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Duct System Design Calculator

Go from the blower’s static pressure to a round size for every supply run and both trunks, with velocities and the pressure each run’s balancing damper will need to absorb.

Method reviewed October 26, 2026. Version 1.0.0. How it works

Your inputs

Unit system

From the air handler or furnace data at the design airflow.

Used to give the supply trunk as a rectangle with the same friction as its round size.

Thinner air at altitude carries less heat per CFM. Leave at 0 for sea-level (standard-air) results.

From the air handler to this outlet: straight length plus the equivalent length of every fitting, from your fitting data.

From the air handler to this outlet: straight length plus the equivalent length of every fitting, from your fitting data.

From the air handler to this outlet: straight length plus the equivalent length of every fitting, from your fitting data.

From the air handler to this outlet: straight length plus the equivalent length of every fitting, from your fitting data.

From the air handler to this outlet: straight length plus the equivalent length of every fitting, from your fitting data.

From the air handler to this outlet: straight length plus the equivalent length of every fitting, from your fitting data.

Result

Design friction rate
0.087in. w.c./100 ft
Available static pressure for ducts
0.24in. w.c.
Total supply airflow
630CFM

Design at 0.087 in. w.c. per 100 ft: 630 CFM supply trunk 12 in round (16 × 8 in), return 12 in, 6 runs sized.

  • Supply trunk: 802 fpm is above the 700 fpm ACCA Manual D recommendation, within the 900 fpm maximum.
  • Return trunk: 802 fpm is above the 700 fpm ACCA Manual D recommendation, within the 900 fpm maximum.
  • Total effective lengths depend on fitting equivalent lengths from your fitting data; this site does not publish fitting tables. Check the room airflows against your room-by-room loads.

Trunks

Supply trunk, round
12in
Supply trunk, rectangular width at 8 in high
16in
Supply trunk velocity
802fpm
Return trunk, round
12in
Return trunk velocity
802fpm

Run sizes (round)

Run 1
7in
Run 2
7in
Run 3
6in
Run 4
6in
Run 5
6in
Run 6
6in

Run velocities

Run 1
561fpm
Run 2
449fpm
Run 3
509fpm
Run 4
509fpm
Run 5
407fpm
Run 6
407fpm

Pressure to absorb with the balancing damper

Run 1
0.01in. w.c.
Run 2
0.071in. w.c.
Run 3
0.036in. w.c.
Run 4
0.053in. w.c.
Run 5
0.091in. w.c.
Run 6
0.097in. w.c.
Show the working
  1. Available static pressure = rated ESP − component drops
    0.6 − 0.36 = 0.24 in. w.c.
  2. Design friction rate = ASP × 100 ÷ (longest supply TEL + longest return TEL)
    0.24 × 100 ÷ (175 + 100) = 0.0873 in. w.c. per 100 ft
  3. Supply and return trunks carry 630 CFM
    round 11.81 in → 12 in at 802 fpm; 16 × 8 in rectangular
  4. Run 1: 150 CFM, 175 ft TEL
    6.91 in → 7 in, 561 fpm, loss 0.143 in. w.c. (allowance 0.153)
  5. Run 2: 120 CFM, 150 ft TEL
    6.36 in → 7 in, 449 fpm, loss 0.082 in. w.c. (allowance 0.153)
  6. Run 3: 100 CFM, 140 ft TEL
    5.94 in → 6 in, 509 fpm, loss 0.116 in. w.c. (allowance 0.153)
  7. Run 4: 100 CFM, 120 ft TEL
    5.94 in → 6 in, 509 fpm, loss 0.1 in. w.c. (allowance 0.153)
  8. Run 5: 80 CFM, 110 ft TEL
    5.47 in → 6 in, 407 fpm, loss 0.061 in. w.c. (allowance 0.153)
  9. Run 6: 80 CFM, 100 ft TEL
    5.47 in → 6 in, 407 fpm, loss 0.056 in. w.c. (allowance 0.153)

The equal-friction method

Start with the blower’s rated external static pressure and subtract what the coil, filter, registers, grilles and other devices use at the design airflow. What is left is the available static pressure for the ducts.

Spread that over the longest supply run plus the longest return run — each measured as total effective length, the straight length plus the equivalent length of every fitting — to get the design friction rate. Every duct is then sized to lose about that much pressure per 100 feet.

Reading the results

Each run is rounded up to the next standard round size, so it loses a little less than the design rate. Shorter runs also need less pressure than the longest one. The difference is what each run’s balancing damper has to absorb so that every room gets its share of air; large numbers mean that run will be noisy or hard to balance and may be worth downsizing.

Velocities are compared with the residential limits ACCA’s Manual D lists for trunks, branches and flexible duct. A trunk that carries the whole airflow is often the first place a velocity limit bites.

What you need to supply

The airflow for each run comes from the room loads — the Room-by-Room Load Estimator gives them. Fitting equivalent lengths come from fitting manufacturers or design references; this site does not publish fitting tables, and they usually make up most of each run’s total effective length.

Worked example

These are the example values the calculator opens with, and the result the engine calculates for them.

Inputs

Blower rated external static pressure
0.6 in. w.c.
Cooling coil pressure drop
0.2 in. w.c.
Filter pressure drop
0.1 in. w.c.
Supply outlet pressure drop
0.03 in. w.c.
Return grille pressure drop
0.03 in. w.c.
Other devices (dampers, humidifier, etc.)
0 in. w.c.
Longest return run — total effective length
100 ft
Duct material
Galvanized steel (sheet metal)
Rectangular trunk height
8 in
Site elevation
0 ft
Number of supply runs
6 runs
Run 1: airflow
150 CFM
Run 1: total effective length
175 ft
Run 2: airflow
120 CFM
Run 2: total effective length
150 ft
Run 3: airflow
100 CFM
Run 3: total effective length
140 ft
Run 4: airflow
100 CFM
Run 4: total effective length
120 ft
Run 5: airflow
80 CFM
Run 5: total effective length
110 ft
Run 6: airflow
80 CFM
Run 6: total effective length
100 ft

Result

Design friction rate
0.087 in. w.c./100 ft
Available static pressure for ducts
0.24 in. w.c.
Total supply airflow
630 CFM

Design at 0.087 in. w.c. per 100 ft: 630 CFM supply trunk 12 in round (16 × 8 in), return 12 in, 6 runs sized.

Assumptions

  • Equal-friction method: design friction rate = available static pressure × 100 ÷ (longest supply + longest return total effective length), as in ACCA’s Manual D checklist; every duct is sized at that rate.
  • Friction from Darcy–Weisbach with the Colebrook friction factor for standard air, corrected for elevation — the same engine as our Duct Size Calculator. Sizes are the next standard round size up.
  • The trunks carry the total supply airflow; the return trunk is sized for the same airflow at the same friction rate.

Limitations

  • Fitting equivalent lengths are not built in. They come from fitting manufacturers or design references and dominate each run’s total effective length.
  • Reducing trunks, takeoff positions, room air distribution and noise are not designed here. This is a sizing aid, not a complete duct design.

Frequently asked questions

Is this a Manual D design?

No. It uses the same equal-friction approach that Manual D describes, but it does not include fitting tables, reducing trunks, takeoff positions or room air distribution, and it is not ACCA-approved software.

What friction rate is normal?

It is calculated, not chosen. Low values mean large ducts; high values mean small ducts and higher velocities. The calculator warns below about 0.06 and above about 0.18 in. w.c. per 100 ft.

Can I use flexible duct?

Yes — choose it as the material. Flexible duct is much rougher than sheet metal, more so when compressed, and the sizes assume it is pulled taut.

Sources

  1. Manual D calculation checklist (Table 3-1 velocity limits) — ACCA (hosted by AHRI). Friction rate = available static pressure × 100 / total effective length; residential velocity limits (rigid trunk 700 recommended / 900 max; flex 600 / 700).
  2. Exhaust Duct Design (ASHRAE Distinguished Lecture; friction per ASHRAE 2021 Handbook p. 21.7) — P. J. Brooks, SMACNA, for ASHRAE (hosted by ETEK). Duct friction loss in the ASHRAE 2021 Handbook uses Darcy–Weisbach with the Colebrook friction factor; absolute roughness is determined experimentally.
  3. Determining the Absolute Roughness of Phenolic Duct — S. Idem & A. Paruchuri, ASHRAE eSociety (April 2019). Galvanized steel duct “average” absolute roughness 0.0003 ft (0.09 mm), ASHRAE Handbook—Fundamentals (2017), Duct Design, Table 1.
  4. Equivalent Diameter of rectangular and oval ducts — The Engineering ToolBox. Huebscher equation De = 1.30·(a·b)^0.625/(a + b)^0.25.
  5. Compression Effects on Pressure Loss in Flexible HVAC Ducts — B. Abushakra, I. S. Walker & M. H. Sherman, Lawrence Berkeley National Laboratory (2002). ASHRAE classes fabric-and-wire flexible duct as “rough”, 0.0035–0.015 ft (1.0–4.6 mm) fully extended; compression raises effective roughness far above that range.

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