Duct sizing calculator for trunk and branch sizes from CFM.

How the duct sizes are calculated.

The calculator runs the equal-friction method from ACCA Manual D in its Speed Sheet form, the same code BuildSolver's assistant uses for a first duct-sizing pass. It picks one design friction rate for the whole system, then sizes the supply trunk for the full airflow and each branch for its share.

Total equivalent length

TEL = longest run × fittings factorDefault factor 1.7 covers about 12 typical residential fittings (Manual D Appendix 4). No fitting-by-fitting takeoff.

Available friction rate

f_avail = ESP × 100 ÷ TEL (in.w.c./100 ft)Available external static pressure after the coil, filter and cabinet, divided by total equivalent length, per 100 ft.

Design friction rate

f = f_avail, limited to 0.05-0.15Above 0.15 the tool sizes at 0.15 so ducts do not get loud, and the result shows both rates. Below 0.05 the tool sizes at 0.05, which needs more static pressure than you entered, and the result then opens with an alert that these ducts will not move the design airflow at the pressure the blower actually has.

Branch airflow

CFM per branch = total CFM ÷ branchesLeave branches blank and the tool sets branches = CFM ÷ 150, rounded, limited to 4-14. A warning appears above 250 CFM per branch.

Required diameter

smallest D where Δp = (12 f × 100 ÷ D) × 0.075 × (V ÷ 1097)² ≤ design friction rateDarcy equation with the Colebrook friction factor for standard air (ASHRAE Fundamentals Ch. 21, Eq. 18 to 20), the same basis as the ASHRAE friction chart. Roughness 0.0003 ft for galvanized round duct and 0.003 ft for fully stretched flex (Table 1). The trunk uses the full design CFM; each branch uses CFM per branch.

Round size

D rounded up to 4, 5, 6, 7, 8, 9, 10, 12, 14, 16, 18 or 20 inCommercial round sizes. A trunk that needs more than 20 in shows as over 20: split the trunk or zone the system.

Velocity check

V = CFM ÷ (π × (D/24)²) (FPM)Checked at the rounded size. Noise warning above 1,200 FPM (galvanized) or 1,000 FPM (flex) in the trunk, and above 800 FPM (galvanized) or 700 FPM (flex) in branches. These noise thresholds are BuildSolver defaults. Above 1,500 FPM in the trunk or 1,200 FPM in a branch, the size goes up one bin.

Rectangular equivalent

D_eq = 1.30 × (a × b)^0.625 ÷ (a + b)^0.25Closest 1-inch rectangular size to the round result, aspect ratio 3:1 or less preferred, 4:1 at most.

A worked example for a 1,200 CFM flex system.

A 3-ton system at 400 CFM per ton gives 1,200 CFM. The longest supply run is 150 ft, the blower leaves 0.50 in.w.c. for the ducts, and the ducts are flex. Branches and fittings factor are left blank, so the defaults apply.

1. The total equivalent length is 150 ft × 1.7 = 255 ft.
2. The available friction rate is 0.50 × 100 ÷ 255 = 0.196 in.w.c./100 ft. That is outside the 0.05-0.15 range, so the design friction rate is 0.150.
3. That gives 8 branches (one per 150 CFM), so 150 CFM each.
4. The trunk comes out 16 in round at 859 FPM, or 18×12 in rectangular. Each branch comes out 7 in round at 561 FPM.

The calculator also shows these warnings.

• Flex sizing assumes fully-stretched. Field installs with sag may require +1 diameter on branches.

Before quoting, confirm the blower delivers 1,200 CFM at that static pressure. Change the inputs above and compare galvanized sheet metal or a higher fittings factor.

Our write-up on the flex duct friction rate works a 1,200 CFM flex system by hand, 0.21 in.w.c. across 200 ft of equivalent length for a 0.105 rate, and shows what compression costs a branch sized to it.

For estimation only · ACCA Manual D equal friction · computed by the calculator above

Standards and data sources.

• ACCA Manual D - residential duct design, equal-friction method. The page uses a whole-system Speed Sheet simplification with no fitting-by-fitting takeoff.
• ACCA Manual D Appendix 4 - equivalent lengths of fittings; the basis for the default 1.7 fittings factor.
• ASHRAE Handbook - Fundamentals 2017, Ch. 21 - Darcy and Colebrook friction loss with Table 1 duct roughness, velocity at the rounded size, and the rectangular-to-round equivalent diameter (Eq. 21-3).
• IECC 403.3.1 - duct insulation in unconditioned spaces. Duct heat gain and loss are not part of this sizing.

Limitations of the quick sizing.

• The whole system is sized at once, so every branch gets the same CFM (total ÷ branches) and a large room gets the same branch size as a closet.
• The trunk carries the full design CFM along its whole length, so the far end comes out about one size large. Step the trunk down after each group of takeoffs.
• Supply side only. Return ducts are not sized here, since they use different velocity targets.
• All fittings are covered by one length factor. Raise it for layouts with many elbows, boots or dampers.
• Flex is assumed fully stretched. Sagging or compressed flex has more friction and may need branches one diameter larger.
• Round sizes stop at 20 in. Larger airflows need parallel trunks or zoning.
• Duct heat gain and loss are not included. Insulate ducts in unconditioned spaces per IECC.
• Preliminary sales-phase sizing, less precise than a full Manual D. Not for permit submission.

Common duct sizing mistakes.

1. Using the blower's rated static pressure. Enter the pressure left after the coil, filter and cabinet. Entering the full blower rating overstates the friction rate, and the ducts come out too small.
2. Measuring the run without fittings. Elbows, boots and takeoffs add equivalent length. A 150 ft run with typical fittings behaves like about 255 ft. Raise the factor when the layout has many registers or zone dampers.
3. Sizing flex like sheet metal. Flex has more friction at the same diameter and often needs one size larger. Sag and tight bends add more on top of that.
4. Guessing CFM from the unit alone. Airflow per ton depends on the latent load. Plan on about 400 CFM per ton in most homes, closer to 350 in humid climates and up to 450 in dry climates. Confirm the blower can deliver that CFM at the available static pressure.
5. Ignoring velocity. Check velocity too. A duct can meet the friction rate and still be noisy, so compare trunk and branch velocity against the noise limits, especially near bedrooms.

Common questions about duct sizing.

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