**Units in hydraulic diameter calculator: **Numbers may be entered in any units, so long as they are consistent. [L] means length units. [L/T] means length per unit time. [L^{3}/T] means flow rate in cubic length per unit time. For instance, if b and c are cm, then area will be cm^{2}. If velocity in cm/s and area in cm^{2},
then flow rate will be cm^{3}/s.

**Cross-Sections and Equations for Rectangular and Annular Hydraulic Diameter Calculator**

For both geometries: Q=VA, Q=Flow Rate, V=Velocity, A=Flow Area

Rectangular: A=Area=bc. P=Perimeter=2(b+c). D=Hydraulic Diameter=4A/P

Annular: A=Area=π(c^{2}-b^{2})/4.
D=Hydraulic Diameter=c-b References

**Using our hydraulic diameter page**

If you have a non-circular duct, the cross-sectional geometry can be converted to
hydraulic diameter using this page. Then, velocity (not flow rate)
can be computed using a) Design of Circular Liquid or Gas
Pipes or b) Design of Circular Water Pipes based
on the hydraulic diameter. However, (a) and (b) will not compute the correct
flow rate since they compute area based on a circular cross-section. Therefore, you
must multiply the velocity by the actual area of the duct. The actual area is
computed on this page.

If you wish to determine the cross-sectional geometry of a non-circular duct, you can use a) Design of Circular Liquid or Gas Pipes or b) Design of Circular Water Pipes if you know the velocity, not the flow rate (flow rate can only be converted to velocity if the actual duct area is used, not the area based on hydraulic diameter). From the hydraulic diameter output from either (a) or (b), this page can be used to determine the geometry of the duct.

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