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SZ-05Sizing · HVAC

Duct size calculator

The round duct diameter for an airflow and your design friction rate, the velocity it gives against the supply or return limit, and the rectangle with the same friction — drawn to scale. One section of sheet-metal duct, not a Manual D design.

Round duct size

Example: 400 cfm at 0.073 in./100 ft (0.31 in. of static pressure over 424 ft, Energy Vanguard’s worked example). Change any number.

Suggested supply duct Example

Suggested 11 inch round duct at 606 feet per minute; exact 10.34 inches.

606 fpm in this ductexact 10.34 in at 0.073 in./100 ft

Rectangle with the same friction: 8 × 12.8 in

See it drawn to scale Change the numbers

Sections and velocityDrawn to scale: sections in inches, velocity in fpm
Duct sections and velocity for the example, drawn to scaleCross-sections to scale: a 11″ round duct with the exact 10.34″ diameter dashed inside, and a 8″ by 12.8″ rectangle with the same friction rate. Velocities: exact 686 fpm, 11″ 606 fpm, 12″ 509 fpm, rectangle 561 fpm; supply limit 900 fpm.ASections, to scalegrid 1 inØ 11″Round0.66 sq ft12.8″8″Rectangle0.71 sq ftBAir velocity, fpm400 cfm ÷ areaSupply limit 900 fpmExact 10.34″686Round 11″606Round 12″5098″ × 12.8″56105001,000
  • Duct section
  • Exact diameter, 10.34″
  • Velocity limit (as reported)
Velocity
606fpm, limit 900
Friction
0.054in./100 ft
One size up
12 in509 fpm

D at FR 0.073 = 10.34 inD at 900 fpm = 9.03 innext whole inch above both = 11 in

Already showing the example.

Read with the result

  • One section of galvanized sheet-metal round duct at standard air (70 °F, sea level). Not flex duct, fittings or a Manual D design.Roughness 0.0003 ft (ASHRAE); air 0.075 lb/ft³ (AMCA).

Change the numbers

For this one duct section, 20–5,000 cfm.

Static pressure available for the ducts × 100 ÷ total effective length. Work it out; don’t assume 0.10.

Work out the friction rate

Available static pressure: blower table minus component losses.

Longest supply + return run, fittings included.

Friction rate0.073pressure × 100 ÷ length
Duct side

Velocity limit: supply 900 fpm, return 700 fpm, as reported from Manual D.

One side of a rectangular duct, e.g. the depth between joists, 2–100 in. Leave empty for round only.

Suggested: 11 in round · 606 fpm. Back to the result

How the math works

The calculator finds the diameter at which the duct loses exactly your friction rate, using the Darcy equation with the Altshul–Tsal friction factor that ASHRAE’s Duct Design chapter gives. Then it checks the velocity and rounds up to a whole inch.

  1. VelocityV = cfm ÷ (π/4 × (D/12)²) fpm
  2. Reynolds numberRe = (V/60) × (D/12) ÷ 0.00015783Kinematic viscosity of standard air at 70 °F, our arithmetic from NASA’s Sutherland formula and AMCA’s density.
  3. Friction factorf′ = 0.11 × (12 × 0.0003 ÷ D + 68 ÷ Re)^0.25; below 0.018, f = 0.85 f′ + 0.0028Altshul–Tsal, with ASHRAE’s “average” roughness for galvanized duct (0.0003 ft).
  4. Friction rateFR = (12 f × 100 ÷ D) × 0.075 × (V ÷ 1097)²Darcy equation in inch-pound units, in. w.c. per 100 ft. The calculator solves it for D by bisection.
  5. Velocity checkD for the limit = 12 × √(4 × cfm ÷ (π × limit))Limits 900 fpm supply and 700 fpm return: Manual D Table N3-1 as reported by Energy Vanguard, not yet confirmed in Manual D itself.
  6. Suggested sizenext whole inch at or above the larger of the two diametersOne size up is shown too. Check which sizes your supplier stocks; we print no list of standard sizes.
  7. RectangleDe = 1.30 × (a × b)^0.625 ÷ (a + b)^0.25Huebscher equation: same friction per 100 ft at the same airflow. The rectangle has more area, so its air moves slower.

Worked examples

  • 400 cfm at 0.073, supplyexact 10.34 in at 686 fpm → 11 in at 606 fpm, FR 0.054
  • 1,200 cfm at 0.10, supply14.59 in would run 1,034 fpm; for 900 fpm D = 15.64 in → 16 in at 859 fpm
  • 12 in round, 8 in sideother side = 15.47 in (check: De(8, 15.47) = 12.00 in)

Values the calculator uses

Duct roughness
0.0003 ft, galvanized sheet metalASHRAE (ashrae.org eSociety, April 2019): "average" absolute roughness of galvanized steel ducts
Air density
0.075 lb/ft³, standard airAMCA International, "The Fundamentals of Airflow" (2020): standard air, 70 °F, sea level
Kinematic viscosity
1.5783 × 10⁻⁴ ft²/s at 70 °FSite arithmetic: NASA Glenn Sutherland viscosity at 70 °F ÷ AMCA standard-air density 0.075 lb/ft³
Velocity limits
Supply 900 fpm · return 700 fpm As reportedEnergy Vanguard, "Duct Design 5 — Sizing the Ducts" (2017), citing ACCA Manual D Table N3-1. Reported by Energy Vanguard as Manual D Table N3-1 values; not yet confirmed in Manual D itself.
Low friction warning
Below 0.06 in. w.c. per 100 ftICC, "An Overview on ACCA’s Residential HVAC System Design Process" (2018 slides): the acceptable-FR wedge's edge is labelled 0.06

Before you use the number

  • Galvanized round duct only. Flex duct is rougher, and compressing it can raise the pressure drop four times or more (LBNL). There is no flex mode, because a single number would mislead.
  • The friction rate is an input, not a guess. Manual D derives it from the static pressure available for the ducts and the total effective length. Assuming 0.10 undersizes ducts (Energy Vanguard; ICC/ACCA).
  • Velocity matters too. If the size that meets the friction rate moves air faster than the limit, go larger. The limits shown are reported values, not code requirements.
  • One section, not a system. Fittings, equivalent lengths, registers, balancing dampers and the blower table are not included. Residential codes reference Manual D for the design.
  • Standard air. 70 °F at sea level. Altitude and temperature change the result; the calculator does not correct for them. A paper duct calculator may read a few percent differently.

Sizing the equipment first? Start with the room load: BTU and AC tonnage calculator.

Sources

Opened and checked October 4, 2026. The worked examples are our own arithmetic.

  1. Compression Effects on Pressure Loss in Flexible HVAC Ducts (Abushakra, Walker, Sherman, 2002)Lawrence Berkeley National Laboratory, via eScholarship
  2. Determining the Absolute Roughness of Phenolic Duct (April 2019)ASHRAE
  3. The Fundamentals of Airflow (webinar slides, 2020)AMCA International
  4. ViscosityNASA Glenn Research Center
  5. Fluid Flow — Equivalent DiameterEngineering ToolBox
  6. Why using spiral duct improves duct design (2017)Snips (BNP Media)
  7. Duct Design 4 — Calculating Friction RateEnergy Vanguard
  8. Duct Design 5 — Sizing the DuctsEnergy Vanguard
  9. An Overview on ACCA’s Residential HVAC System Design Process (2018 slides)International Code Council
  10. Manual D Residential Duct DesignAir Conditioning Contractors of America
  11. Advanced Strategy Guideline: Air Distribution Basics and Duct Design (2011)U.S. DOE Building America, via OSTI

Tools for the job

Tool listHVAC + Cross-trade4 itemsSpecs read

As an Amazon Associate, Trade Price Book earns from qualifying purchases. Amazon links are marked “(paid link)”; prices aren’t shown here.

  1. Digital manifolds and probesFieldpiece JL3KH6

    JL3KH6 Job Link System Probes Wireless Charge & Air Test Kit

    • Six probes in a padded case: 2 pressure probes, 2 pipe-clamp temperature probes and 2 psychrometers.

    Specs: Fieldpiece product page, read

    View on Amazon for Fieldpiece JL3KH6(paid link)

  2. Airflow and static pressureFieldpiece STA2

    STA2 In-Duct Hot Wire Anemometer

    • Air velocity 40 to 3,960 fpm at 1 fpm resolution; accuracy ±(5% + 1 digit) of reading or ±(1% + 1 digit) of full scale.

    Specs: Fieldpiece product page, read

    View on Amazon for Fieldpiece STA2(paid link)

  3. Airflow and static pressureFluke 922

    922 Airflow Meter/Micromanometer

    • Differential and static pressure to ±16 inH2O (±4,000 Pa) with 0.001 inH2O resolution; accuracy ±1% + 0.01 inH2O.

    Specs: Fluke product page, read

    View on Amazon for Fluke 922(paid link)

  4. Laser distance metersBosch GLM400CL

    BLAZE Outdoor 400 Ft. Connected Lithium-Ion Laser Measure with Camera

    • Range up to 400 ft with accuracy up to ±1/16 in.

    Specs: Bosch product page, read

    View on Amazon for Bosch GLM400CL(paid link)

How tools are chosen for gear listsAll 20 HVAC tools