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How pipe size is measured

Updated 2026-09-04 · 7 min read

Sizing and flow
Isometric illustration of a caliper measuring a copper pipe end
A caliper across a copper tube end: the outside diameter is the measurable number.

Measure the outside, not the bore

You cannot measure a nominal size, because it is not a dimension. What you can measure is the outside diameter, and every naming system maps it back. In copper tube size the rule is simple enough to hold in your head: the outside diameter is the nominal size plus 1/8". A tube that measures 0.875" is a 3/4" line.

Iron pipe size does not follow that rule and needs the table. A pipe measuring 1.050" is 3/4" steel — the same nominal name, a quarter of an inch bigger across.

PEX and CPVC are copper tube size

Both follow CTS, so the plus-an-eighth rule works on them. That is deliberate: it lets them share transition fittings with copper.

Three ways to take the measurement

  1. 1

    Calipers, if the end is exposed

    Jaws across the outside, square to the axis. Take it twice at 90 degrees — an old line dented by a wrench reads oval, and the larger of the two is the honest one. Two decimal places is enough.

  2. 2

    String, when the pipe is in a wall

    Wrap a string once around, mark the overlap, measure the length and divide by 3.1416. A 2.75" circumference is a 0.875" OD, so a 3/4" copper line. This is the field method that works with one hand in a joist bay.

  3. 3

    A fitting, when neither is possible

    Slip a known coupling over the pipe. If a 3/4" copper coupling goes on, it is 3/4" copper. Cheap, definitive, and it tells you the material at the same time.

Sizing and flow
Isometric detail illustration for the how to measure pipe size guide
Jaws square to the axis, taken twice at ninety degrees.

Reading the OD back to a size

Outside diameter to nominal size, inches
Measured ODIf CTS (copper, PEX, CPVC)If IPS (steel, PVC)
0.6251/2"—
0.840—1/2"
0.8753/4"—
1.050—3/4"
1.1251"—
1.315—1"
1.3751-1/4"—
1.660—1-1/4"
1.6251-1/2"—
1.900—1-1/2"
2.1252"—
2.375—2"

Notice how the two columns interleave. A measurement of 1.050" is unambiguous — no CTS size lands there — but 1.375" and 1.315" are close enough that a careless read swaps a copper 1-1/4" for a steel 1". Material first, then size.

Where it goes wrong

Insulation is the commonest error: a foam sleeve adds three-quarters of an inch and turns a 1/2" line into an imaginary 1-1/4" one. Strip it back before you measure.

The second is measuring a fitting instead of a pipe. A coupling, an elbow or a solder cup is always larger than the tube it joins. Measure on the straight, at least two inches from any joint.

Old galvanised reads oversize

Decades of scale build up inside and corrosion swells the outside. If the OD reads between two table values on old steel, take the smaller nominal — and note that the bore is probably far below the table figure, which is exactly why it is being replaced.

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