Pipe Spec Quick Reference: SDR, DR and C900 Ratings

By David Ortego, Manager · Reviewed by David Ortego, Manager · Updated October 20, 2026

SDR and DR are the ratio of a pipe's outside diameter to its wall thickness. A lower number means a thicker wall and a higher rating. SDR 26 is stiffer than SDR 35, and C900 DR14 is rated for more pressure than DR18. Every number below has a source.

Jump to: SDR and DR · SDR 26 vs SDR 35 · SDR 35 vs Schedule 40 · C900 DR18 vs DR14 · HDPE DR

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What SDR and DR mean

SDR stands for standard dimension ratio. DR stands for dimension ratio. Both are the outside diameter divided by the minimum wall thickness. The idea is the same for PVC and HDPE: the lower the number, the thicker the wall.

SDR 26 vs SDR 35

SDR 26 and SDR 35 are PVC gravity sewer pipe made to ASTM D3034. SDR 26 has a thicker wall and higher stiffness.

Pipe	Min. pipe stiffness	Typical use
SDR 35	46 psi	Gravity sewer and drainage
SDR 26	115 psi	Gravity sewer where higher stiffness is needed

Source: Westlake Pipe, gasketed sewer PVC pipe SDR 35/26 (checked October 2026).

In the City of Houston, the 2026 IDM calls for outside manhole drops built of SDR 26 PVC (ASTM D 3034) (§8.2.01.J.2.f.(4)(b)) and doesn't allow SDR 35 PVC for public sewers near Metro guided rapid transit tracks (§8.2.01.L.1.e).

SDR 35 vs Schedule 40

They're different systems. SDR 35 is gravity sewer pipe, sized and rated by stiffness, with a wall that scales with the diameter. Schedule 40 is sized by a schedule system, and its wall thickness is set by nominal size, not a fixed ratio. SDR 35 isn't the same as Schedule 40, so don't swap one for the other without checking your plans.

C900 DR18 vs DR14

C900 is PVC pressure pipe made to AWWA C900, in ductile iron outside-diameter sizes.

DR	Pressure class	Typical use
DR25	165 psi	Lower-pressure water lines
DR18	235 psi	Water mains
DR14	305 psi	Higher-pressure water mains, fire lines

Source: Uni-Bell PVC Pipe Association, An Engineering Primer on AWWA C900 PVC Pipe (checked October 2026).

City of Houston note. The 2026 IDM (§7.2.01.C) calls for AWWA C900 DR 18 for PVC restrained joints and "250 psi AWWA C900 DR 14" for vertical offsets. The 250 psi figure is the City's wording; the Uni-Bell primer lists DR14 at a 305 psi pressure class. Follow the plans and specs on your job.

What is C900 DR18 pipe?

C900 DR18 is AWWA C900 PVC pressure pipe with a dimension ratio of 18, which gives it a 235 psi pressure class per the Uni-Bell primer. It's made in ductile iron outside-diameter sizes, so ductile iron MJ fittings fit it. The City of Houston calls for C900 DR 18 for PVC restrained joints. See our water & sewer pipe and fittings.

HDPE DR ratings

For solid wall HDPE pressure pipe, DR works the same way: a lower DR means a thicker wall and a higher pressure rating. The exact pressure rating also depends on the material designation and temperature, so use the manufacturer's sheet for your pipe. See our HDPE pipe page.

Butt fusion vs electrofusion

Butt fusion heats the ends of two pipes and presses them together. Electrofusion uses a fitting with built-in heating wires to fuse the joint.

D&D Utility Supply supplies pipe and fittings only. We don't do fusion or fabrication.

Need this pipe?

We stock:

C900: DR18 and DR14, 4" to 16", 20 ft joints
SDR 26 and SDR 35: 4" to 16", 14 ft joints
Fittings: Sigma, 4" to 16"

We're glad to prepare quotes and takeoffs as a service. Please check the quantities against your plans before you order.

Call (832) 482-2483 Ask a spec question or get a quote

Pipe spec FAQs

What does SDR stand for?

SDR stands for standard dimension ratio: the pipe's outside diameter divided by its minimum wall thickness. A lower SDR means a thicker wall.

What is the difference between SDR 26 and SDR 35?

Both are ASTM D3034 PVC gravity sewer pipe. SDR 26 has a thicker wall and a minimum pipe stiffness of 115 psi. SDR 35 has a minimum pipe stiffness of 46 psi (Westlake Pipe).

Can SDR 35 be used for pressure lines?

No. SDR 35 is designed for non-pressure gravity sewer and drainage. Use a pressure-rated pipe such as C900 for pressure lines.

What is the pressure rating of C900 DR18 vs DR14?

Per the Uni-Bell primer on AWWA C900, DR18 has a 235 psi pressure class and DR14 has a 305 psi pressure class.

Which C900 DR does the City of Houston require for restrained joints?

The 2026 IDM (§7.2.01.C) calls for AWWA C900 DR 18 for PVC restrained joints, and 250 psi AWWA C900 DR 14 for vertical offsets.

Do you stock SDR 26, SDR 35, C900 DR18 and DR14?

Yes. We stock all four from 4" to 16". C900 comes in 20 ft joints and SDR pipe in 14 ft joints.

Need SDR 26, SDR 35 or C900 on a job?

Call (832) 482-2483 Email your takeoff for a quote

D&D Utility Supply · 14630 Blue Ash Dr., Houston, TX 77090 · Monday–Friday, 7 AM–4 PM