Calculators

Duct and airflow calculators

Most comfort complaints on a correctly sized system come back to air: not enough CFM at the register, a blower pushing against too much static pressure, or duct that leaks into the attic. These calculators cover the design side and the test side of that.

Start with the design CFM, size the trunk and branches by equal friction, convert to rectangular where the framing demands it, and check the static pressure budget the blower has left. The fan law and leakage calculators are for the service call and the code inspection.

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Which one you need

Every tool in this section

  • Duct sizing

    Duct Sizing Calculator

    Supply trunk and branch diameters from design CFM, longest run, available static pressure and duct material.

    Returns round sizes with rectangular equivalents, velocities and the design friction rate. Sales-phase sizing; room-by-room Manual D is part of the Pro plan.

    Based on ACCA Manual D (equal friction)

  • Airflow

    HVAC CFM Calculator

    Design airflow from cooling tons, a sensible BTU/h load, or a furnace output and temperature rise.

    Uses Qs = 1.08 x CFM x dT at sea-level air and flags CFM per ton, supply dT or rise outside the usual range.

    Based on ASHRAE sensible heat equation

  • Ventilation

    Duct Leakage Test Calculator

    CFM25 per 100 sq ft of floor area and the allowed total leakage for your code edition.

    Rough-in and postconstruction limits of the 2021 IECC, the 2024 table by ducted returns and system size. Leakage to outside gets its number but no pass or fail.

    Based on 2021 IECC R403.3.6, 2024 IECC Table R403.3.8

  • Airflow

    Static Pressure Calculator

    Total external static pressure from supply and return readings, or the available static pressure left for the ducts.

    Adds the drop of a coil or filter the rating leaves out when your probe sits beyond it, then compares the pressure at the rating points with the blower's rated maximum. The design mode subtracts the filter, coil, grille and damper drops you enter and returns the friction rate over the total effective length of the longest supply and return runs.

    Based on Manufacturer blower data, ACCA Manual D method

  • Airflow

    Fan Law Calculator

    New CFM, static pressure and brake horsepower for a blower speed change, and the speed for a target airflow.

    Applies the three fan laws to a known point on the same fan and duct system. The density mode converts static pressure and horsepower between standard air and the air at your altitude and temperature.

    Based on Fan laws (HVAC School), Aerovent FE-1600 density correction

  • Duct sizing

    Rectangular to Round Duct Calculator

    The round duct equal in friction and airflow to a rectangle, or the rectangle for a round size with one side fixed.

    Inside dimensions in, equivalent round diameter and the next stock size out. An optional CFM adds the velocity in both shapes and the friction rate for galvanized duct. It converts a duct you already have; sizing from CFM is the duct sizing calculator.

    Based on ASHRAE Fundamentals Ch. 21, Huebscher equivalent diameter

  • Duct sizing

    Duct Velocity and Friction Loss Calculator

    Friction rate and velocity of one round duct, the CFM a size carries at a friction rate, or the size for an airflow.

    Galvanized round or flex fully stretched, standard air, straight duct. Checks the velocity against the Manual D Table 3-1 limits for a supply or return trunk or branch. A whole trunk and branch layout from the system CFM is the duct sizing calculator.

    Based on ASHRAE Fundamentals Ch. 21 (Darcy, Colebrook), ACCA Manual D Table 3-1

Other calculators

Every calculator here lists the standard it works from and what it leaves out. The numbers are for sales-phase sizing and field checks, not a permit submittal. Methodology lists the standard behind each one.