Duct sizing calculator

How it works

This is an equal-friction ductulator. You give the airflow and the friction loss you can afford per 100 ft of duct. The tool finds the round diameter that carries that airflow at exactly that friction rate.

Friction is calculated with the method in the ASHRAE Handbook of Fundamentals duct design chapter, as reproduced by LBNL and Autodesk:

• Darcy equation (IP form): Δp per 100 ft = 12 · f · (100 ÷ D) · ρ · (V ÷ 1097)², with D in inches and V in fpm.
• Altshul–Tsal friction factor: f′ = 0.11 · (12ε ÷ D + 68 ÷ Re)^0.25. If f′ is 0.018 or more, f = f′; otherwise f = 0.85 f′ + 0.0028.
• Reynolds number for standard air: Re = 8.56 · D · V.
• Standard air density ρ = 0.075 lb/ft³, and roughness ε = 0.0003 ft (0.09 mm) for galvanized steel. The same roughness appears in the ASHRAE Duct Fitting Database friction example in ASHRAE lecture slides.

The result shows the exact diameter, then the next whole-inch size with its velocity (CFM ÷ area) and its actual friction rate. Rectangular sizes come from the Huebscher equivalent diameter, De = 1.30 (a·b)^0.625 ÷ (a + b)^0.25. That formula gives the rectangle with the same friction loss as the round duct at the same airflow. The tool lists widths for heights of 6, 8, 10, 12 and 14 in, up to a 4:1 aspect ratio. Above 900 fpm it adds a noise warning.

Choosing the friction rate (Manual D)

The friction rate is not a fixed number. ACCA Manual D works it out from the blower and the duct layout, as shown in ACCA’s Manual D verification brochure:

1. External static pressure (ESP) the blower delivers at design CFM, from the manufacturer’s blower table.
2. Subtract device pressure losses: coil, filter and other devices from manufacturer data, plus a standard 0.03 in. w.c. each for supply outlet, return grille and balancing damper. The result is the available static pressure (ASP).
3. Total effective length (TEL): the longest supply run plus the longest return run, each counting straight duct plus the equivalent length of every fitting.
4. Friction rate = ASP × 100 ÷ TEL.

ACCA’s example: ESP 0.70 in. w.c. minus 0.50 in. of device losses (coil 0.23, filter 0.18, three 0.03 items) gives ASP 0.20. With TEL 200 ft, FR = 0.20 × 100 ÷ 200 = 0.10 in. w.c. per 100 ft. ACCA’s friction rate chart runs from 0.06 to 0.18.

Worked example

The tool opens at 400 CFM and 0.08 in. w.c./100 ft:

• Exact round size: 10.2 in.
• Next whole inch, 11 in: 400 ÷ (π × (11 ÷ 24)²) = 606 fpm, with 0.054 in. w.c./100 ft actual friction.
• Rectangular equivalents: 16 × 6 or 11 × 8 in.

With ACCA’s 0.10 friction rate, the same 400 CFM needs 9.7 in round, so 10 in at 733 fpm and 0.086 in./100 ft.

Check of the Huebscher formula: 12 × 8 in gives 1.30 × 96^0.625 ÷ 20^0.25 = 10.66 in equivalent round.

Reading the result: velocity

Size by friction first, then check velocity. The ACCA brochure reproduces Manual D Table 3-1, Recommended Velocity (fpm):

Face velocities in the same table: supply outlet sized for throw, 700; return grille 500; filter grille 300. Manual D sizes returns by friction rate and then enlarges them where needed to meet these lower velocities. In the example, 11 in at 606 fpm is just over the 600 recommended for a supply branch and well under the 900 rigid maximum.

Flex duct

The calculator assumes galvanized steel. Flexible duct is far rougher: the ASHRAE roughness categories reproduced by LBNL put fully extended flex in the “rough” class, ε = 0.0035–0.015 ft. That is about 12 to 50 times the 0.0003 ft used here, so flex needs a larger diameter than this tool shows for the same CFM and friction rate. Flex also has lower velocity limits in the table above. Manual D sizes each run “based on the design CFM, friction rate, and the duct material used”. Use the flex friction chart from Manual D or the duct maker rather than this metal result.

Common mistakes

• Picking 0.10 by habit. Compute FR from your blower’s ESP and your actual TEL.
• Using straight length instead of TEL. TEL includes the equivalent length of every elbow, take-off and reducer, not just the duct.
• Sizing the return at the supply velocity. Returns have lower velocity limits.
• Treating rectangular as equal area. A 12 × 8 in duct has 96 in² but behaves like a 10.66 in round, which has only 89 in². Use the Huebscher sizes.
• Sizing flex from a metal chart.
• Wrong airflow. Get room CFM from the load calculation, and check system airflow with the airflow calculator. Low indoor airflow also skews charge readings, so confirm it before using the superheat calculator or the target superheat calculator.