HVAC & Energy
Duct Size Calculator
Compare round and rectangular duct geometry with an explicit velocity or friction criterion. Solve a required size or reverse a known cross-section to airflow, then inspect friction rate, mean velocity and straight-run pressure loss.
Change any input to update the calculation. Unit changes preserve the physical quantity.
Inputs
Used when sizing a rectangular duct; reverse mode uses the entered width and height.
ASHRAE round-duct chart reference uses 0.09 mm; replace with the actual duct surface condition.
Dry-air scenario near room temperature; enter project-specific fluid properties if needed.
Calculated locally in your browser. No sign-up or input upload.
Results
Airflow
500 CFM235.974 L/s849.505 m³/hRound inside diameter
274.067 mm10.79 inMean velocity
4 m/s13.123 ft/sStraight-duct friction rate
0.722 Pa/m0.088 in.wg/100 ftStraight-run pressure loss
7.222 Pa0.007 kPaHydraulic diameter
274.067 mm10.79 inEqual-area round diameter
274.067 mm10.79 inEqual-friction round diameter at this flow
274.067 mm10.79 inReynolds number
72,600.553Darcy friction factor
0.02062Calculation breakdown
- Sizing criterion
- Mean velocity; friction rate reported separately
Assumptions & checks
Straight, rigid, uniform ducts only. Fittings, grilles, branches, noise criteria, leakage and flexible-duct compression are excluded; this does not replace a duct-system design.
Rectangular friction uses a hydraulic-diameter approximation. Laminar and transitional noncircular resistance is especially uncertain. Size results are calculated dimensions, not rounded stock sizes.
Formula
Q = vA and Darcy pressure loss with iterative Colebrook friction. Rectangular hydraulic diameter is 4A/perimeter. Equal-area and equal-friction round sizes solve different comparisons and are reported separately.
Worked example
Using the default values shown in the calculator, the same formula gives the following result. This is a quick sanity check for the calculation, not a design recommendation.
Inputs
- Cross-section
- Round
- Governing calculation criterion
- Mean velocity
- Design airflow
- 500 CFM
- Target mean velocity
- 4 m/s
- Target straight-duct friction rate
- 0.8 Pa/m
- Rectangular width / height ratio
- 2
- Straight-run duct length
- 10 m
- Absolute internal roughness
- 0.09 mm
- Air density
- 1.2 kg/m³
- Air kinematic viscosity
- 15.1 cSt
Result
- Airflow
- 500 CFM
- Round inside diameter
- 274.067 mm
- Mean velocity
- 4 m/s
- Straight-duct friction rate
- 0.722 Pa/m
- Straight-run pressure loss
- 7.222 Pa
- Hydraulic diameter
- 274.067 mm
- Equal-area round diameter
- 274.067 mm
- Equal-friction round diameter at this flow
- 274.067 mm
- Reynolds number
- 72,600.553
- Darcy friction factor
- 0.02062
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Frequently asked questions
Is this only CFM divided by velocity?
Velocity mode uses continuity, but friction mode solves Darcy/Colebrook resistance numerically. Both modes report the other criterion so you can compare constraints.
Are equal-area and equal-friction diameters interchangeable?
No. Equal area preserves cross-sectional area. Equal friction matches this model's straight-run pressure loss at the same flow and fluid properties.
Does the pressure loss include elbows and diffusers?
No. It covers the entered straight length only. Add component losses and review the complete fan/duct system separately.
Reference data & method sources
- ASHRAE Handbook: Duct Design
Darcy pressure loss, Colebrook friction and hydraulic diameter. Rectangular duct resistance is an approximation; fittings and flexible-duct compression need separate treatment.