Schedule 20 vs. Schedule 40 Wall Thickness Chart

Choosing between Schedule 20 and Schedule 40 pipe can feel straightforward until you actually look at the numbers. The wall thickness difference between these two schedules affects everything from pressure capacity to freight cost, and picking the wrong one can mean either overspending on material you don't need or, worse, a failure under pressure. A reliable wall thickness chart comparing Schedule 20 and Schedule 40 is one of the most referenced tools in piping design, yet many charts floating around online contain errors or omit critical pipe sizes. This guide puts accurate data in one place, explains what the numbers actually mean in practice, and helps you make a confident selection for your specific application. Whether you're speccing a low-pressure drainage line or a pressurized industrial system, the wall thickness you choose has real consequences for safety, budget, and long-term performance.

Pipe Schedule & Wall Thickness Defined

Pipe schedules were introduced by the American Standards Association (now ASME/ANSI) as a standardized way to communicate wall thickness across the industry. The schedule number itself is derived from a formula: Schedule = (1000 × P) / S, where P is the internal working pressure in psi and S is the allowable stress of the material in psi. This means higher schedule numbers correspond to thicker walls capable of handling greater internal pressures.

What trips people up is that two pipes with the same schedule number but different nominal sizes will have different wall thicknesses. A 2-inch Schedule 40 pipe has a wall thickness of 0.154 inches, while a 6-inch Schedule 40 pipe has a wall of 0.280 inches. The schedule number is not a fixed dimension: it's a ratio-based designation that scales with pipe diameter.

Defining Nominal Pipe Size (NPS)

Nominal Pipe Size is one of the most misunderstood terms in piping. NPS does not refer to any actual measured dimension of the pipe. For NPS 1/8 through NPS 12, the nominal size is loosely based on the inside diameter, but it doesn't match it exactly. The outside diameter (OD), however, is fixed for each NPS regardless of schedule. A 4-inch NPS pipe always has an OD of 4.500 inches whether it's Schedule 10, 20, 40, or 80. What changes is the wall thickness, which in turn changes the inside diameter and the weight per foot.

This distinction matters because when you're comparing Schedule 20 to Schedule 40, you're comparing two pipes with identical outside diameters but different wall thicknesses and therefore different internal bores. A thicker wall reduces the internal flow area, which can affect flow velocity calculations in hydraulic design.

Schedule Numbers & Pressure Ratings

A common mistake is assuming the schedule number directly tells you the maximum pressure rating. It doesn't: not without knowing the material, the temperature, and the applicable code. A Schedule 40 carbon steel pipe rated per ASME B31.1 at ambient temperature will have a very different allowable pressure than a Schedule 40 stainless steel pipe at 800°F.

The schedule number gives you geometry. Pressure ratings come from combining that geometry with material properties and design factors specified by codes like ASME B31.3 (Process Piping) or B31.1 (Power Piping). Two engineers can look at the same Schedule 40 pipe and arrive at different allowable pressures depending on which code governs their project.

Schedule 20 vs. Schedule 40 Wall Thickness Chart

Comparative Data for Common Pipe Diameters

Here's the data most people are searching for. All dimensions follow ASME B36.10M for carbon and alloy steel pipe. Note that Schedule 20 is not defined for small pipe sizes (below NPS 8 in most standards), which is a detail many online charts get wrong.

If you need pipe smaller than NPS 8, Schedule 20 typically isn't an option under ASME B36.10M. You'd be looking at Schedule 10 or standard Schedule 40 for those sizes.

Weight Per Foot Variations

The weight difference between schedules is significant, especially on large-diameter pipe where material costs and shipping logistics matter enormously. You can estimate weight per foot for steel pipe using the formula: W = 10.69 × (OD - t) × t, where W is in pounds per foot, OD is outside diameter in inches, and t is wall thickness in inches.

On a 24-inch line, switching from Schedule 40 to Schedule 20 saves roughly 31 pounds per foot. Over a 1,000-foot run, that's 31,000 pounds of steel you're not buying, not welding, and not shipping. The cost implications are massive.

Applications & Structural Differences

Typical Uses for Schedule 20 Thin-Wall Piping

Schedule 20 pipe shows up most often in low-pressure or gravity-flow systems. Think cooling water return lines, large-diameter HVAC ductwork in industrial plants, or non-critical drainage systems where the internal pressure stays well below 150 psi. It's also common in fire protection sprinkler mains where the system pressure is relatively low and the pipe size is large.

Because Schedule 20 starts at NPS 8, it's inherently a large-pipe specification. These applications tend to involve high-volume, low-pressure fluid transport where minimizing material weight and cost is a priority. Structural loads on these pipes are typically managed through proper support spacing rather than relying on wall thickness.

Standard Industrial Applications for Schedule 40

Schedule 40 is the workhorse of the piping world. It covers NPS 1/8 all the way through NPS 24 and beyond, making it the default specification for general-purpose service. Process plants, compressed air systems, steam lines (at moderate temperatures), and potable water distribution all commonly use Schedule 40.

The thicker wall provides a larger corrosion allowance, which is critical in systems handling corrosive fluids or operating over a 20-plus-year design life. If your pipe will see any combination of moderate pressure, temperature cycling, or corrosive media, Schedule 40 is usually the minimum starting point. Many engineers default to it precisely because it provides a comfortable margin for the unknowns that inevitably show up in real-world service.

Material Considerations: Steel vs. PVC

The schedule designation system applies to both steel and PVC pipe, but the wall thicknesses are not identical across materials. PVC pipe schedules follow ASTM D1785, while steel pipe follows ASME B36.10M. A Schedule 40 PVC pipe and a Schedule 40 steel pipe of the same NPS will share the same OD and wall thickness dimensions, but their pressure ratings will be drastically different because PVC has a much lower allowable stress than steel.

For example, a 2-inch Schedule 40 PVC pipe is rated for 280 psi at 73°F per ASTM standards, while a 2-inch Schedule 40 carbon steel pipe (ASTM A53 Grade B) can handle well over 1,000 psi at ambient temperature under ASME B31.1. The geometry is the same; the capability is not.

PVC Schedule 20 pipe is rarely encountered in practice. Most PVC applications use Schedule 40 or Schedule 80, with SDR (Standard Dimension Ratio) pipe being more common for thin-wall PVC applications. If someone hands you a spec calling for "Schedule 20 PVC," double-check the source: they may actually mean SDR 21 or SDR 26, which are different classification systems entirely.

Selection Guide: Sch 20 vs. Sch 40

Evaluating Internal Pressure Requirements

Start with your design pressure and temperature. Look up the allowable stress for your pipe material at that temperature using ASME B31.3 Table A-1 (or the applicable code for your project). Then use the Barlow formula to determine the minimum required wall thickness: t = (P × OD) / (2 × S × E), where P is design pressure, OD is outside diameter, S is allowable stress, and E is the longitudinal joint factor (1.0 for seamless, 0.85 for ERW).

Add your corrosion allowance to that calculated thickness, then select the schedule that meets or exceeds the total. If Schedule 20 provides enough wall after accounting for corrosion, there's no engineering reason to pay for Schedule 40. But if the calculation is close, err on the heavier side: manufacturing tolerances per ASTM A530 allow wall thickness to be up to 12.5% below nominal on seamless pipe.

Cost Impact of Wall Thickness Selection

The price difference between Schedule 20 and Schedule 40 is not just about raw material. Thicker pipe requires more weld filler metal, longer welding times, and potentially higher preheat requirements. On a large-diameter project, the labor savings from using Schedule 20 can exceed the material savings.

Shipping is another factor people underestimate. Trucking costs are often driven by weight, not volume, for steel pipe. A truckload of 24-inch Schedule 40 pipe hits weight limits much faster than Schedule 20, meaning you need more trucks for the same linear footage. On one recent refinery project, switching from Schedule 40 to Schedule 20 on a 16-inch cooling water line saved roughly 15% on total installed cost, with freight accounting for nearly a third of those savings.

Making the Right Call

The comparison between Schedule 20 and Schedule 40 wall thickness comes down to a simple question: does your application need the extra metal? If you're running a low-pressure, non-corrosive service on large-diameter pipe, Schedule 20 saves real money without sacrificing safety. If you're dealing with moderate pressures, corrosive fluids, or smaller pipe sizes where Schedule 20 doesn't exist, Schedule 40 is the proven standard for good reason.

Always run the wall thickness calculation yourself rather than relying on rules of thumb. Verify your chart data against ASME B36.10M directly, because errors in online reference tables are more common than you'd expect. And remember that the schedule number is just geometry: the actual pressure rating depends on your material, your temperature, and your governing code. Get those inputs right, and the correct schedule practically selects itself.