Duct Velocity Calculator turns CFM and inside duct size into FPM, checks the result against Manual D limits for trunks, branches and returns, then shows friction plus a stock size.
Use inside dimensions. Lined duct loses the liner thickness on each side.
Duct Section, Material and Length
Airflow Capacity by Square Size
| Size | At 700 FPM | At 900 FPM | Friction at Max (in/100 ft) |
|---|---|---|---|
| 6 in × 6 in | 175 CFM | 225 CFM | 0.234 |
| 8 in × 8 in | 311 CFM | 400 CFM | 0.164 |
| 10 in × 10 in | 486 CFM | 625 CFM | 0.124 |
| 12 in × 12 in | 700 CFM | 900 CFM | 0.100 |
| 14 in × 14 in | 953 CFM | 1,225 CFM | 0.082 |
| 16 in × 16 in | 1,244 CFM | 1,600 CFM | 0.070 |
| 20 in × 20 in | 1,944 CFM | 2,500 CFM | 0.053 |
| 24 in × 24 in | 2,800 CFM | 3,600 CFM | 0.043 |
Checking HVAC Duct Speed With the Duct Velocity Calculator
The Duct Velocity Calculator finds how fast air moves through a round or rectangular duct from its airflow and inside size. It then checks that speed against the residential limits in ACCA Manual D and estimates the friction loss for the run.
HVAC installers use it to see if a trunk or branch will be noisy before it goes in. Homeowners use it to check a contractor’s duct sizes, and designers flip it around to size a duct for a target speed.
Velocity, Airflow or Duct Size
The Solve For menu has three modes. Velocity takes a duct size and airflow, Airflow takes a size and speed, and Duct Size takes airflow and a target speed, then works out the diameter or the height at a fixed width.
US units use inches, CFM and FPM, while metric uses millimeters with m³/h or L/s and m/s. A second panel sets the duct section, from supply trunk to filter grille, plus the duct material and run length for the friction estimate.
The Formula Behind the Duct Velocity Calculator
Velocity is airflow divided by the duct’s cross-sectional area. With area in square inches, the 144 converts it to square feet, as the Engineering ToolBox duct equations show.
$$V = \frac{Q \times 144}{A}$$
Here V is velocity in FPM, Q is airflow in CFM and A is area in square inches. A 12 in by 8 in duct has 96 sq in, so 450 CFM moves at 675 FPM.
A common slip is entering the outside size of lined duct or duct board. The liner takes up its thickness on each side, so the open area is smaller and the real velocity higher than the outside dimensions suggest.
Each dimension can run up to 200 in, airflow up to 1,000,000 CFM and velocity up to 10,000 FPM. Every size, airflow and speed must be above zero, while run length can be left blank or set to zero, which simply skips the total friction loss.
Manual D Velocity Limits by Duct Section
The first card compares your velocity with ACCA Manual D Table 3-1 for the section you chose. A supply trunk is recommended at 700 FPM with a 900 FPM maximum, a supply branch at 600 FPM, a return trunk at 600 FPM with a 700 FPM maximum, and a return branch at 400 FPM.
Below the recommended speed the card turns green, between the two it turns amber, and over the maximum it turns red. The default 675 FPM supply trunk sits at 75% of the 900 FPM cap.
Flex supply ducts get a lower cap. Manual D summaries hold supply flex to 700 FPM against 900 FPM for rigid duct, so choosing flex for a supply trunk or branch drops the maximum to 700 FPM, and the same 675 FPM then reads 96% of the limit.
For a filter grille, the card checks face velocity against 300 FPM. That figure comes from HVAC School, which cites Manual D as recommending 300 FPM as the maximum across a filter grille face.
Building scientist Allison Bailes at Energy Vanguard shows why the limits matter. In his Manual D walkthrough, 400 CFM at about 725 FPM is fine for a supply, but the same airflow on the return side needs a 12 in duct to stay under 700 FPM.
Manual D certified designer Tracy Savoy notes that design software often leaves branches and returns at a 900 FPM default. That is well above the 600 FPM branch and 700 FPM return limits, and he warns that return branches much above 500 FPM can get noisy.
Friction Loss From Darcy and Colebrook
The second card estimates friction per 100 ft with the Darcy equation, using the Colebrook equation for the friction factor. Velocity pressure for standard air at 0.075 lb/ft³ comes from the familiar ASHRAE relation.
$$P_v = \left( \frac{V}{4005} \right)^2$$
Duct roughness changes the answer a lot. The tool uses 0.0003 ft for galvanized steel, the ASHRAE value, 0.003 ft for duct board or lined duct, and 3 mm, about 0.01 ft, for flex duct pulled fully straight.
Published tables do not agree on flex. Some list fully stretched flex at 0.003 ft, so the tool takes the rougher 3 mm figure, and its alert warns that sagging or compressed flex loses more. The card adds no fitting losses, so elbows and boots still need to be counted separately.
The default 12 × 8 in steel trunk at 450 CFM loses 0.077 in w.g. per 100 ft, or 0.039 in w.g. over a 50 ft run. Compare the total with the blower’s available static pressure rather than with friction alone.
Equivalent Round Size and Aspect Ratio
The third card gives the free area, the equivalent round diameter and the aspect ratio for a rectangular duct. The equivalent round comes from the Huebscher formula, which matches friction at equal airflow.
$$D_e = \frac{1.30 \times (ab)^{0.625}}{(a + b)^{0.25}}$$
Huebscher is defined for equal friction, not equal area. The 12 × 8 in duct has an equivalent round of 10.7 in, but that round duct holds only about 89 sq in, so the same 450 CFM would run near 727 FPM in it, not 675.
So use the equivalent diameter for pressure loss, and the real rectangular area for velocity, noise and grille selection. In Duct Size mode, this card shows the next stock round size up, or the height rounded up to the next inch.
The fourth card adds velocity pressure, mass flow and the Reynolds number. Air in nearly every residential duct runs above about 4,000, which means turbulent flow where the Colebrook equation applies.
Measurement Mistakes That Skew Duct Velocity
Using a branch or return limit for a supply trunk, or the other way around, gives the wrong verdict. Set Duct Section to match the duct you measured, since a return branch is held to far lower speeds than a trunk.
Treating flex like metal hides most of its friction. Choose flex as the material whenever the run is flexible duct, which also applies the 700 FPM supply cap, and remember the tool assumes it is pulled fully tight.
Checking only the trunk misses the noisy part. Tracy Savoy’s point about branches left at 900 FPM shows that branch ducts are where the default settings most often run too fast.
Common Questions About Duct Airflow and Speed
How many CFM is in a 10×10 duct?
It depends on the speed. A 10 × 10 in duct has 100 sq in, so it moves about 486 CFM at the 700 FPM Manual D recommends for a supply trunk, and 625 CFM at the 900 FPM maximum.
As a return trunk with a 700 FPM cap, the same duct tops out near 486 CFM. The capacity table in the tool lists square sizes from 6 to 24 in for whichever section you pick.
How do I convert velocity to CFM?
Multiply velocity in FPM by the duct area in square feet. A 12 × 8 in duct is 0.667 sq ft, so air moving at 900 FPM through it carries 600 CFM.
The Airflow mode does this for you, with the area worked out from the inside size. The CFM Calculator covers airflow for rooms and fans as well as ducts.
What is 1 CFM equal to?
One cubic foot of air per minute equals about 1.70 m³/h or 0.472 L/s. The tool uses those factors when you switch between its US and two metric unit settings.
CFM is a volume rate, not a speed, so the same CFM moves faster in a smaller duct. The Air Changes per Hour Calculator turns CFM into how often a room’s air is replaced.
What does 1200 CFM mean for duct size?
It means 1,200 cubic feet of air every minute. To keep that in a supply trunk at the 700 FPM Manual D recommends, the duct needs about 247 sq in of free area, such as a 22 × 12 in trunk.
At the 900 FPM maximum, the area drops to about 192 sq in. The return side has to be larger again, and the Return Air Duct Size Calculator sizes it for the full system airflow.