Wall Thickness in Plastic Piping: History and Fundamentals

Choosing the right plastic pipe isn’t just about diameter size it’s about wall thickness, pressure performance, and media compatibility. But terms like Schedule, SDR, IPS, or DIPS often appear without sufficient context, leading to confusion in the field and even costly mistakes.

To truly understand these terms, it helps to know where they came from, how they work, and why they matter. This article breaks down the historical origins, dimensional logic, and real-world implications of IPS, DIPS, Schedule, and SDR systems in thermoplastic piping.

IPS (Iron Pipe Size): The Foundation of Dimensional Consistency

Where It Came From:

• The Iron Pipe Size (IPS) system originated in the early 20th century for steel pipes used in water, gas, and steam systems.
• It standardized the outside diameter (OD) of pipes, ensuring compatibility among fittings, valves, and systems even as wall thicknesses varied.

Why It Matters in Plastics:

• When plastic piping was developed in the mid-1900s, it adopted the same IPS OD system to ensure easy replacement of metal piping and compatibility with legacy fittings.
• This decision explains why a 2” PVC pipe, for example, has the same OD (2.375″) as a 2” steel pipe.

[IPS is not a wall thickness system. It’s a dimensional baseline in which wall thickness systems are based.]

Ductile Iron Pipe Size (DIPS): Municipal Sizing Legacy

Where It Came From:

• DIPS evolved from cast iron and ductile iron pipe standards used in municipal water and sewer infrastructure.
• These systems standardized a larger outside diameter than IPS to maintain strength and flow capacity for buried utilities while carrying the load of soil on top of the pipes.

Where You’ll See It:

• DIPS is commonly used in HDPE pipe systems designed for municipal water and wastewater applications, where the pipe must match existing ductile iron infrastructure.
• A 6” DIPS pipe, for instance, has an OD of 6.90″, compared to 6.625″ for a 6” IPS pipe.

Why It Matters:

• The difference is small but critical. DIPS-sized pipe cannot be joined to IPS fittings without special adapters or transition components.

Schedule Pipe: A Durable Standard with Historical Depth

Where It Came From:

• The Schedule system was developed in the early 20th century under the ASME B36.10 specification for steel piping standards.
• Before numbered schedules, pipe was classified as Standard Weight (STD), Extra Strong (XS), or Double Extra Strong (XXS), which are broad groupings based on pressure and wall thickness.
• To provide a more specific range of options, ASME introduced Schedule numbers (10, 20, 40, 80, etc.). These numbers correspond to the ratio of the design stress to the allowable material stress.

Why It’s Still Widely Used:

• Schedule pipe remains a staple in thermoplastics, especially in fittings, valves, and legacy systems.
• Many manufacturers, including Asahi/America, continue to offer Schedule 80 components because of their mechanical strength, chemical resistance, and well-established dimensional consistency.
• For small to mid-sized piping systems, Schedule 40/80 remains simple, robust, and widely understood by engineers, contractors, and inspectors.

But There’s a Catch:

• Schedule thickness does not scale proportionally. A 2″ Schedule 80 PVC pipe might handle over 400 psi, but a 12″ Schedule 80 PVC pipe might be rated below 100psi.
• As pipe size increases, wall thickness increases in inches, not in ratio, which leads to diminishing pressure ratings at larger sizes.

[Schedule is simple but not logical across sizes.]

Standard Dimension Ratio (SDR): Engineering-Driven Design

Why It Was Invented:

• The SDR system was developed in Europe in the 1950s to address inconsistencies in Schedule piping pressure ratings in thermoplastic systems. The idea was to use material strength as a fundamental part of the calculation and provide the same pressure rating to the designer.
• SDR stands for Standard Dimension Ratio and is calculated as: SDR = Outside Diameter ÷ Wall Thickness
• This means the wall thickness scales proportionally with pipe size, offering predictable hydraulic performance and consistent pressure ratings across diameters.

How It Benefits Plastic Systems:

• SDR simplifies engineering: a 4” SDR 17 pipe and a 12” SDR 17 pipe made from the same material will have the same pressure rating and behavior.
• SDR uses the material strength as a core component to pressure rating. Because of this, each material may have different pressure ratings for identical SDR’s.
  • Polypropylene SDR 33 is nominally rated for 45psi
  • PVDF SDR 33 is nominally rated for 150psi

Why SDR Is Not Used for Metal Pipes

Failure Behavior: Brittle vs Ductile

• Plastic pipes tend to fail by slow crack growth or deformation under long-term stress. This makes wall thickness ratios (like SDR) especially important for managing pressure over time.
• Metal pipes, especially steel, fail differently—through yielding, fatigue, or corrosion. Their strength depends more on material properties and safety factors than on geometric proportions alone.
• SDR evolved to suit thermoplastic materials, where long-term pressure performance depends heavily on the wall thickness-to-OD ratio. In contrast, metal pipes are traditionally sized using fixed wall thickness tables (like Schedule 40 or 80), and because metals don’t deform or fail the same way, there was no need for an SDR-style approach.

Ultimately, wall thickness standards are more than just technical terminology; they represent the foundation of reliability in plastic piping. Understanding the history and intent behind IPS, DIPS, Schedule, and SDR allows engineers, contractors, and designers to make informed choices that ensure proper fit, reliable performance, and long service life. In the field, that knowledge can mean the difference between a flawless installation and an expensive, time-consuming correction, making wall thickness not just a design detail, but a critical cornerstone of system success.