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Calcimator

Duct Sizing Calculator

Size round or rectangular ductwork using the equal friction method based on CFM and desired friction rate.

Inputs

CFM
"WC/100ft

Residential: 0.08" WC/100 ft; Commercial: 0.06–0.10"; VAV systems: 0.06–0.08" WC/100 ft

SMACNA best practice: ≤4:1 aspect ratio; >4:1 increases friction loss significantly

Results

Round Duct Diameter

22 inches

≈ 6 credit cards

Rectangular Width0 inches
Rectangular Height0 inches
Air Velocity303 FPM
Pressure Drop0.08 "WC/100ft
How to Use This Calculator
  1. Enter Airflow, Friction Rate, and Duct Shape.
  2. Set Aspect Ratio (W:H).
  3. Review the Round Duct Diameter (inches) result.
  4. Use Rectangular Width (inches) and Rectangular Height (inches) to inform your decision.

How the result changes with Airflow

AirflowRound Duct Diameter
10,04560 inches
35,033104 inches
65,018134 inches
90,005154 inches

What each input means

Airflow
Required airflow through the duct section in cubic feet per minute.
Friction Rate
Design friction rate in inches of water column per 100 feet. SMACNA HVAC Duct Construction Standards recommends 0.08–0.10" WC/100 ft for residential and 0.06–0.10" for commercial.
Duct Shape
Duct cross-section shape: 1 = Round, 2 = Rectangular.
Aspect Ratio (W:H)
Width-to-height ratio for rectangular ducts. SMACNA recommends keeping aspect ratio below 4:1 to minimize friction and fabrication costs; above 5:1 significantly increases pressure loss.

How this is calculated

Worked example, using the default values

  1. Identify Input Parameters
    4 parameters
    Airflow = 800, Friction Rate = 0.08, Duct Shape = 1, Aspect Ratio (W:H) = 2 = 4 input(s) provided
  2. Calculate Round Duct Diameter
    22 = 22
  3. Calculate Rectangular Width
    0 = 0
  4. Calculate Rectangular Height
    0 = 0

Engine last updated . Checked against 1 independently-derived test — how we verify calculators.

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