14.32 FPM 9.17 FPM
10 CFM 114.59 FPM 50.93 FPM 28.65 FPM 18.33 FPM
15 CFM 171.89 FPM 76.39 FPM 42.97 FPM 27.50 FPM
20 CFM 229.18 FPM 101.86 FPM 57.30 FPM 36.67 FPM
30 CFM 343.77 FPM 152.79 FPM 85.94 FPM 55.00 FPM
40 CFM 458.37 FPM 203.72 FPM 114.59 FPM 73.34 FPM
50 CFM 572.96 FPM 254.65 FPM 143.24 FPM 91.67 FPM
60 CFM 687.55 FPM 305.58 FPM 171.89 FPM 110.01 FPM
70 CFM 802.14 FPM 356.51 FPM 200.54 FPM 128.34 FPM
80 CFM 916.73 FPM 407.44 FPM 229.18 FPM 146.68 FPM
90 CFM 1031.32 FPM 458.37 FPM 257.83 FPM 165.01 FPM
100 CFM 1145.92 FPM 509.30 FPM 286.48 FPM 183.35 FPM
150 CFM 1718.87 FPM 763.94 FPM 429.72 FPM 275.02 FPM
200 CFM 2291.83 FPM 1018.59 FPM 572.96 FPM 366.69 FPM
250 CFM 2864.79 FPM 1273.24 FPM 716.20 FPM 458.37 FPM
300 CFM 3437.75 FPM 1527.89 FPM 859.44 FPM 550.04 FPM
350 CFM 4010.70 FPM 1782.54 FPM 1002.68 FPM 641.71 FPM
400 CFM 4583.66 FPM 2037.18 FPM 1145.92 FPM 733.39 FPM
450 CFM 5156.62 FPM 2291.83 FPM 1289.16 FPM 825.06 FPM
500 CFM 5729.58 FPM 2546.48 FPM 1432.39 FPM 916.73 FPM
600 CFM 6875.49 FPM 3055.77 FPM 1718.87 FPM 1100.08 FPM
700 CFM 8021.41 FPM 3565.07 FPM 2005.35 FPM 1283.43 FPM
800 CFM 9167.32 FPM 4074.37 FPM 2291.83 FPM 1466.77 FPM
900 CFM 10,313.24 FPM 4583.66 FPM 2578.31 FPM 1650.12 FPM
1000 CFM 11,459.16 FPM 5092.96 FPM 2864.79 FPM 1833.46 FPM
1500 CFM 17,188.73 FPM 7639.44 FPM 4297.18 FPM 2750.20 FPM
2000 CFM 22,918.31 FPM 10,185.92 FPM 5729.58 FPM 3666.93 FPM
2500 CFM 28,647.89 FPM 12,732.40 FPM 7161.97 FPM 4583.66 FPM
3000 CFM 34,377.47 FPM 15,278.87 FPM 8594.37 FPM 5500.39 FPM
3500 CFM 40,107.05 FPM 17,825.35 FPM 10,026.76 FPM 6417.13 FPM
4000 CFM 45,836.62 FPM 20,371.83 FPM 11,459.16 FPM 7333.86 FPM
4500 CFM 51,566.20 FPM 22,918.31 FPM 12,891.55 FPM 8250.59 FPM
5000 CFM 57,295.78 FPM 25,464.79 FPM 14,323.94 FPM 9167.32 FPM

Here’s some insight from this chart:

  • To produce 1 CFM, we would need 11.46 FPM, 5.09 FPM, 2.86 FPM, and 1.83 FPM air velocity in 4-inch, 6-inch, 8-inch, and 10-inch round duct, respectively. We can clearly see that we need lower FPM per CFM the larger the duct we use.
  • For 100 CFM, we need between 183.35 FPM and 1145.92 FPM, if we use 4-10 inch round ducts.
  • For 1000 CFM, we need 1650.12 FPM to 11,459.16 FPM, if we use 4-10 inch round ducts. This is quite an enormous air velocity (11,459.16 FPM is equal to 130 miles per hour (mph)).

Now, let’s have a look at the 2nd part for rectangular air ducts, starting with the 2nd calculator:

 

Alright, let’s try to figure out how many FPM we need to produce 600 CFM with a 10×12 rectangular duct as an example. We insert ‘600’ in the CFM input field, slide the 1st duct width slider to ’10’, and 2nd duct height slider to ’12’. We get this result (2nd screenshot):

cfm to fpm for 10x12 duct

We can immediately see that we need 720 feet per minute to produce 600 cubic feet per minute airflow through a 10×12 inch rectangular duct.

Let’s have a look at the formula behind this conversion:

CFM To FPM Formula For Rectangular Ducts

With rectangular ducts, the cross-section of the duct is calculated using a formula for the area of a rectangle. We also need to convert inches (duct dimensions) to feet, and we get this CFM to FPM formula for rectangular ducts:

FPM (Rectangular Ducts) = CFM × 144 / (Duct Width (Inches) × Duct Height (Inches))

Basically, with rectangular ducts, we need to first multiply CFM by 144 (for inches to feet conversion), and then divide the product by the product of duct width and duct height (both in inches), to adequately convert CFM to FPM.

Quick Example: How many FPM do we need to produce 1500 CFM airflow in a 10×14 duct? Let’s just insert these numbers in the rectangular duct equation above like this (with result):

FPM (1500 CFM, 10×14 Duct) = 1500 CFM × 144 / (10 Inches × 14 Inches) = 1542.86 FPM

You can check this 1542.86 FPM result with the 2nd calculator above.

Let’s have a look at the 2nd conversion chart for rectangular ducts:

CFM To FPM For Rectangular Ducts

CFM (Cubic Feet Per Minute): FPM In 4×10 Duct: FPM In 6×12 Duct: FPM In 8×12 Duct: FPM In 12×12 Duct:
1 CFM 3.60 FPM 2 FPM 1.5 FPM 1 FPM
2 CFM 7.20 FPM 4 FPM 3 FPM 2 FPM
3 CFM 10.80 FPM 6 FPM 4.5 FPM 3 FPM
4 CFM 14.40 FPM 8 FPM 6 FPM 4 FPM
5 CFM 18 FPM 10 FPM 7.5 FPM 5 FPM
10 CFM 36 FPM 20 FPM 15 FPM 10 FPM
15 CFM 54 FPM 30 FPM 22.5 FPM 15 FPM
20 CFM 72 FPM 40 FPM 30 FPM 20 FPM
30 CFM 108 FPM 60 FPM 45 FPM 30 FPM
40 CFM 144 FPM 80 FPM 60 FPM 40 FPM
50 CFM 180 FPM 100 FPM 75 FPM 50 FPM
60 CFM 216 FPM 120 FPM 90 FPM 60 FPM
70 CFM 252 FPM 140 FPM 105 FPM 70 FPM
80 CFM 288 FPM 160 FPM 120 FPM 80 FPM
90 CFM 324 FPM 180 FPM 135 FPM 90 FPM
100 CFM 360 FPM 200 FPM 150 FPM 100 FPM
150 CFM 540 FPM 300 FPM 225 FPM 150 FPM
200 CFM 720 FPM 400 FPM 300 FPM 200 FPM
250 CFM 900 FPM 5000 FPM 375 FPM 250 FPM
300 CFM 1080 FPM 600 FPM 450 FPM 300 FPM
350 CFM 1260 FPM 700 FPM 525 FPM 350 FPM
400 CFM 1440 FPM 800 FPM 600 FPM 400 FPM
450 CFM 1620 FPM 900 FPM 675 FPM 450 FPM
500 CFM 1800 FPM 1000 FPM 750 FPM 500 FPM
600 CFM 2160 FPM 1200 FPM 900 FPM 600 FPM
700 CFM 2520 FPM 1400 FPM 1050 FPM 700 FPM
800 CFM 2880 FPM 1600 FPM 1200 FPM 800 FPM
900 CFM 3240 FPM 1800 FPM 1350 FPM 900 FPM
1000 CFM 3600 FPM 2000 FPM 1500 FPM 1000 FPM
1500 CFM 5400 FPM 3000 FPM 2250 FPM 1500 FPM
2000 CFM 7200 FPM 4000 FPM 3000 FPM 2000 FPM
2500 CFM 9000 FPM 5000 FPM 3750 FPM 2500 FPM
3000 CFM 10,800 FPM 6000 FPM 4500 FPM 3000 FPM
3500 CFM 12,600 FPM 7000 FPM 5250 FPM 3500 FPM
4000 CFM 14,400 FPM 8000 FPM 6000 FPM 4000 FPM
4500 CFM 16,200 FPM 9000 FPM 6750 FPM 4500 FPM
5000 CFM 18,000 FPM 10,000 FPM 7500 FPM 5000 FPM

In this chart, we see how many FPM we need to produce certain CFMs in 4×10, 6×12, 8×12, and 12×12 rectangular ducts. You can see that, to produce 1 CFM of airflow, we need 3.60 FPM, 2 FPM, 1.5 FPM, and 1 FPM air velocity in 4×10, 6×12, 8×12, and 12×12 rectangular ducts, respectively. You can see that 1 CFM = 1 FPM in a 12×12 duct.

Using these 2 calculators, formulas, and conversion chart, you are now fully equipped to convert CFM to FPM for round and rectangular ducts. If you need a bit of help from our side, you can use the comment section below, give us a few numbers, and we can do some math together. Thank you.

 

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