PVC and Copper Pipe Sizes: How to Tell What You Are Holding

Two naming systems that were never meant to meet

Ask why a 1/2 inch pipe is not half an inch and you get one answer, usually about inside diameters and old iron. It is half of the story, and it is the wrong half for the question people actually have, which is what is this thing in my wall.

The useful answer is that there are two systems, and which one you are in decides everything:

• Copper tube size (CTS) — copper, CPVC for hot and cold supply, PEX.
• Iron pipe size (IPS) — PVC, CPVC schedule 80, steel, black iron.

Neither names a dimension you can measure. But one of them at least follows a rule.

Copper is the easy half: nominal plus an eighth, every time

Copper water tube has one law and it does not break it.

The outside diameter is the nominal size plus exactly 1/8 inch. A 1/2 inch tube is 0.625. Three quarters is 0.875. One inch is 1.125. Two inches is 2.125. Every size in the first table above, from 1/4 to 3 inches, without a single exception — which is worth saying plainly because almost nothing else in American construction sizing is that clean.

That means you can do it in your head, in both directions. Measure 1.375 across, subtract an eighth, and you have a 1-1/4 inch tube. It also means the eighth is the whole gap: the nominal figure is close to the bore of a middling wall, which is what it was once meant to describe, and it stopped being an exact anything a long time ago.

Iron pipe size is not a rule, it is a list

Now try the same trick on PVC and it falls apart immediately.

A 1/2 inch PVC pipe is 0.840 across — 0.340 over its name, not 0.125. Three quarters is 1.050, which is 0.300 over. One inch is 1.315, which is 0.315 over. Then 1-1/4 jumps to 0.410 over, and by 3 and 4 inches the pipe is running a full half inch over its nominal size.

There is no pattern there because there is no rule. Iron pipe size is a list of historical outside diameters, inherited from wrought iron pipe and frozen in place so that a century and a half of threads and fittings would keep working. The usual explanation is that the sizes were originally chosen so the bore came out near the nominal figure at the wall thicknesses of the day, and that the names survived after the walls got thinner. That account is repeated everywhere; we have not traced it to a primary source and are flagging it as the received story rather than as verified fact. What is verifiable is the column of numbers, and the column of numbers has no arithmetic in it.

The collision: a copper tube measures like the PVC one size down

Put the two lists on one axis, which is what the third table does, and a pattern appears that neither list shows on its own.

A copper tube lands close to the PVC pipe one nominal size smaller than itself. Copper 3/4 is 0.875 and PVC 1/2 is 0.840. Copper 1-1/4 is 1.375 and PVC 1 is 1.315. Copper 1/2 is 0.625 and PVC 3/8 is 0.675. The offset is not a coincidence of two sizes; it holds the length of the range.

It runs both ways depending where you are in the range. From 3/4 inch up the copper is usually the larger of the pair; down at 3/8 and 1/2 inch the PVC one size below comes out bigger than the copper, which is the same trap wearing the other hat.

Five of these pairs sit closer together than one mark on a tape measure. A tape is marked in sixteenths and a sixteenth is 0.0625. The two tightest are 0.035 apart — a 3/4 inch copper tube against a 1/2 inch PVC pipe, and a 1-1/2 inch copper tube against a 1-1/4 inch PVC — which is half of one tape mark. You cannot see it, you cannot feel it, and if you are standing in a hardware aisle holding a cut-off from the crawlspace, you will get it wrong.

That is the real reason the question keeps being asked. It is not that people cannot read a chart. It is that the two right answers are closer together than the tool they are measuring with.

So measure around the pipe, not across it

Here is the fix, and it is one line of arithmetic.

Wrap a tape around the pipe and read the circumference instead. Circumference is π times diameter, so every difference gets multiplied by 3.14 before you have to read it.

Those two pipes that were 0.035 apart across are 2.64 and 2.75 inches around — a gap of 0.110, which is nearly two tape marks and perfectly readable. The 0.060 gap between PVC 1 inch and copper 1-1/4 becomes 0.188 around, three marks. A measurement you could not make becomes one you can, using the same tape, because you turned it ninety degrees.

The wrap figure is in the third table for every size. Pull the tape snug, keep it square to the pipe rather than spiralling, and read to the nearest sixteenth.

Schedule 40 and schedule 80 are the same size on the outside

This one surprises people who assume a heavier pipe is a bigger pipe.

At every size in the table, schedule 40 and schedule 80 have the identical outside diameter. The extra wall of schedule 80 grows inward. That is why the fittings interchange, and why a schedule 80 pipe glues into a schedule 40 socket without complaint.

What it costs you is bore. A 1/2 inch schedule 40 pipe has a 0.622 inch bore; the schedule 80 has 0.546. Square those and you find schedule 80 gives up about 23 percent of its flow area at that size. The penalty shrinks as the pipe grows — 19 percent at 3/4, 13 percent at 1-1/2, 10 percent at 4 inches — because the wall grows more slowly than the pipe does.

So schedule 80 is a pressure decision, not a size decision. Reach for it when the pressure rating or the physical abuse calls for it, and know that on a small line you are narrowing the pipe noticeably to get there. In the aisle, schedule 40 is white and schedule 80 is dark gray.

K, L and M are the same tube

Copper has the same trick with a different name. Types K, L and M share an outside diameter at every size and differ only in wall thickness, so all three take the same fittings.

At 3/4 inch the walls run 0.065 for K, 0.045 for L and 0.032 for M. Run those through and type M has about 19 percent more bore area than type K, from a tube that measures the same on the outside. Thicker is not bigger; thicker is narrower.

Which to use is a code and application question — K underground, L the common choice inside a house, M where it is permitted and the pressure allows. Identifying what you have is easier than that: the print line is color-coded. K is green, L is blue, M is red, and DWV is yellow. Hard tube also carries the type incised into the metal, so it survives when the print wears off.

What this page will not do for you

Three limits, said plainly.

These are nominal dimensions. Real pipe carries a manufacturing tolerance, and PVC in particular is published as an average outside diameter and a minimum wall rather than as exact figures. Where a few thousandths decide whether a fitting seats, measure the actual piece.

This page tells you what a pipe is, not what size you need. Sizing a water distribution system runs off fixture units, available pressure and developed length; drainage runs off fixture unit loads and slope. Both live in the plumbing provisions of the code, and neither is a chart lookup.

And identifying the pipe is not the same as knowing what it may carry. The same outside diameter can be a potable water line, a drain, or something that should never have been plumbed in that material. If you are working out what is behind a wall in a house you did not build, the size is the beginning of the question rather than the end of it.