It’s an aisle in your local plumbing supply store with stacks of steel piping that all appear to be the same from across the room. But in fact they are different, varying in strength and weight. You’re there looking for 2 inch pipe, which is being used as a water line…dozens of variations on the shelves. They has the same outside diameter but different wall thicknesses.
Confusing right? It’s designed that way. Nominal sizes and complicated schedules trap inexperienced DIYers and seasoned tradesmen alike. Here’s a chart (above) that will help explain the system by showing how you can have the same outer diameter while just changing the wall thickness. You should of not purchase any pipe until you understand this.
How to Choose the Right Pipe
To further add to the confusion, actual physical sizes do not match the nominal pipe size. The nominal size of any given pipe less than fourteen inches really has nothing to do with what is actualy physically present. Ask someone for a two-inch pipe and you will most likely recieve one that is more like two and three-eighths inches around externally.
The reason is that historically, iron was extruded through dies which lost some amount of material at the walls. Today’s pipe manufacturing maintain the outside diameter so they could fit into fittings properly but changed the wall thickness to alter inside diameter. Therefore, the pipe is what is left over once the wall is removed from fixed outer shell.
The schedule system is also important since it determines how much pressure a pipe will hold. Most common residential water lines is schedule 40, which balances cost, internal flow area, and strength well. You could step up to say schedule 80, which is a lot more wall thickness. More metal = pipe that holds more pressure (like in chemical processing or maybe steam line).
Here’s the gotcha: there’s a price to pay immediately and in tangible form. The inside diameter is smaller due to the increased wall thickness. This means there is less cross-sectional area to push stuff through. For moving large volumes of fluid, this can be bad news for how well the system work. It is a small detail but it makes a huge difference in hydraulics.
The other wrinkle here involves material selection. The industry’s workhorse is carbon steel, which offers good strength and cost. However, it will also corrode over time if it is not properly coated against exposure to elements. Stainless steel addresses the corrosion issue (particularly when used in food-grade or marine applications). However, it costs more and frequently features thinner walls given equal schedule designations.
To be certain that you’re comparing apples to apples, look for the “S” suffix on stainless schedule numbers. If you use the carbon steel dimensions for a stainless application without consulting actual alloy specs, you risk dangerous misjudgments about pressure. There is no such thing as trusting a chart when assessing whether your system will hold pressure. Factors like material grade, joint type, and temperature all contribute to pressure ratings.
In the field, this is a quick rule of thumb: always inspect the mill stamp on each piece of pipe prior to cutting. One of the most common mistakes are mixing up Schedule 80 pipe with a Schedule 40 fitting. This will decrease the pressure rating by at least 30% if not more. If you’re using corroded or used pipe, always confirm what the wall thickness still is after corrosion. An easy way to do this is with an ultrasound gauge; this will let you know if there’s too much thinning of metal to be considered reliable.
Bottom line, these charts serve as dimensional references only, not engineering certificates. They’ll tell you what it looks like. But a qualified engineer has to tell you whether or not it’s going to hold up in the particular conditions which you’re using it. Respect the limits of the metal you’re working with; read the stamp, measure the wall.
