HVAC Duct Sizing Calculator (Ductulator)

Equal-friction method · Round & rectangular · Velocity check · PDF + CSV

Size sheet-metal duct from a known airflow (CFM) and target friction rate — the same equal-friction approach built into a field Ductulator wheel. Returns required equivalent round diameter, suggested standard stock size, rectangular solutions for a fixed side, and a velocity check against residential/commercial guidelines. This is not a Manual J/D load calculator.

First HVAC tool on TestTalkHQ. This opens the HVAC vertical. It is listed on the Interactive Tools Hub and in the free-calculators HVAC section — deliberately not filed under Electrical or Machining & Fabrication.

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How to use: Enter CFM and friction rate (chips for common residential/commercial rates). Choose Round or Rectangular (set the fixed side when joist bay / clearance constrains one dimension). Pick a velocity application, then Size the duct. Export a PDF result card, PDF reference table, or CSV. Always round up to stock sizes your supplier carries.

Shop HVAC & Duct Tools

Genuine airflow and sheet-metal tools from the affiliate catalog — anemometer and manometer for field checks, snips and TurboShear for fabricating duct. No physical Ductulator wheel or mastic/foil-tape SKUs were in the CSV at publish time, so those are omitted rather than substituted with unrelated welding gear.

Fieldpiece STA2 In-Duct Hot Wire Anemometer

Fieldpiece STA2 In-Duct Anemometer

Hot-wire anemometer for in-duct velocity checks — confirm the FPM your sized run is actually moving.

View on Amazon →

Testo 512-2 Differential Pressure Manometer

Testo 512-2 Manometer

Differential pressure manometer for static pressure and friction-rate field checks on duct systems.

View on Amazon →

Malco M2006 Left Offset Snips

10" left-offset aviation snips for cutting galvanized duct and sheet-metal fittings in the field.

View on Amazon →

Malco TS1 TurboShear

Drill-powered sheet-metal shear attachment for long straight cuts in galvanized duct stock.

View on Amazon →

As an Amazon Associate, TestTalkHQ earns from qualifying purchases. No Ductulator wheel, mastic, or foil tape appeared in the affiliate CSV at publish time.

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Sizing Only — What This Tool Does Not Do

It does not calculate required room CFM — that comes from ACCA Manual J (loads) and Manual D (duct design). It sizes straight runs only and does not add fitting losses (elbows, takeoffs, transitions). Calculated diameters often land between stock increments; the tool suggests the next standard size up so you never undersize airflow. Read the full guide or troubleshooting.

Companion Reading

Learn the equal-friction method in the Duct Sizing Guide. Fix no-solution and velocity-flag issues in troubleshooting. Decide shape with Round vs Rectangular Duct.

HVAC Duct Sizing Calculator (Ductulator)