# Duct Size Calculator (Flow Rate × Velocity) Size a duct cross-section from the air flow rate Q and the design mean velocity v: area A = Q/v. Circular duct diameter D = √(4A/π). Rectangular duct with aspect ratio r = a/b gives b = √(A/r) and a = r·b, plus the ASHRAE equivalent diameter D_eq = 1.30·(a·b)^0.625/(a+b)^0.25. Flow rate in m³/s/m³/h/CFM, velocity in m/s; dimensions reported in mm and inches. > Canonical page: https://elysiatools.com/en/tools/duct-size-calculator - **Category:** Math & Numbers - **Keywords:** duct sizing, ductwork, airflow, velocity, circular duct, rectangular duct, equivalent diameter, HVAC, CFM ## Overview The Duct Size Calculator determines the required duct cross-sectional area from airflow and design velocity, then calculates dimensions for circular or rectangular ducts. Enter airflow in m³/s, m³/h, or CFM and velocity in m/s to receive dimensions in millimetres and inches. ## Inputs - **Duct Shape** (select): Duct cross-section shape: Rectangular or Circular. - **Air Flow Rate Q** (number): Volumetric air flow rate Q. Enter in the selected Flow Unit. - **Flow Unit** (select) - **Design Velocity v** (number): Design mean air velocity v in m/s. Typical: low-pressure 3–5 m/s, main 6–8 m/s. - **Aspect Ratio (a/b)** (number): Rectangular duct aspect ratio r = a/b (long side over short side). Used only when Duct Shape = Rectangular. Typical 1–4. Default 2. - **Decimal Places** (number) ## When to use - Estimate a duct cross-section for a specified airflow and design velocity. - Compare circular and rectangular duct dimensions for HVAC layouts. - Calculate rectangular duct dimensions using a chosen aspect ratio and equivalent diameter. ## How it works - Enter the airflow rate and select m³/s, m³/h, or CFM as the flow unit. - Enter the design mean air velocity in m/s. - Choose a circular or rectangular duct shape; for rectangular ducts, set the aspect ratio a/b. - The calculator finds area using A = Q/v and reports the resulting diameter or width and height in millimetres and inches. ## Use cases - HVAC design estimates for supply and return ductwork. - Preliminary sizing of circular ducts from CFM or metric airflow values. - Selecting rectangular duct proportions and reviewing their equivalent diameter. ## Frequently asked questions ### What inputs does the calculator require? It requires an airflow rate and a design velocity. You can also select the duct shape and, for rectangular ducts, an aspect ratio. ### Which airflow units are supported? The calculator supports cubic metres per second (m³/s), cubic metres per hour (m³/h), and cubic feet per minute (CFM). ### What velocity unit should I enter? Enter the design mean air velocity in metres per second (m/s). ### How are circular duct dimensions calculated? The calculator first finds area with A = Q/v, then calculates diameter using D = √(4A/π). ### What does the rectangular aspect ratio mean? It is the long side divided by the short side, expressed as r = a/b. 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