Preliminary numbers for estimating and for checking a drawing. Straight duct only, standard air. The duct design, the equipment manufacturer and the local authority having jurisdiction come first.

Maintained by BuildSolver · Updated October 17, 2026

Calculator · HVAC ducts

Duct velocity and friction loss calculator

At 0.10 in. w.c. per 100 ft, a 6 in. galvanized round duct carries 110 CFM and the same size in flex 93 CFM. A 1,000 CFM trunk meets 0.10 in 14 in. metal and still runs at 935 FPM, over the 900 FPM Manual D maximum. This page does what a slide-rule duct calculator does for one round duct: give it two of size, airflow and friction rate, and it works out the rest and checks the velocity. To lay out a whole trunk and branch system from the total airflow, use the duct sizing calculator.

Choose the friction and velocity of a size at an airflow, the airflow a size carries at a friction rate, or the size for an airflow and a friction rate.
Pick galvanized round or flex and where the duct sits, then enter the two known values.
The result shows the friction rate, the velocity and the airflow or size, and checks the velocity against the Manual D Table 3-1 limits for that duct.

One straight round duct, galvanized or flex fully stretched, standard air. Fittings and compressed flex are not covered.

What to work out

The three jobs of a slide-rule duct calculator. Each one takes two of size, airflow and friction rate and works out the rest.

Duct

Flex is rougher inside, so it loses more pressure than metal of the same size.

Where the duct sits

Sets the Manual D velocity limits the result checks against. It does not change the friction.

Round diameter, in. (inside)

Inside diameter. Flex is sold by its inside diameter.

Airflow, CFM

The airflow in this one duct, not the system total.

Straight length, ft (optional)

Leave blank for the rate per 100 ft alone. With a length the result adds the loss of that straight run.

Example result for the default inputs

8 in. flex duct, fully stretched, supply branch, 150 CFM.

Friction rate

0.0562 in. w.c./100 ft

Velocity

430 FPM

Velocity pressure

0.0115 in. w.c.

Calculation details
Friction, Darcy and Colebrook	0.0562 in. w.c. per 100 ft in flex duct, fully stretched, standard air, Reynolds number 29,427.
Velocity	V = 150 CFM / 0.349 sq ft = 430 FPM.
Velocity pressure	0.0115 in. w.c. at 430 FPM, standard air. This is what a pitot tube reads in the duct, and it is not a loss.
Manual D velocity, Table 3-1	430 FPM is within the 600 FPM recommended for a flex supply branch (maximum 700 FPM).
Same size in galvanized round	0.0427 in. w.c. per 100 ft at the same airflow.

The flex figures are for a straight, fully stretched run. Flex that is compressed, sagging or bent loses more, and this page does not model that.

For estimation purposes only. Consult a licensed engineer for final designs.

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How friction and velocity are worked

The friction rate of a round duct comes from the Darcy equation with the Colebrook friction factor, the method of the ASHRAE duct design chapter and the one its friction chart is drawn from. Velocity is the airflow over the inside area. The airflow and the size modes solve the same equation the other way round.

Velocity: V = CFM / (pi x D^2 / 576) (FPM, D in inches)150 CFM in an 8 in. duct moves at 430 FPM. The area of an 8 in. round is 0.349 sq ft.
Friction rate: dp = (12 f x 100 / D) x rho x (V / 1097)^2 (in. w.c. per 100 ft)Darcy, ASHRAE Fundamentals Ch. 21 Eq. 18, with rho = 0.075 lb/ft3 for standard air. At 150 CFM an 8 in. flex duct loses 0.0562 and an 8 in. galvanized duct 0.0427 in. w.c. per 100 ft.
Friction factor: 1/sqrt(f) = -2 log10( 12e / (3.7 D) + 2.51 / (Re sqrt(f)) ), Re = 8.56 D VColebrook, Eq. 19 and 20, solved by iteration. The roughness e is 0.0003 ft for galvanized round and 0.003 ft for flex fully extended, from Table 1 of the same chapter.
Airflow at a friction rate: the CFM at which dp equals the target, found by bisectionFriction rises with airflow, so there is one answer. At 0.10 a 6 in. duct carries 110 CFM in galvanized round and 93 CFM in flex, rounded down.
Size for a friction rate: the D at which dp equals the target, then the next stock size up1,000 CFM at 0.10 in galvanized round needs 13.7 in., so the stock size is 14 in. The stock list here runs from 4 to 20 in.
Velocity pressure: pv = rho x (V / 1097)^2 (in. w.c.)The kinetic pressure of the moving air, the same term as in the Darcy equation. 430 FPM gives 0.0115 in. w.c. It is what a pitot tube reads, not a loss.

Airflow per duct size at 0.10

The airflow each size carries at a friction rate of 0.10 in. w.c. per 100 ft, from the same function as the form, rounded down. The velocity column is for galvanized round.

Airflow in CFM per round duct size at 0.10 in. w.c. per 100 ft, galvanized and flex
Round size	Galvanized, CFM	Velocity, FPM	Flex, CFM
4 in.	37	424	31
5 in.	67	491	57
6 in.	110	560	93
7 in.	167	625	141
8 in.	239	685	202
10 in.	435	798	367
12 in.	708	901	596
14 in.	1,068	999	898
16 in.	1,523	1,091	1,280

ACCA's own worked house is designed at 0.10 with flex supply branches. For its rooms of 150, 100, 80, 65, 40 CFM this page picks 8 in., 7 in., 6 in., 6 in., 5 in., the sizes in ACCA's table for those rooms. For the den and the living room ACCA chose one size larger than the smallest that holds 0.10. The default above is its first bedroom: 150 CFM in 8 in. flex at 0.0562 in. w.c. per 100 ft and 430 FPM.

Standards and data sources

ASHRAE Handbook, Fundamentals, Chapter 21, Duct Design - the Darcy and Colebrook friction loss (Eq. 18 to 20), the Reynolds number for standard air and the duct roughness of Table 1.
ANSI/ACCA Manual D, Table 3-1 - recommended and maximum velocities for supply and return trunks and branches, rigid and flex.
ACCA, Verifying ACCA Manual D Procedures - the published copy of Table 3-1 used here, the friction rate worksheet (ASP x 100 / TEL) and a worked house at 0.10 with flex branches. This page picks ACCA's size for five of its rooms; for the den and the living room ACCA chose one size larger.

Limitations of this calculator

One straight round duct at a time. The duct sizing calculator lays out a trunk and branches from the system airflow, and the rectangular to round calculator handles rectangular sheet metal through its round equivalent.
Straight duct only. Elbows, takeoffs, boots, transitions and dampers add their own loss, which Manual D counts as equivalent length. The loss over a length is the straight run alone.
Flex is modeled straight and fully stretched. Compressed, sagging or kinked flex loses more, and this page does not model that.
Standard air, 0.075 lb/ft3. Altitude and air temperature change the density and the friction rate.
Galvanized round and flex only. Duct board, lined duct, spiral with a different roughness and oval duct are not modeled.
Diameters from 3 to 36 in., airflow from 10 to 20,000 CFM, and velocities from 100 to 6,000 FPM. That window is this page's working range, not a standard, and inputs outside it are refused.
Stock round sizes come from a residential list from 4 to 20 in. Your supplier may stock 3 in., 11 in., 13 in. or sizes above 20 in.
The velocity check uses the Manual D Table 3-1 values for the duct position you pick. The supply outlet and return grille face velocities are a different check, made on the grille.
Preliminary numbers for estimating and for checking a drawing. A permit-grade duct design is a full Manual D calculation, and the installer and the local authority having jurisdiction have the final word.

Common mistakes with duct friction and velocity

Sizing on friction alone. 1,000 CFM at 0.10 in galvanized round needs 13.7 in., so 14 in. meets the friction rate at 0.0885. It also runs at 935 FPM, over the 900 FPM Manual D maximum for a rigid supply trunk. A 16 in. trunk brings it to 716 FPM. Check the velocity as well as the friction.
Reading the rate as the loss of the run. The friction rate is per 100 ft. 0.0562 in. w.c. per 100 ft over a 25 ft branch is 0.0140 in. w.c. of straight duct, before the fittings.
Leaving the fittings out of the length. Manual D divides the available static pressure by the total effective length, which is the measured run plus the equivalent length of every elbow, takeoff and boot. Fittings often make up more of it than the straight duct does.
Using metal numbers for flex. Flex is rougher inside. At 150 CFM an 8 in. flex duct loses 0.0562 in. w.c. per 100 ft and 8 in. metal 0.0427. At 0.10 an 8 in. duct carries 239 CFM in metal and 202 CFM in flex.
Pulling flex tight on paper and not on site. The flex figures assume a straight, fully stretched run. Extra length bunched up in an attic, sags between supports and tight bends all raise the loss above the number on this page.
Entering the system airflow for a branch. Each duct carries its own share. A branch to one room carries that room's CFM from the load calculation, and a trunk carries the sum of the branches beyond it.

Common questions about duct friction and velocity

How many CFM can a 6 inch duct carry?

At a friction rate of 0.10 in. w.c. per 100 ft, a 6 in. galvanized round duct carries 110 CFM and a 6 in. flex duct, fully stretched, 93 CFM. At a higher friction rate it carries more. The Manual D maximum of 900 FPM for a rigid supply branch would allow 176 CFM, so at 0.10 the friction rate is the limit, not the velocity.

What friction rate should I use for duct design?

Manual D works it out for each system: the available static pressure, which is the blower's external static pressure minus the losses of the coil, filter, grilles and dampers, times 100, divided by the total effective length of the longest supply and return runs. ACCA's own worked example has 0.20 in. w.c. available over 200 ft of total effective length, which gives 0.10.

What is the maximum velocity in a supply duct?

Manual D Table 3-1 sets the maximum at 900 FPM for rigid supply trunks and branches and 700 FPM for flex, and 700 FPM for return trunks and branches. The recommended values are lower: 700 FPM for a rigid supply trunk, 600 FPM for supply branches and 400 FPM for return branches.

How do you calculate air velocity in a round duct?

Divide the airflow by the inside area in square feet. An 8 in. round has 0.349 sq ft, so 150 CFM moves at 430 FPM. The formula is V = CFM / (pi x D^2 / 576) with D in inches.

Why does flex duct lose more pressure than metal duct?

Its inside surface is rougher. The ASHRAE duct design chapter gives 0.003 ft of roughness for flex fully extended against 0.0003 ft for galvanized round, so at 150 CFM an 8 in. flex duct loses 0.0562 in. w.c. per 100 ft against 0.0427 for metal. Compressed or sagging flex loses more again.

Does the friction rate include elbows and fittings?

No. The rate is for straight duct. Manual D adds the equivalent length of each elbow, takeoff and boot to the measured run to get the total effective length, and the design friction rate is worked over that total.

What is velocity pressure?

The pressure of the air's motion, rho x (V / 1097)^2 in in. w.c. for standard air. At 430 FPM it is 0.0115 in. w.c. A pitot tube traverse reads it to measure airflow. It is not lost in a straight duct of constant size.

How is this different from the duct sizing calculator?

This page works one round duct at a time, the way a slide-rule duct calculator does: two of size, airflow and friction rate in, the rest out, with the Manual D velocity check. The duct sizing calculator starts from the system airflow, the static pressure and the longest run, and sizes a trunk and branches.

Can I use this for a rectangular duct?

Use the round equivalent. The rectangular to round duct calculator gives the round diameter with the same friction at the same airflow, and the friction rate it shows is the one this page works. Velocity in the rectangle is lower than in its round equivalent, because the rectangle has more area.