# Oval Rectangle

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## Convert rectangular and oval duct geometry to a equivalent circular diameter - online calculator with imperial and SI-units

The equivalent diameter is the diameter of a circular duct or pipe that for equal flow gives the same pressure loss or resistance as an equivalent rectangular duct or pipe. A round shape results in lower pressure drops and less fan horsepower to move the air and, consequently, smaller equipment.

The equivalent diameter of a rectangular tube or duct can be calculated as (Huebscher)

de = 1.30 (a b)0.625 / (a + b)0.25 (1)

where

de = equivalent diameter (mm, inches)

a= length of major or minor side (mm, inches)

b= length of minor or major side (mm, inches)

• Hydraulic diameter - the 'characteristic length' used to calculate the dimensionless Reynolds Number

### Example - Equivalent Diameter

The equivalent diameter of a 300 mm x 500 mm rectangular duct can be calculated as

de = 1.30 ((300 mm) (500 mm))0.625 / ((300 mm) + (500 mm))0.25

= 420 mm

### Rectangular to Equivalent Circular Duct Calculator

The calculator below is based on formula (1). The formula is generic and any unit can be used.

Equivalent diameters for some common mm rectangular duct dimensions:

For full table with larger dimensions - rotate the screen!

Circular equivalent diameter - de(mm)
Duct
side
- a -
(mm)
Duct side - b(mm)
100150200250300400500600800100012001400160018002000
100109133152168183207227
150133164189210229261287310
200152189219244266305337365
250168210246273299343381414470
300183229266299328378420457520574
400207260305343378437488531609674731
500227287337381420488547598687762827886
6003103654144575315986567558409149801041
8004144705206096877558759761066114612191286
1000517574674762840976109311961289137314511523
12006207318279141066119613121416151115981680
14007818869801146128914161530163517321822
160093910411219137315111635174918541952
180010961286145115981732185419682073
2000152316801822195220732186

print equivalent diameter diagram!

Equivalent diameters of some common inches rectangular ducts:

For full table - rotate the screen!

Circular equivalent diameter (inches)
Length
- b -
(inches)
Length - a - (inches)
4568101216
44.44.95.36.1
54.95.566.97.6
65.366.67.68.49.1
86.16.97.68.69.810.712.2
107.68.49.810.91213.7
129.110.71213.115.1
1612.213.715.117.5

print equivalent diameter diagram!

### Oval Equivalent Diameter (Ellipse)

Flat oval ducts have smaller height requirements than round ducts and retain most of the advantages of the round ducts. Fittings for flat oval ducts are difficult to fabricate or modify in the field.

The equivalent diameter of a oval duct or tube (ellipse) can be calculated as (Heyt & Diaz)

de = 1.55 A0.625 / P0.25 (2)

where

A = cross-sectional area oval duct (m2, in2)

P = perimeter oval duct (m, inches)

The cross-sectional area of an oval duct can be expressed as

A = π a b / 4

where

a = major dimension of the flat oval duct (m, in)

b = minor dimension of the flat oval duct (m, in)

The perimeter of an oval duct (ellipse) can be approximated to

### Oval Rectangle Clipart

P ≈ 2 π (1/2 ((a / 2)2 + (b / 2)2))1/2 (2b)

### Oblong Ducts

Equivalent diameters of oblong ducts or pipes can be expressed as

de = 1.55 (π b2 / 4 + a b - b2)0.625/(π b + 2 a - 2 b)0.25 (3)

### Hydraulic Diameter

Note! - equivalent diameter is not the same as hydraulic diameter. The hydraulic diameter express the rate between the area section of the duct or tube, and the wetted perimeter of the duct or tube. Hydraulic diameter is used to determine if the flow is laminar or turbulent and to calculate the pressure loss.

## Related Topics

• Fluid Mechanics - The study of fluids - liquids and gases. Involves velocity, pressure, density and temperature as functions of space and time
• Ventilation - Systems for ventilation and air handling - air change rates, ducts and pressure drops, charts and diagrams and more

## Related Documents

### Oval Rectangle Above Ground Pool

• Air Ducts - Friction Loss Diagram - A major loss diagram for air ducts - Imperial units ranging 10 - 100 000 cfm
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• Smaller Rectangles within a Large Rectangle - The maximum number of smaller rectangles - or squares - within a larger rectangle (or square)