Duct Calculator: HVAC Sizing

Now sizes flex and duct board, not just metal. The material changes the size. Type in a duct you already have to see what it carries. Free trial, then subscription.

The digital ductulator. Equal-friction duct sizing in your pocket, no cardboard slide rule.

Type in the airflow and get the duct size. Or type in a duct that's already there and find out what it actually carries. Every calculation runs on the device, with no account and nothing to load.

Every subscription starts with a free trial, so you can put it through a real job before you pay for anything. After the trial Ductulator needs a subscription, and the tools below are what it unlocks. Cancel anytime in Settings.

WHAT YOUR SUBSCRIPTION UNLOCKS

SIZE A NEW DUCT

• The exact diameter the airflow needs, plus the nearest standard round size
• Fix one side of a rectangular duct and get the matching other side
• Equivalent round diameter (Huebscher), to cross-check against round runs
• Air velocity, flagged when it runs above the residential guide

CHECK A DUCT THAT'S ALREADY THERE

• Type in a round diameter, or a width and a height
• Read back its velocity, its friction rate, and the airflow it will carry
• Adequate or undersized, judged against whichever limit binds first

THE MATERIAL CHANGES THE ANSWER

• Galvanized, duct board or flexible duct, each with its own ASHRAE roughness
• Flex needs a full standard size more than metal at the same airflow. Size a flex run as if it were sheet metal and it always goes in over the friction rate it was designed for, by up to double

BUILT ON REAL PHYSICS

• Darcy-Weisbach with the Colebrook friction factor, solved on the spot. It is the method behind the ASHRAE friction chart and professional ductulators
• Standard air, with supply (0.10 in wg/100ft) and return (0.08) defaults built in, or set your own friction rate
• A friction rate you enter is yours. Switching between supply and return never overwrites it, because Manual D designs one system to one rate

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