Understanding your result
The headline is the air velocity in the duct, with the verdict against the limit and the friction rate in the sub-line. The bars put the duct’s velocity next to the limit you chose, so the margin is visible. The stats repeat the velocity, the friction per 100 ft and the airflow the duct carries at the limit, which is the useful figure when you are deciding whether an existing duct can take more.
The table adds the duct’s cross-section, its equivalent round diameter (the same as the diameter for a round duct, smaller than the equal-area size for a rectangle) and the friction rate. A friction rate above about 0.15 in wc per 100 ft, on a long run, uses up the blower’s static pressure before the air reaches the far registers; the equivalent length calculator shows how much pressure the blower has to spend.
If the duct is over the limit, the duct size calculator picks the size that would carry the airflow quietly, and the return grille calculator does the same for the grille that ends a return duct. If the ducts are the right size but the house still does not cool, the duct leakage calculator is the next check.
How we calculate this
Velocity is airflow divided by area; the 144 converts square inches to square feet so that cubic feet per minute over square feet gives feet per minute. The equivalent diameter formula is ASHRAE’s for rectangular ducts, and the friction fit is the standard equation behind the ASHRAE friction chart for galvanised duct, the same one the duct size calculator uses, so the two tools agree. In metric mode the velocity is shown in metres per second (1 FPM = 0.00508 m/s) and the friction in pascals per 100 ft; the factors are on the unit conversions page, and the duct size chart lists velocities for each stock size at common airflows.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Velocity limits | 900 FPM trunk, 700 branch, 600 return duct, 500 grille face | Residential quiet-design conventions (ACCA Manual D and common practice) |
| Friction formula | 0.109136 × Q^1.9 ÷ D^5.02 | Fit to the ASHRAE friction chart, galvanised duct, standard air |
| Equivalent diameter | ASHRAE 1.30 (wh)^0.625 ÷ (w+h)^0.25 | ASHRAE Handbook, Fundamentals |
| Air | Standard (sea level, 70 °F) | Velocity is unaffected; friction and capacity fall slightly at altitude |
Assumptions last reviewed October 8, 2026.
The calculator treats the duct as clean, straight galvanised metal; flex duct at the same size has about 50% more friction and more again if it sags, lined duct more still, and dust build-up narrows old returns. It does not add the losses of fittings, which the equivalent length calculator does. The guide to duct sizing basics explains how velocity, friction and static pressure fit together, and the guide on MERV filters and airflow covers the filter, the other big restriction in the air path.
Two worked examples
A 10 inch round trunk at 400 CFM
400 CFM through a 10-inch round duct, compared with the supply trunk limit of 900 FPM.
- Area: π × 10² ÷ 4 = 78.5 sq in
- Velocity: 400 × 144 ÷ 78.5 = 733 FPM, within the limit
- Friction: 0.109136 × 400^1.9 ÷ 10^5.02 = 0.092 in wc per 100 ft
- At the 900 FPM limit this duct carries 491 CFM
There is about 90 CFM of headroom before the trunk reaches the limit, so one more small bedroom could be added to it; against the 700 FPM branch limit the same duct is already 5% over, which shows how much the choice of limit matters.
A return duct that is too small
1,200 CFM (a 3-ton system) through a 20 × 8 in rectangular return, compared with the 600 FPM return limit.
- Area: 20 × 8 = 160 sq in
- Velocity: 1,200 × 144 ÷ 160 = 1,080 FPM, 80% above the limit
- Equivalent round: 13.5 in; friction 0.165 in wc per 100 ft
- At 600 FPM this duct carries only 667 CFM
This is the classic undersized return: a joist bay panned over with sheet metal and asked to carry a whole system’s air. It is loud, it costs the blower 0.165 in wc for every 100 ft, and the system cannot move its rated airflow. The fixes are a second return of the same size or a 14 × 20 duct, which the duct size calculator confirms at the return limit.
Where to find your inputs
Airflow. For a trunk, 400 CFM per ton of the system it serves (350 humid, 450 dry); for a branch, the room’s CFM from the CFM per room calculator; for a return, the whole system’s airflow divided by the number of returns.
Dimensions. Measure the duct’s outside and subtract twice the metal thickness, which is negligible, or read the size stamped on round duct. For insulated flex, the nominal size is the inside diameter.
Duct type. Trunk, branch, return or grille face; each has its own limit.
Common mistakes
- Using equal area for rectangular ducts. A 12 × 8 duct is equivalent to 10.7 in round, not 11 in; use the equivalent diameter for friction.
- Checking velocity and ignoring friction. A long run at a high friction rate starves the far rooms even if the velocity is acceptable.
- Adding a room to a trunk without checking capacity. The new room takes its air from the others; check CFM at the limit first.
- Treating flex like metal. Flex at the same size runs the same velocity but much higher friction.
- Forgetting the return. A perfect supply system on an undersized return is still a loud, starved system.
- Measuring the outside of insulated duct. Use the inside diameter; the insulation adds an inch or more to the outside.
Questions people ask
- How do I calculate air velocity in a duct?
- Divide the airflow by the duct's cross-section area. Velocity in feet per minute equals CFM × 144 divided by the area in square inches. A 10-inch round duct has an area of 78.5 sq in, so 400 CFM moves at 400 × 144 ÷ 78.5 = 733 FPM; a 12 × 8 rectangular duct has 96 sq in, so the same 400 CFM moves at 600 FPM. In metric, divide m³/h by 3,600 and by the area in square metres for metres per second.
- What is a good air velocity for residential ductwork?
- 600 to 900 feet per minute in supply trunks, 500 to 700 in branch ducts, 400 to 600 in return ducts, and 300 to 500 at grille faces, with the lower figures in bedrooms and anywhere noise matters. Below about 300 FPM the duct is larger than it needs to be; above 900 the air rushes audibly and the friction climbs steeply, because pressure loss rises with roughly the square of velocity.
- Why is my duct noisy?
- Almost always velocity. Doubling the airflow through a duct roughly quadruples its pressure loss and sends the velocity past the limit where the rushing becomes audible, and registers whistle when their face velocity passes about 600 FPM. Other causes are a dirty filter forcing the blower to work against high static, an undersized return (the commonest), and sheet metal that pops as the blower starts. The calculator shows how far over the limit a duct is.
- How much air can an existing duct carry?
- The calculator's last row answers that for the limit you choose. A 10-inch round duct carries about 490 CFM at the 900 FPM trunk limit, 380 at the 700 FPM branch limit and 330 at the 600 FPM return limit. If you want to add a room to a system, check that the trunk feeding it has that much spare capacity at the limit; if not, the new room will take air from the others.
- What is the equivalent round diameter of a rectangular duct?
- The diameter of the round duct that would carry the same airflow with the same friction. It is smaller than the diameter of a round duct with equal area, because a rectangle has more wall for the air to rub on. A 12 × 8 duct has the area of an 11-inch round duct but is equivalent to a 10.7-inch one. The calculator uses the ASHRAE equation to work it out and then applies the friction formula to it.
- Does velocity matter for return ducts?
- More than for supply ducts. Return grilles sit in living spaces with nothing but a grille between the blower and your ears, so returns are kept to 600 FPM in the duct and 300 to 500 at the grille face. An undersized return also raises the system's static pressure, cuts the airflow and makes the blower work harder, which is why the return grille calculator and the duct size calculator treat returns with their own limits.