Calcmeister

Air duct size calculator

Measure the airflow (CFM) of your HVAC system and enter your desired duct velocity. Choose round or rectangular duct shape and specify the aspect ratio for rectangular ducts.

CFM

Total airflow in cubic feet per minute.

FPM

Desired air velocity in feet per minute. Typical values: 600–900 for supply ducts, 400–600 for return ducts.

:1

Only for rectangular ducts. Ratio of width to height.

%

Extra capacity added to airflow for safety.

Equivalent diameter
– in
Duct diameter (round)
– in
Duct width (rectangular)
– in
Duct height (rectangular)
– in
Cross-sectional area
– ft²
Actual velocity
– FPM

    How It Is Calculated

    This calculator uses the velocity method to determine the required duct size for a given airflow and desired air velocity. The steps are:

    1. Read the inputs: airflow (Q) in CFM, duct shape (round or rectangular), design velocity (V) in FPM, aspect ratio (AR) for rectangular ducts, and safety factor (SF) in percent.
    2. Compute the adjusted airflow: Q_adj = Q × (1 + SF / 100).
    3. Calculate the required cross-sectional area in square feet: A_ft² = Q_adj / V.
    4. Convert area to square inches: A_in² = A_ft² × 144.
    5. If the duct is round:
      • Duct diameter (D) in inches = 2 × √(A_in² / π).
      • Equivalent diameter = D.
      • Width and height are set to 0.
    6. If the duct is rectangular:
      • Height (H) in inches = √(A_in² / AR).
      • Width (W) in inches = AR × H.
      • Equivalent diameter (D_eq) in inches = 1.3 × (W × H)0.625 / (W + H)0.25 (Huebscher equation).
      • Duct diameter (round) is set to 0.
    7. Cross-sectional area (A_ft²) is rounded to 2 decimals.
    8. Actual velocity = V (since velocity method is used), rounded to 0 decimals.

    Q_adj = Q × (1 + SF / 100)

    A_ft² = Q_adj / V

    A_in² = A_ft² × 144

    Round: D = 2 × √(A_in² / π)

    Rectangular: H = √(A_in² / AR), W = AR × H

    D_eq = 1.3 × (W × H)0.625 / (W + H)0.25

    Assumptions

    Worked Example

    Given: Airflow = 1000 CFM, Duct shape = Round, Design velocity = 800 FPM, Aspect ratio = 1:1 (not used), Safety factor = 10%.

    1. Adjusted airflow = 1000 × (1 + 10/100) = 1100 CFM.
    2. Required area = 1100 / 800 = 1.375 ft².
    3. Area in in² = 1.375 × 144 = 198 in².
    4. Round duct diameter = 2 × √(198 / π) ≈ 15.9 in.
    5. Equivalent diameter = 15.9 in.
    6. Cross-sectional area = 1.38 ft² (rounded).
    7. Actual velocity = 800 FPM.

    Results: Equivalent diameter = 15.9 in, Duct diameter (round) = 15.9 in, Duct width = 0 in, Duct height = 0 in, Cross-sectional area = 1.38 ft², Actual velocity = 800 FPM.

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    Questions and answers

    What is the difference between equivalent diameter and actual duct diameter?

    Equivalent diameter is the diameter of a round duct that has the same friction loss per unit length as a rectangular duct of given dimensions. For round ducts, the equivalent diameter equals the actual diameter.

    Why is a safety factor added to the airflow?

    A safety factor accounts for future system changes, filter loading, or minor leaks, ensuring the duct can handle slightly higher airflow without excessive velocity or pressure drop.

    Can I use this calculator for both supply and return ducts?

    Yes, but you should adjust the design velocity accordingly: supply ducts typically use 600–900 FPM, while return ducts use 400–600 FPM to reduce noise.

    What if my calculated duct size doesn't match standard sizes?

    Round up to the next standard size (e.g., from 15.9 to 16 inches) to ensure adequate airflow. For rectangular ducts, adjust width and height while keeping the same cross-sectional area.

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    Last reviewed: 2026-10-18. Results are estimates for planning. Check them before you rely on them for design, purchase or safety decisions.