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

When working with ducts, you might need to convert FPM to CFM. Basically, we want to know how much airflow (measured in CFM or Cubic Feet Per Minute) we get at a certain airflow velocity (measured in FPM or Feet Per Minute).

Now, the relationship between FPM and CFM is determined by the cross-section of the duct (area of the duct, either for flexible round ducts or rectangular ducts). To help you convert FPM to CFM for all kinds of ductwork, we created these 2 separate toolboxes (calculator + chart) that simply determining CFMs:

  1. Flexible Round Duct FPM To CFM Calculator + Chart. Here, you insert the diameter of a round duct (5-inch, 6-inch, 8-inch, 10-inch, etc.) and FPM, and this 1st calculator will convert them to determine CFM immediately. We also calculated CFMs for 5-30 inch ducts at 1-2000 FPMs, and summarized all the results in the 1st conversion chart.
  2. Rectangular Duct FPM To CFM Calculator + Chart. With rectangular ducts, the cross-section is calculated using the a × b equation. In this 2nd calculator, you insert these a and b (Example: 10×12 duct has a = 10 inches and b = 12 inches sides) and FPM, and you will get CFM right away. We also created the 2nd conversion chart (a bunch of calculations, honestly) with FPM to CFM conversions for more than 50 rectangular duct sizes.

As we see there are 2 sections to this article. First, we will focus on FPM to CFM for flexible round ducts (calculator + example + explanation + conversion chart). We will then proceed with the same helpful toolbox for rectangular ducts.

fpm to cfm for round vs rectangular ducts
We have to 1st look at round ducts and 2nd at rectangular ducts. This is because the area (cross-section) that is key to FPM to CFM conversion is calculated differently for each duct type.

Note: If you will still need a bit of help after these 2 toolboxes, you can always use the comment section below, give us some numbers, and we will do the math together.

Here is the 1st section, starting with the 1st calculator:

 

In this 1st calculator, you see that if you increase the diameter of the round duct, the CFMs increase as well.

Quick Example: How many CFMs do we get at 500 FPM using a 10-inch round duct? Simple. Insert ‘500’ in the FPM input field, slide the duct diameter to ’10’, and you get this result (1st screenshot):

flexible round duct fpm to cfm conversion with an online calculator

We can immediately see that an 8-inch round duct at 500 FPM airflow velocity can generate 272.71 CFM of airflow. If you need this conversion in reverse – CFM to FPM – you can use this CFM to FPM calculator for round and rectangular ducts.

Let’s have a look under the hood here to see how math works for round ducts:

How To Convert FPM To CFM For Round Flexible Ducts

Namely, for all FPM to CFM conversion, we start with this basic equation:

CFM = FPM × Area

When we are dealing with round ducts, the area or cross-section is basically the area of a circle (with π × r2/2). Round ducts are named after their diameter (8-inch duct means that diameter is 8 inches). On top of that, we are talking inches here, but both FPM and CFM are in feet, not inches. That’s why we need to include 12 inches = 1 foot conversion here as well, and we get this formula:

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

Quick Example: Let’s say we have a very high 2000 FPM air velocity traveling through an 8-inch flexible duct. How many CFMs can we get? We just insert these numbers into the flexible round duct formula above like this:

CFM (Round Duct, 2000 CFM) = 2000 FPM × π × 82 / (4 × 144) = 698.13 CFM

We can see that an 8-inch 2000 FPM round duct produces 698.13 CFM of airflow. You can check this result with the 1st calculator above.

Since it’s not all that easy to input all these numbers in your hand calculator, we created that calculator that does everything automatically. To simplify things even further, we have calculated CFMs for 5-30 inch round ducts at 1-2000 FPMs, and summarized all the results in this 1st chart:

FPM To CFM Conversion Chart For Flexible Round Ducts

Flexible Duct Diameter (d): CFM At 1 FPM: CFM At 100 FPM: CFM At 500 FPM: CFM At 1000 FPM: CFM At 1500 FPM: CFM At 2000 FPM:
5-Inch Round Duct 0.14 CFM 13.64 CFM 68.18 CFM 136.35 CFM 204.53 CFM 272.71 CFM
6-Inch Round Duct 0.20 CFM 19.63 CFM 98.17 CFM 196.35 CFM 294.52 CFM 392.70 CFM
7-Inch Round Duct 0.27 CFM 26.73 CFM 133.63 CFM 267.25 CFM 400.88 CFM 534.51 CFM
8-Inch Round Duct 0.35 CFM 34.91 CFM 174.53 CFM 349.07 CFM 523.60 CFM 698.13 CFM
9-Inch Round Duct 0.44 CFM 44.18 CFM 220.89 CFM 441.79 CFM 662.68 CFM 883.57 CFM
10-Inch Round Duct 0.55 CFM 54.54 CFM 272.71 CFM 545.42 CFM 818.12 CFM 1090.83 CFM
12-Inch Round Duct 0.79 CFM 78.54 CFM 392.70 CFM 785.40 CFM 1178.10 CFM 1570.80 CFM
14-Inch Round Duct 1.07 CFM 106.90 CFM 534.51 CFM 1069.01 CFM 1603.52 CFM 2138.03 CFM
16-Inch Round Duct 1.40 CFM 139.63 CFM 698.13 CFM 1396.26 CFM 2094.40 CFM 2792.53 CFM
18-Inch Round Duct 1.77 CFM 176.71 CFM 883.57 CFM 1767.15 CFM 2650.72 CFM 3534.29 CFM
20-Inch Round Duct 2.18 CFM 218.17 CFM 1090.83 CFM 2181.66 CFM 3272.49 CFM 4363.32 CFM
22-Inch Round Duct 2.64 CFM 263.98 CFM 1319.91 CFM 2639.81 CFM 3959.72 CFM 5279.62 CFM
24-Inch Round Duct 3.14 CFM 314.16 CFM 1570.80 CFM 3141.59 CFM 4712.39 CFM 6283.19 CFM
26-Inch Round Duct 3.69 CFM 368.70 CFM 1843.50 CFM 3687.01 CFM 5530.51 CFM 7374.02 CFM
28-Inch Round Duct 4.28 CFM 427.61 CFM 2138.03 CFM 4276.06 CFM 6414.09 CFM 8552.11 CFM
30-Inch Round Duct 4.91 CFM 490.87 CFM 2454.37 CFM 4908.74 CFM 7363.11 CFM 9817.48 CFM

Alright, this was a bunch of calculations. Here’s what we can see from this chart:

  • At 1 FPM, we can see that we get below 1 CFM with 12-inch and smaller ducts. With 14-inch and bigger ducts, we get above 1 CFM (up to almost 5 CFM if we are using that big 30-inch duct).
  • At 100 FPM, we get some sensible CFMs as well.
  • 500 FPM is the air velocity we most commonly use for HVAC systems (500 FPM is equal to 5.7 miles per hour or mph, just to get an idea). We can see that we can get anywhere from 68.18 CFM to 2454.37 CFM at 500 FPM.
  • At the very high 2000 FPM airflow velocity, we can almost get to 10,000 CFMs if we use the big 30-inch duct. For medium-sized 8-inch, 10-inch, and 12-inch ducts, we get around 1000 CFMs.

Now, there are the flexible round ducts. Let’s now proceed in a similar manner with the 2nd part – FPM to CFM for rectangular ducts – starting with the 2nd calculator:

 

Let’s solve one example to illustrate how this works:

How many CFM can an 8×12 inch duct generate at 500 FPM? We insert ‘500’ FPM in the 1st field, slide the duct width to ‘8’, and slide the duct height to ’12’. We get this result (2nd screenshot):

fpm to cfm conversion for rectangular ducts

We see that an 8×12 rectangular duct at 500 FPM produces 333.33 CFM of airflow.

Let’s look at how we can convert FPM to CFM for rectangular ducts manually (the math):

How To Convert FPM To CFM For Rectangular Ducts

Again, the difference between flexible round ducts and rectangular ducts is just how you calculate the area. With round ducts, we calculate the area of a circle, based on the duct diameter. On the other hand, with rectangular ducts we calculate the area of a rectangle, based on the duct width and duct height. Here is the full FPM to CFM rectangular duct formula:

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

Let’s take a 10×12 duct as an example. How many CFM it generates at 500 feet per minute? Let’s insert these numbers in the equation like this (with result):

CFM (10×12 Duct, 500 FPM) = 500 FPM × 10 Inches × 12 Inches / 144 = 416.67 CFM

We can see that the 10×12 duct generates 416.67 CFM of airflow at 500 FPM air speed.

Now, we did a huge amount of calculations for these rectangular ducts at 1 FPM, 500 FPM, and 2000 FPM, and gathered all the results in this feet per minute to cubic feet per minute rectangular duct conversion chart:

FPM To CFM Conversion Chart For Rectangular Ducts

Rectangular Duct Dimensions: CFM At 1 FPM: CFM At 500 FPM: CFM At 2000 FPM:
6×4 Duct: 0.17 CFM 83.33 CFM 333.33 CFM
8×4 Duct: 0.22 CFM 111.11 CFM 444.44 CFM
10×4 Duct: 0.28 CFM 138.89 CFM 555.56 CFM
12×4 Duct: 0.33 CFM 166.67 CFM 666.67 CFM
14×4 Duct: 0.39 CFM 194.44 CFM 777.78 CFM
16×4 Duct: 0.44 CFM 222.22 CFM 888.89 CFM
18×4 Duct: 0.50 CFM 250.00 CFM 1000.00 CFM
20×4 Duct: 0.56 CFM 277.78 CFM 1111.11 CFM
22×4 Duct: 0.61 CFM 305.56 CFM 1222.22 CFM
24×4 Duct: 0.67 CFM 333.33 CFM 1333.33 CFM
4×6 Duct: 0.17 CFM 83.33 CFM 333.33 CFM
6×6 Duct: 0.25 CFM 125.00 CFM 500.00 CFM
8×6 Duct: 0.33 CFM 166.67 CFM 666.67 CFM
10×6 Duct: 0.42 CFM 208.33 CFM 833.33 CFM
12×6 Duct: 0.50 CFM 250.00 CFM 1000.00 CFM
14×6 Duct: 0.58 CFM 291.67 CFM 1166.67 CFM
16×6 Duct: 0.67 CFM 333.33 CFM 1333.33 CFM
18×6 Duct: 0.75 CFM 375.00 CFM 1500.00 CFM
20×6 Duct: 0.83 CFM 416.67 CFM 1666.67 CFM
22×6 Duct: 0.92 CFM 458.33 CFM 1833.33 CFM
24×6 Duct: 1.00 CFM 500.00 CFM 2000.00 CFM
26×6 Duct: 1.08 CFM 541.67 CFM 2166.67 CFM
28×6 Duct: 1.17 CFM 583.33 CFM 2333.33 CFM
30×6 Duct: 1.25 CFM 625.00 CFM 2500.00 CFM
4×8 Duct: 0.22 CFM 111.11 CFM 444.44 CFM
6×8 Duct: 0.33 CFM 166.67 CFM 666.67 CFM
8×8 Duct: 0.44 CFM 222.22 CFM 888.89 CFM
10×8 Duct: 0.56 CFM 277.78 CFM 1111.11 CFM
12×8 Duct: 0.67 CFM 333.33 CFM 1333.33 CFM
14×8 Duct: 0.78 CFM 388.89 CFM 1555.56 CFM
16×8 Duct: 0.89 CFM 444.44 CFM 1777.78 CFM
18×8 Duct: 1.00 CFM 500.00 CFM 2000.00 CFM
20×8 Duct: 1.11 CFM 555.56 CFM 2222.22 CFM
22×8 Duct: 1.22 CFM 611.11 CFM 2444.44 CFM
24×8 Duct: 1.33 CFM 666.67 CFM 2666.67 CFM
26×8 Duct: 1.44 CFM 722.22 CFM 2888.89 CFM
28×8 Duct: 1.56 CFM 777.78 CFM 3111.11 CFM
30×8 Duct: 1.67 CFM 833.33 CFM 3333.33 CFM
32×8 Duct: 1.78 CFM 888.89 CFM 3555.56 CFM
34×8 Duct: 1.89 CFM 944.44 CFM 3777.78 CFM
36×8 Duct: 2.00 CFM 1000.00 CFM 4000.00 CFM
4×10 Duct: 0.28 CFM 138.89 CFM 555.56 CFM
6×10 Duct: 0.42 CFM 208.33 CFM 833.33 CFM
8×10 Duct: 0.56 CFM 277.78 CFM 1111.11 CFM
10×10 Duct: 0.69 CFM 347.22 CFM 1388.89 CFM
12×10 Duct: 0.83 CFM 416.67 CFM 1666.67 CFM
14×10 Duct: 0.97 CFM 486.11 CFM 1944.44 CFM
16×10 Duct: 1.11 CFM 555.56 CFM 2222.22 CFM
18×10 Duct: 1.25 CFM 625.00 CFM 2500.00 CFM
20×10 Duct: 1.39 CFM 694.44 CFM 2777.78 CFM
22×10 Duct: 1.53 CFM 763.89 CFM 3055.56 CFM
24×10 Duct: 1.67 CFM 833.33 CFM 3333.33 CFM
26×10 Duct: 1.81 CFM 902.78 CFM 3611.11 CFM
28×10 Duct: 1.94 CFM 972.22 CFM 3888.89 CFM
30×10 Duct: 2.08 CFM 1041.67 CFM 4166.67 CFM
32×10 Duct: 2.22 CFM 1111.11 CFM 4444.44 CFM
34×10 Duct: 2.36 CFM 1180.56 CFM 4722.22 CFM
36×10 Duct: 2.50 CFM 1250.00 CFM 5000.00 CFM
38×10 Duct: 2.64 CFM 1319.44 CFM 5277.78 CFM
40×10 Duct: 2.78 CFM 1388.89 CFM 5555.56 CFM
4×12 Duct: 0.33 CFM 166.67 CFM 666.67 CFM
6×12 Duct: 0.50 CFM 250.00 CFM 1000.00 CFM
8×12 Duct: 0.67 CFM 333.33 CFM 1333.33 CFM
10×12 Duct: 0.83 CFM 416.67 CFM 1666.67 CFM
12×12 Duct: 1.00 CFM 500.00 CFM 2000.00 CFM
14×12 Duct: 1.17 CFM 583.33 CFM 2333.33 CFM
16×12 Duct: 1.33 CFM 666.67 CFM 2666.67 CFM
18×12 Duct: 1.50 CFM 750.00 CFM 3000.00 CFM
20×12 Duct: 1.67 CFM 833.33 CFM 3333.33 CFM
22×12 Duct: 1.83 CFM 916.67 CFM 3666.67 CFM
24×12 Duct: 2.00 CFM 1000.00 CFM 4000.00 CFM
26×12 Duct: 2.17 CFM 1083.33 CFM 4333.33 CFM
28×12 Duct: 2.33 CFM 1166.67 CFM 4666.67 CFM
30×12 Duct: 2.50 CFM 1250.00 CFM 5000.00 CFM
32×12 Duct: 2.67 CFM 1333.33 CFM 5333.33 CFM
34×12 Duct: 2.83 CFM 1416.67 CFM 5666.67 CFM
36×12 Duct: 3.00 CFM 1500.00 CFM 6000.00 CFM
38×12 Duct: 3.17 CFM 1583.33 CFM 6333.33 CFM
40×12 Duct: 3.33 CFM 1666.67 CFM 6666.67 CFM
42×12 Duct: 3.50 CFM 1750.00 CFM 7000.00 CFM

Alright, we see that for rectangular ducts, 1 FPM is equal to anywhere from 0.17 CFM to 3.50 CFM, depending on the size of the duct cross-section.

At 500 FPM air velocity used for air conditioning systems and furnaces, we see that CFM range from below 100 CFM to almost 2000 CFM.

At the high 2000 CFM, you can get up to 7000 CFM using the big 42×12 duct.

All in all, we hope that you know have all you need to convert any FPM to CFM. If you do need a bit of hand, just use the comment section below, tell us about the ducts you’re working with and some numbers, and we will see how we can solve the FPM to CFM conversion.

Thank you.

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