Standard dimensions for piping reducers and expanders are recommended by ASME/ANSI standards.
Piping reducer
Piping reducers and expanders are tapered pipe joints used to join two pipes of different sizes together.
When the upstream pipe is larger in size, the joint is called a pipe reducer. As the flow goes from a large pipe to a 'reduced' pipe size.
Similarly, when the upstream pipe is smaller, flow goes from a small pipe to an 'expanded' pipe size. Hence in this case the tapered joint is called an 'expander' in this case.
Standard dimension calculator
You can use these calculators to quickly get standard dimensions for different sizes of piping reducers/expanders.
 Standard dimensions calculator for piping reducer / expander
 Standard pipe dimensions calculator as per ANSI standards
Standard reducer dimensions
Nominal Pipe Size
(inches) 
Outside Diameter
(mm) 
Length
(mm) 


Inch  D  P  H 
3/4 x 1/2  26.7  21.3  38 
3/4 x 3/8  26.7  17.1  38 
1 x 3/4  33.4  26.7  51 
1 x 1/2  33.4  21.3  51 
1 1/4 x 1  42.2  33.4  51 
1 1/4 x 3/4  42.2  26.7  51 
1 1/4 x 1/2  42.2  21.3  51 
1 1/2 x 1 1/2  48.3  42.2  64 
1 1/2 x 1  48.3  33.4  64 
1 1/2 x 3/4  48.3  26.7  64 
1 1/2 x 1/2  48.3  21.3  64 
2 x 1 1/2  60.3  48.2  76 
2 x 1 1/4  60.3  42.2  76 
2 x 1  60.3  33.4  76 
2 x 3/4  60.3  26.7  76 
2 1/2 x 2  73  60.3  89 
2 1/2 x 1 1/2  73  48.3  89 
2 1/2 x 1 1/4  73  42.2  89 
2 1/2 x 1  73  33.4  89 
3 x 2 1/2  88.9  73  89 
3 x 2  88.9  60.3  89 
3 x 1 1/2  88.9  48.3  89 
3 x 1 1/4  88.9  42.2  89 
3 1/2 x 3  101.6  88.9  102 
3 1/2 x 21/2  101.6  73  102 
3 1/2 x 2  101.6  60.3  102 
3 1/2 x 1 1/2  101.6  48.3  102 
3 1/2 x 1 1/4  101.6  42.2  102 
4 x 3 1/2  114.3  101.6  102 
4 x 3  114.3  88.9  102 
4 x 2 1/2  114.3  73  102 
4 x 2  114.3  60.3  102 
4 x 1 1/2  114.3  48.3  102 
5 x 4  141.3  114.3  127 
5 x 3 1/2  141.3  101.6  127 
5 x 3  141.3  88.9  127 
5 x 2 1/2  141.3  73  127 
5 x 2  141.3  60.3  127 
6 x 5  168.3  141.3  140 
6 x 4  168.3  114.3  140 
6 x 3 1/2  168.3  101.6  140 
6 x 3  168.3  88.9  140 
6 x 2 1/2  168.3  73  140 
8 x 6  219.1  168.3  152 
8 x 5  219.1  141.3  152 
8 x 4  219.1  114.3  152 
8 x 3 1/2  219.1  101.6  152 
10 x 8  273.1  219.1  178 
10 x 6  273.1  168.1  178 
10 x 5  273.1  141.3  178 
10 x 4  273.1  114.3  178 
12 x 10  323.9  273.1  203 
12 x 8  323.9  219.1  203 
12 x 6  323.9  168.3  203 
12 x 5  323.9  141.3  203 
14 x 12  355.6  323.9  330 
14 x 10  355.6  273.1  330 
14 x 8  355.6  219.1  330 
14 x 6  355.6  168.3  330 
16 x 14  406.4  355.6  356 
16 x 12  406.4  323.9  356 
16 x 10  406.4  273.1  356 
16 x 8  406.4  219.1  356 
16 x 6  406.4  168.3  356 
18 x 16  457  406.4  381 
18 x 14  457  355.6  381 
18 x 12  457  323.9  381 
18 x 10  457  273.1  381 
18 x 8  457  219.1  381 
20 x 18  508  457  508 
20 x 16  508  406.4  508 
20 x 14  508  355.6  508 
20 x 12  508  323.9  508 
20 x 10  508  273.1  508 
20 x 8  508  219.1  508 
22 x 20  559  508  508 
22 x 18  559  457  508 
22 x 16  559  406.4  508 
22 x 14  559  355.6  508 
22 x 12  559  323.9  508 
24 x 10  559  273.1  508 
24 x 22  610  559  508 
24 x 20  610  508  508 
24 x 18  610  457  508 
24 x 16  610  406.4  508 
24 x 14  610  355.6  508 
24 x 12  610  323.9  508 
24 x 10  610  273.1  508 
Standards for piping reducers
B16.1 (2005)  Gray Iron Pipe Flanges and Flanged Fittings: Classes 25, 125, and 250
This Standard covers Classes 25, 125, and 250 Gray Iron Pipe Flanges and Flanged Fittings. It includes: (a) pressuretemperature ratings; (b) sizes and method of designating openings of reducing fittings; (c) markings; (d) materials; (e) dimensions and tolerances; (f) bolting and gaskets; (g) pressure testing.
B16.5 (2009) – Pipe flanges and flanged fittings (2009)
This standard covers pressuretemperature ratings, materials, dimensions, tolerances, marking, testing and methods of designating for pipe flanges and flanged fittings
Here are more ASME/ANSI piping standards for reference.