Nominal Pipe Size Chart: What NPS, DN and Schedule Actually Mean

1. What a Pipe Size Actually Names

Nominal pipe size is a designation, not a dimension. The word nominal is doing real work in that sentence, and the commonest error in reading a pipe chart is to treat it as decoration and assume the number describes the pipe. It does not describe the outside diameter, it does not describe the bore, and it does not describe the wall.

Take the size everyone knows. A 2-inch pipe has an outside diameter of 2.375 inch, which is 60.33 mm, and a bore of 2.067 inch under Schedule 40. The outside diameter is 18.75 per cent larger than the name, the bore is 3.4 per cent larger than the name, and neither of them is two inches. A 1/2-inch pipe is 21.34 mm across the outside and has a bore of 0.622 inch, so the bore is 24.4 per cent larger than the name. Nothing here is a mistake, and nothing here is a rounding error. The name simply belongs to a different quantity than the one it appears to describe.

The label is a survivor of the iron pipe trade. When the sizes were fixed in the nineteenth century, they approximated the bore of the pipe as it was then made, so a 2-inch pipe really did have a bore near two inches because the wall of the period was thin. The wall has since been standardized separately and thickened in the process, and the bore has moved inward while the name stayed where it was. The outside diameter was frozen at the same time, and that decision is the reason the whole system still works: because the outside diameter is identical across every wall thickness at a given nominal size, a fitting, a thread or a flange made for that size fits all of them.

So a pipe size names a family, not a cylinder. Within one nominal size, the outside diameter is fixed and the bore is whatever is left after the wall. Everything else on this page follows from those two facts.

The three designations, kept apart

• NPS is the North American label, written as a number of inches, with the sizes below 1 inch written as fractions. It appears in the tables on this page.
• DN is the ISO label, written as a whole number with no unit. It is a reference designation from a preferred series, and it is not a length in millimeters, as the fourth section shows.
• Schedule is not a size designation at all. It selects a wall thickness for a size that has already been named, which is why a complete specification needs both: NPS 2 Schedule 80 is a different pipe from NPS 2 Schedule 40, and neither can be written with a schedule alone.

2. The Outside Diameter, and Why It Never Changes

The outside diameter is the one dimension that does not change when the wall changes, and it is therefore the only reliable way to identify an unknown pipe. The table below gives it for every nominal size from 1/8 inch to 24 inch, in inches as the standard states it and in millimeters converted exactly.

Two rules govern that table, and both are worth memorising because they let you check any row without the rest of the chart.

Above NPS 14, the designation is the outside diameter. A 16-inch pipe is 16.000 inch across, a 24-inch pipe is 24.000 inch, and the equality is exact rather than approximate. That is the reason the outside-diameter column looks like it has a discontinuity at 14: below that point the numbers climb in irregular steps inherited from the old iron sizes, and above it they simply count.

Below NPS 14, the designation is smaller than the outside diameter, by an irregular amount. Twelve inch pipe is 12.750 inch outside against a nominal 12.000, so the excess is 0.750 inch. At NPS 4 the excess is 0.500 inch. At NPS 1 it is 0.315 inch. There is no formula tying the excess to the nominal size, which is why the table cannot be reconstructed from the name and has to be read.

The millimeter column is an exact conversion and not a separate standard, so the two decimals it carries are a statement about the inch value and nothing more. 1.315 inch is 33.40 mm because the inch is defined as exactly 25.4 millimeters, not because a metric committee chose that number. Where a supplier lists 26.67 mm for the size ASME calls 1.050 inch, the difference is a standards question rather than a conversion one, and the fourth section takes it up.

3. How Far the Name Is From the Bore

If the name approximated the bore once, it is worth measuring how far the two have drifted apart, because the drift is not uniform and it is largest where intuition is weakest.

The pattern is that the name tracks the bore best in the middle of the range and loses contact at both ends. Around NPS 8 to 12 the two agree to within half a per cent: an 8-inch pipe has a bore of 7.981 inch, which is -0.2 per cent away from the name, and a 12-inch pipe has a bore of 11.938 inch, which is -0.5 per cent away. That middle band is where the name still earns its keep as a statement about capacity.

Below it the name understates the bore sharply. A 1-inch pipe carries a bore of 1.049 inch, 4.9 per cent larger than the name. A 1/2-inch pipe carries a bore of 0.622 inch, 24.4 per cent larger. And NPS 1/8, the smallest size listed, has a bore of 0.269 inch against a nominal 0.125, which is 115.2 per cent larger: the hole is more than twice the size the name suggests. The reason is that the wall cannot shrink in proportion at very small sizes, since a pipe still has to be strong enough to thread, so the bore is forced upward relative to a nominal size that shrinks faster.

Above NPS 12 the direction reverses. Because the designation snaps to the outside diameter at 14 and the wall keeps growing, the bore falls progressively behind the name: 22.624 inch against 24, -5.7 per cent. A 24-inch pipe has a bore well under two feet.

The practical conclusion is that the nominal size is a good guide to bore in the middle of the range and a poor one at either end, which is exactly the opposite of the usual assumption that small sizes are the simple case. Where capacity matters, compute the bore from the outside diameter and the wall rather than reading it off the name, or take it from the table in the fifth section.

4. DN Is Not Millimeters

The metric designation for pipe size is DN, and the single most useful thing to know about it is that the number carries no unit. DN 25 is not 25 millimeters of anything. It is a reference label drawn from a preferred number series, formally defined by ISO 6708 as an alphanumeric designation for reference purposes, and its relation to the actual dimensions of the pipe changes with size.

The table below puts DN beside the two dimensions that actually exist on the pipe, and expresses DN as a percentage of each. Read the two percentage columns together and the behavior becomes visible.

At the small threaded sizes DN sits close to the bore: at NPS 1/2 it is 94.9 per cent of the bore and 58.3 per cent of the outside diameter. At the large welded sizes it has migrated to the outside diameter instead: by NPS 24 it is 98.4 per cent of the outside diameter and 104.4 per cent of the bore. The label did not change; the pipe did. DN 15 names a bore-like dimension on a half-inch pipe and DN 600 names an outside-diameter-like dimension on a 24-inch one, because the series was built to follow whichever dimension dominated the trade in that size range.

This is why DN and NPS are interchangeable as names but not as measurements. DN 50 and NPS 2 name the same size, and both are correct, but a specification that gives only one of them and expects the other to supply the dimensions is a specification with a gap in it.

Where the two standards disagree about the outside diameter

There is a second layer of confusion, and it explains a great deal of the disagreement between supplier datasheets. For the small threaded sizes the ISO and European tables do not use the same outside diameter as the ASME table. The size ASME B36.10M gives as 26.67 mm is given as 26.9 mm in the ISO 6708 and EN 10255 columns, and the difference persists across the sizes from 3/4 inch up to 1-1/4 inch before the two converge at 1-1/2 inch and above.

So a European catalogue and an American one can both be right, both say DN 20, and disagree by a fraction of a millimeter about the outside diameter of the pipe. Where a job crosses standards, the standard has to be named and not just the size. That single omission accounts for more mismatched fittings than any arithmetic error.

5. What the Schedule Number Buys

A schedule fixes a wall thickness for a nominal size. The pair is what specifies a pipe, and the table below gives both wall thicknesses in general use together with the bore each one leaves.

The bore is derived, not tabulated independently: it is the outside diameter minus twice the wall, which is the only arithmetic on this page that a reader needs to reproduce. Because the outside diameter is fixed, every increase in schedule is paid for entirely out of the bore, and the cost is larger than the linear numbers suggest, because flow area goes with the square of the bore.

The clearest example is at NPS 2, where the change from Schedule 40 to Schedule 80 takes the bore from 2.067 inch to 1.939 inch. That is a change of 6.2 per cent in diameter and 12.0 per cent in flow area, from one nominal size to the same nominal size. A line resized from Schedule 40 to Schedule 80 for pressure reasons loses more than a tenth of its capacity without any change to the pipe designation, and that is the sort of loss that does not appear on a drawing.

Two cautions about the table. It is the carbon steel table, and the stainless series uses the same outside diameters with a separate set of walls designated 5S, 10S, 40S and 80S, which coincide with the plain schedules at the small sizes and diverge above NPS 8. And the schedule numbers continue well beyond 80, through 100, 120, 140 and 160, which are used for high-pressure work and are deliberately not listed here because they are not interchangeable with the two common ones and are rarely wanted at a glance.

One more point about what a schedule is not. It is not a pressure class. The wall thickness determines the pressure rating together with the diameter, the material and the temperature, so Schedule 40 pipe in two different sizes has two different ratings, and Schedule 40 in carbon steel and in a low-strength alloy have different ratings again. A schedule is an input to a pressure calculation, never a substitute for one.

6. Schedule 40 Stops Being STD at NPS 12

Before the schedule numbers there were weight classes: standard weight, extra strong and double extra strong, written STD, XS and XXS. The schedule system was built on top of them, and at the small sizes the two systems coincide exactly, which is why STD and Schedule 40 are so often treated as synonyms.

They stop being synonyms at NPS 12, and the table below shows where and by how much.

The mechanism is visible in the two wall columns. Schedule 40 wall keeps growing with size, while STD wall freezes at 0.375 inch from NPS 12 upward. At NPS 10 the two are still identical, and the difference is zero. At NPS 12, Schedule 40 is 0.406 inch and STD is 0.375, and the Schedule 40 pipe is 8.0 per cent heavier. Because Schedule 40 continues to thicken while STD stays put, the gap widens with every size above: 32.3 per cent at NPS 16, and 81.0 per cent at NPS 24, where Schedule 40 is 171.48 lb/ft against 94.72 lb/ft.

The extra-strong class behaves the same way in the other direction. XS equals Schedule 80 only up to NPS 8, and above that XS freezes at half an inch while Schedule 80 keeps thickening. By NPS 24 the two are 1.219 inch and half an inch, a difference of nearly a factor of two and a half.

This matters because the error is invisible on a drawing. A line list that says 12 inch STD and a purchase order that says 12 inch Schedule 40 are ordering different weights of pipe at the same nominal size, and the difference is eight per cent of the steel. At 24 inch it is most of the steel. Where a drawing carries one of these class letters on a pipe larger than NPS 10, it is worth confirming which the designer meant, because the drawing may be correct and the abbreviation may be doing something the reader does not expect.

7. Weight Follows From the Geometry

Pipe weight is not tabulated from a separate standard. It follows from the annulus area and the density of the material, and stating the derivation is what makes it possible to check any figure on any chart.

A = π · t · (D − t), where A is the cross-sectional area of the steel, D the outside diameter and t the wall, with D and t in the same unit. Weight is A times the density, and in the two usual unit systems that gives lb/ft = 10.6901 · t · (D − t) with D and t in inches, and kg/m = 0.0246615 · t · (D − t) with D and t in millimeters.

The two constants are not independent facts to be memorised. Both encode the same density of 7850 kilograms per cubic meter, which is 490 pounds per cubic foot: the imperial constant is pi times twelve times 490 divided by 1728, and the metric one is pi times 7.85 thousandths. Set one beside the other and the reason pipe charts disagree in the last digit becomes clear. A table that computes with a constant of 10.68 instead of 10.6901 is using a density about one tenth of a per cent lower, which is 7841 rather than 7850, and that is the entire difference between the 53.52 lb/ft that some charts print for 12-inch Schedule 40 and the 53.58 that others do.

So a one-digit disagreement between two weight columns is a density convention and can be ignored, while a disagreement larger than about half a per cent is a wall thickness difference and cannot. This page uses 7850 kilograms per cubic meter throughout, which is the conventional figure for carbon steel, so its weights sit within a tenth of a per cent of the published columns without matching any of them exactly.

Two cautions. The formula gives the plain-end weight of the steel alone, so it excludes coatings, linings, end finishes and couplings, all of which matter for a freight estimate. And it is a mass per unit length, so it says nothing about whether the pipe is strong enough for anything; that is a separate calculation that starts from the wall and the diameter.

8. Identifying a Pipe You Already Have

The workflow is short, and the only thing that has to be remembered is which dimension to trust.

1. Measure the outside diameter and look it up. This is the only dimension that is unique to a nominal size and identical across every schedule, so the outside diameter alone settles the nominal size. A measurement of 2.375 inch is NPS 2 whatever the wall turns out to be.
2. Measure the wall, then subtract twice it. A calliper on a cut end works; an ultrasonic gauge through the wall avoids cutting. Bore is outside diameter minus two walls, and the schedule follows from the wall once the size is known.
3. Do not identify by bore alone. Because a schedule is paid for out of the bore, a bore measurement is ambiguous: 2.067 inch at NPS 2 is Schedule 40, and a slightly smaller bore at the same outside diameter is Schedule 80. Measuring the bore and working backwards gives two answers.
4. Check the material system before trusting the chart. Copper tube is not on this chart at all. Its outside diameter is the nominal size plus one eighth of an inch, and the type letter K, L or M sets the wall, so a half-inch copper tube is 15.88 mm across by coincidence of a different rule. Round tube and pipe look alike and are named differently.

For the unit arithmetic behind any of these measurements, the length conversion guide covers the inch and millimeter factors, and the converters for inches to millimeters and millimeters to inches handle single values. Where a bore is being converted into a flow area, the area conversion guide covers the squared units.

The three other legacy size scales on this site fail in the same way and for the same reason, which is worth noticing because the pattern is general. Sheet metal gauge numbers rise as the metal gets thinner, and mean a different thickness in every metal. The drill bit size chart interleaves four naming systems that overlap rather than nest. The AWG wire gauge chart runs backwards for the same historical reason pipe sizes do, and hides a rounding decision that makes four published charts disagree. In every case the name is a label and the dimension has to be looked up.

For the general method behind any conversion of this kind, and for the families where no factor exists at all, see How to Convert Units. For exact factors across every category, the unit conversion chart collects them in one place.

Frequently Asked Questions