Inputs
For oval ducts, use major and minor diameters.
a and b are full diameters, not radii.
* All dimensions must be positive real numbers.
Results
Equivalent Round Diameter
—
mm / in
—
Section Properties
Area: —
Perimeter: —
Hydraulic diameter: —
| # | Shape | Method | Inputs | De (mm) | De (in) | Area (cm²) | Perimeter (mm) | Hydraulic D (mm) | Source |
|---|
Example Data
These examples demonstrate common duct sizes and both calculation methods.
| # | Shape | Inputs (mm) | Method | Equivalent Round De (mm) | Note |
|---|
Formulas Used
- Equal Area (EA): For any shape with area
A, the equivalent round diameter isD_e = √(4A/π).Rectangular:A = W \u00d7 H. Oval (ellipse):A = πab/4. - Equal Friction (EF):
Rectangular (ASHRAE):
D_e = 1.30 \u00d7 (WH)^{0.625} / (W + H)^{0.25}.Oval (ellipse, approximation via hydraulic diameter): perimeter by RamanujanP \u2248 \u03c0 \u00d7 \left[3(a+b) - \u221a\big((3a+b)(a+3b)\big)\right], areaA = \u03c0 ab/4, hydraulic diameterD_h = 4A/P, takeD_e \u2248 D_h. - Hydraulic diameter:
D_h = 4A / P. Rectangular perimeter:P = 2(W+H). - Unit conversions:
1 in = 25.4 mm. Areas shown as cm² for readability.
How to Use
- Select the duct shape and your preferred units.
- Enter dimensions: W and H for rectangular, or a and b for oval.
- Choose the method: Equal Area or Equal Friction.
- Click Compute & add row to record the result.
- Use Load examples to populate typical sizes automatically.
- Download your table as CSV or PDF for reporting.
FAQs
It is the diameter of a circular duct that represents a noncircular duct by matching a chosen criterion such as equal area or equal frictional pressure loss.
Equal Area preserves cross‑sectional area and is helpful for flow capacity comparisons.
Equal Friction preserves pressure‑loss behavior and is commonly used for sizing ducts to maintain similar friction loss per length.
Yes. For oval ducts, enter major axis a and minor axis b. For Equal Friction mode, this tool uses the hydraulic diameter with Ramanujan’s perimeter approximation for practical accuracy.
Millimetres and inches are supported. Inputs are converted internally to millimetres to compute geometry. Results are displayed in both millimetres and inches for convenience.
Not necessarily. Equal Area matches cross‑section only; actual airflow depends on system pressure and velocity. Equal Friction better matches pressure loss behavior for comparable duct lengths and roughness.
The rectangular formula is the standard ASHRAE correlation and performs well across typical aspect ratios. The oval approach via hydraulic diameter is an engineering approximation appropriate for concept and preliminary sizing.