Nominal pipe size is a label, not a measurement
The most useful thing to know before reading any nominal pipe size (NPS) chart is that the number is a name, not a dimension. Ask for a one-inch pipe and nothing on it measures one inch: the outside diameter is 1.315 in and the bore is something else again, set by the wall thickness. The nominal number is a shared reference so that every wall thickness of a given size keeps the same outside diameter and the same fittings interchange. This converter always puts the true outside diameter next to the label so the two are never confused.
NPS and its outside diameter
For sizes of NPS 12 and smaller the outside diameter is larger than the nominal number by a fixed, size-specific amount; from NPS 14 upward the outside diameter is simply set equal to the nominal number. So NPS 2 is 2.375 in (60.3 mm) on the outside, NPS 4 is 4.500 in, NPS 6 is 6.625 in — but NPS 14 really is 14.000 in. That step in the rule is the source of a lot of confusion, and the tool handles it for you by applying the correct outside diameter to each size.
NPS, DN and nominal bore are the same idea
NPS is the inch-based designation common in North America. DN (diametre nominel) is the metric designation used across much of the rest of the world, and nominal bore or NB is just another name for that same metric label. They run in step: DN 15 is NPS ½, DN 25 is NPS 1, DN 50 is NPS 2, DN 80 is NPS 3, DN 100 is NPS 4, DN 150 is NPS 6. None of them is a measured millimetre or inch figure — a quick rule of thumb is DN ≈ 25 × NPS, which works for the round sizes but breaks on the small and fractional ones, so a side-by-side reference is safer.
| NPS (in) | DN | OD (in) | OD (mm) |
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Pipe versus tube: don't mix the systems
Pipe is sized by a nominal designation whose number matches no measured dimension, with the outside diameter fixed for each size and the wall thickness (the schedule) doing the varying. Tube is sized by its true outside diameter: a one-inch tube genuinely measures 1.000 in across the outside, and you state the wall separately. Because of that you can't pair them by name. If a part is called out by an exact outside diameter — 1 in, 28 mm — it is tube; if it is called out as NPS 1 or DN 25 it is pipe, and its real outside diameter is a different number. The converter labels what it gives you so you know which system you are in.
Inches and millimetres
The inch–millimetre relationship is exact, not approximate:
So the 2.375 in outside diameter of an NPS 2 pipe is 2.375 × 25.4 = 60.325 mm, normally rounded to 60.3 mm. The published metric outside diameters are these exact conversions rounded to a tenth of a millimetre, which is why you'll see 60.3 rather than 60.325 on a metric drawing. The standalone length converter on this page does the same exact arithmetic for any value you type.
Inside diameter and bore area (only with a wall)
The inside diameter is not fixed by the nominal size, because it depends on the wall thickness you actually have — the same NPS 2 outside diameter can hide quite different bores. So this tool only reports an inside diameter when you give it a wall thickness, and it computes the bore area from that:
That is a deliberate choice: rather than reproduce a per-schedule wall-thickness table and risk handing you the wrong bore, the converter asks for the one number — your wall — that turns a fixed outside diameter into a real inside diameter.
Worked examples
Example 1 — NPS to metric
Start from NPS 2: outside diameter 2.375 in = 60.3 mm, metric designation DN 50. Open it directly with ?from=nps&val=2&out=all.
Example 2 — bore from a wall
NPS ½ (OD 0.840 in) with a 0.109 in wall gives ID = 0.840 − 2 × 0.109 = 0.622 in and a bore area of about 0.304 in² — the same 0.622 in bore the gas and flow tools ask for.
Pipe size conversion FAQ
Is NPS 2 the same as a 2-inch outside diameter?
No. Nominal pipe size is a label, not a measurement. An NPS 2 pipe has an outside diameter of 2.375 in (60.3 mm), not 2 in. The nominal number is a trade designation that lets every wall thickness for that size share one outside diameter so fittings interchange. The actual two-inch figure only survives as a rough historical link to the bore of old, thin-walled water pipe. This tool always shows the true outside diameter alongside the nominal label.
What is the difference between NPS, DN and nominal bore?
They are the same idea in different units. NPS (nominal pipe size) is the inch-based designation used in North America. DN (diametre nominel) is the metric designation used in much of the rest of the world, and nominal bore or NB is another name for that same metric designation. DN 50 is the same pipe family as NPS 2, DN 25 matches NPS 1, and so on. None equals a measured diameter; they are reference labels, which is why this converter lists all of them next to the real outside diameter.
Why does a 14-inch pipe measure 14 inches but a 2-inch pipe does not?
For NPS 14 and larger the outside diameter is set equal to the nominal number, so NPS 14 really is 14.000 in OD. For NPS 12 and smaller the outside diameter is larger than the nominal number by a fixed, size-specific amount — NPS 2 is 2.375 in, NPS 4 is 4.500 in. The break happens because the smaller sizes inherited their designations from an older system while the large sizes were standardised later on a simpler rule. The converter applies the correct outside diameter for each size automatically.
What is the difference between pipe and tube?
Pipe is sized by a nominal designation (NPS or DN) whose number does not match any measured dimension, and the outside diameter is fixed per size while the wall (schedule) varies. Tube is sized by its true outside diameter: a 1-inch tube really is 1.000 in across the outside, with the wall specified separately. So you can't mix the two by name. If a part is described by an exact outside diameter such as 1 in or 28 mm it is tube; if it is described as NPS 1 or DN 25 it is pipe, and its real outside diameter is something else.
How do I convert inches to millimetres for pipe sizes?
One inch equals 25.4 mm exactly, so multiply inches by 25.4 to get millimetres or divide millimetres by 25.4 to get inches. For example a 2.375-in outside diameter is 2.375 × 25.4 = 60.325 mm, usually rounded to 60.3 mm. The standalone inch-to-mm converter on this page does that exact arithmetic for any length you type, and the pipe size section applies it to the outside diameter for you.
Can this tool give me the inside diameter of a pipe?
Only if you tell it the wall thickness, because the inside diameter is not fixed by the nominal size — it depends on the schedule or wall you actually have. Enter a wall thickness and the tool computes inside diameter as outside diameter minus twice the wall, and the bore area from that. Without a wall thickness there is no single correct inside diameter for a given NPS, which is exactly why this converter never invents one.
Is this pipe size conversion calculator free and private?
Yes. It's free, needs no signup or download, and runs entirely in your browser, so nothing you enter leaves your device. You can copy a shareable link that reopens the converter with your size and units already filled in, and it keeps working offline once the page has loaded.