SDR Pipe – Definition, vs. DR, Chart, and Applications

Sarah
DeGuzman

The SDR pipe stands for “Standard Dimensional Ratio” and describes the pipe’s geometry. The nominal outer diameter to nominal wall thickness ratio is known as the SDR. This article defines SDR pipe, reviews SDR vs. DR, and reviews typical applications of SDR pipe. 

Gasketed SDR Pipe
Courtesy: NAPCO Pipe

SDR Definition

Standard dimension ratio (SDR) is a method of rating a pipe’s durability against pressure. The legal dimension ratio describes the correlation between the pipe dimension and the thickness of the pipe wall. 

For example, SDR 21 denotes that the PE pipe’s outer diameter is twenty-one times its wall thickness. As a result, the lower the allowable pipe pressure, the higher the SDR. SDRs run from 5 to 50, with the lesser the number, the thicker the pipe, and the smaller the diameter. In addition, lines with a lower SDR can withstand higher pressures. The maximum pressure rating for the SDR pipe remains the same at different diameters. Since SDR is a diameter to thickness ratio, all lines with the same SDR have the same pressure rating.

SDR Formula

To determine the SDR of the pipe, engineers use the relatively simple formula of Standard Diameter Ratio = OD/mn. The term “OD” refers to the pipe’s average outside diameter, while “mn” refers to the line’s minimum wall thickness. If these numbers aren’t available, one can use a round micrometer to physically measure the pipe’s open end. Sometimes product markings show DR/SDR on the exterior.

This table shows the minimum wall thickness and calculated ID in inches of SDR 13.5, SDR 17, SDR 21, SDR 26, SDR 32.5, SDR 41, and SDR 64 for nominal sizes from one to ten.

SDR Chart

This chart presents SDR pipe dimensions for SDR 64, 41, 32.5, 26, 21, 17, and 13.5

SDR PipeNominal
Pipe Size
(inches)
Average
Outside
Diameter
(inches)
Minimum
Wall
Thickness
(inches)
Calculated
ID
(inches)
SDR 641/8
1/4
3/8
1/2
3/4
1
1 1/4
1 1/2
2
3
4
5
6
8
10
0.405
0.54
0.675
0.84
1.05
1.315
1.66
1.9
2.375
3.5
4.5
5.563
6.625
8.625
10.75










0.07
0.087
0.104
0.135
0.168










4.36
5.39
6.42
8.36
10.41






SDR 41





1/8
1/4
3/8
1/2
3/4
1
1 1/4
1 1/2
2
3
4
5
6
8
10
0.405
0.54
0.675
0.84
1.05
1.315
1.66
1.9
2.375
3.5
4.5
5.563
6.625
8.625
10.75









0.085
0.11
0.138
0.162
0.21
0.262









3.33
4.28
5.29
6.30
8.20
10.23


SDR 32.5




1/8
1/4
3/8
1/2
3/4
1
1 1/4
1 1/2
2
3
4
5
6
8
10
0.405
0.54
0.675
0.84
1.05
1.315
1.66
1.9
2.375
3.5
4.5
5.563
6.625
8.625
10.75






0.06
0.06
0.073
0.108
0.138
0.171
0.204
0.265
0.331






1.56
1.78
2.23
3.28
4.22
5.22
6.22
8.09
10.09
SDR 261/8
1/4
3/8
1/2
3/4
1
1 1/4
1 1/2
2
3
4
5
6
8
10
0.405
0.54
0.675
0.84
1.05
1.315
1.66
1.9
2.375
3.5
4.5
5.563
6.625
8.625
10.75





0.06
0.064
0.073
0.091
0.135
0.173
0.214
0.255
0.332
0.413





1.21
1.53
1.75
2.19
3.23
4.15
5.14
6.12
7.96
9.92
SDR 211/8
1/4
3/8
1/2
3/4
1
1 1/4
1 1/2
2
3
4
5
6
8
10
0.405
0.54
0.675
0.84
1.05
1.315
1.66
1.9
2.375
3.5
4.5
5.563
6.625
8.625
10.75




0.06
0.063
0.079
0.09
0.113
0.167
0.214
0.265
0.316
0.41
0.511




0.95
1.19
1.50
1.72
2.15
3.17
4.07
5.03
5.99
7.80
9.73
SDR 171/8
1/4
3/8
1/2
3/4
1
1 1/4
1 1/2
2
3
4
5
6
8
10
0.405
0.54
0.675
0.84
1.05
1.315
1.66
1.9
2.375
3.5
4.5
5.563
6.625
8.625
10.75




0.062
0.077
0.098
0.112
0.14
0.206
0.265
0.327
0.39
0.508
0.632




0.93
1.16
1.46
1.68
2.10
3.09
3.97
4.91
5.85
7.61
9.49
SDR 13.51/8
1/4
3/8
1/2
3/4
1
1 1/4
1 1/2
2
3
4
5
6
8
10
0.405
0.54
0.675
0.84
1.05
1.315
1.66
1.9
2.375
3.5
4.5
5.563
6.625
8.625
10.75
0.06
0.06
0.06
0.62
0.078
0.097
0.123
0.141
0.176
0.259
0.333
0.412
0.491

0.35
0.46
0.58
0.72
0.89


1.62
2.02
2.98
3.83
4.74
5.64

SDR vs. DR

The terms DR and SDR are widely used in the pipe industry. DR stands for Dimension Ratio, the average outside diameter of a PE pipe divided by its minimum wall thickness. Also, DR applies only to solid-wall products and has no relevance for profile-wall products.  Profile wall pipes present a smooth interior wall but include ribs, corrugations, and other shapes to brace the pipe against deformation.

Profile Pipe, Courtesy: India Mart

Even though the terms DR and SDR are synonymous, the critical difference is that DR is used for any number, while SDR is used for a particular series of numbers. This “preferred numbers” series is based on a geometric progression developed by a French engineer named Charles Renard. These numbers are often called “Renard’s Numbers.” Renard’s numbers are as follows: 13.5, 17, 21, 26, 32.5, 41, 51, and 64. If a pipe’s ratio isn’t one of Renard’s Numbers, it can’t be referred to as SDR; it can only be called DR.

The development of dimension ratios and standard dimension ratios was based on convenience rather than need. They were established to provide worldwide uniformity in the specification of plastic pipes. These provide a straightforward way of showing product dimensions to preserve stable mechanical qualities regardless of pipe size since they define a constant ratio between outer diameter and wall thickness. In other words, regardless of pipe size, pressure capacity and pipe stiffness stay constant for a particular DR or SDR.

Common Application of SDR Pipe

SDR pipes and associated fittings are used for various applications like agriculture, irrigation, water supply, industrial process lines, swimming pools, and firefighting mains. SDR pipes often provide stormwater and drainage applications. SDR-17 polyethelene pipes are suitable for gravity sewage applications, while thinner tubes like SDR 20 are ideal for shorter and smaller diameter applications.

Light lines severely deflect during installation and pipelining. SDR35 PVC sewage pipe, like other PVC pipe systems, is resistant to a wide range of chemicals and conditions, making it an excellent long-term investment for water utility infrastructure projects. It is effective against corrosive substances such as sewage gas, sulfuric acid, alkaline chemicals, and acidic soils. It is also highly resistant to scouring, abrasions, and gouging.