Fluid Flow - Equivalent Diameter

Calculate the equivalent diameter of rectangular, oval, and oblong ducts or pipes. The equivalent diameter is the diameter of a circular duct that produces the same pressure loss as an equivalent rectangular duct for equal flow.

Rectangular Duct Equivalent Diameter Calculator

Formula (Huebscher):

de = 1.30 (a × b)^0.625 / (a + b)^0.25

Where:

de = equivalent diameter (mm or inches)
a = length of major or minor side (mm or inches)
b = length of minor or major side (mm or inches)

A round shape results in lower pressure drops and less fan horsepower.

Calculator

Inputs

Duct Side a (mm)

Duct Side b (mm)

Results

Equivalent Diameter-- mm

Duct Side a (mm)	100	150	200	250	300	400	500	600	800	1000	1200	1400
100	109	133	152	168	183	207	227
150	133	164	189	210	229	261	287	310
200	152	189	219	244	266	305	337	365
250	168	210	246	273	299	343	381	414	470
300	183	229	266	299	328	378	420	457	520	574
400	207	260	305	343	378	437	488	531	609	674	731
500	227	287	337	381	420	488	547	598	687	762	827	886
600	310	365	414	457	531	598	656	755	840	914	980	1041
800	414	470	520	609	687	755	875	976	1066	1146	1219	1286
1000	517	574	674	762	840	976	1093	1196	1289	1373	1451	1523
1200	620	731	827	914	1066	1196	1312	1416	1511	1598	1680
1400	781	886	980	1146	1289	1416	1530	1635	1732	1822
1600	939	1041	1219	1373	1511	1635	1749	1854	1952
1800	1096	1286	1451	1598	1732	1854	1968	2073
2000	1523	1680	1822	1952	2073	2186

Oval Duct (Ellipse) Equivalent Diameter

Formula (Heyt & Diaz):

de = 1.55 × A^0.625 / P^0.25

Where:

A = cross-sectional area (m² or in²)
P = perimeter (m or inches)
A = π × a × b / 4
P ≈ 2π × √((a/2)² + (b/2)²) / 2

Where a = major dimension, b = minor dimension

Oblong Duct Equivalent Diameter

Formula:

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

Where a and b are the dimensions of the oblong duct.