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NPS to DN Conversion Chart

Nominal Pipe Size in inches matched to metric DN designations, with the pipe outside diameter in inches and millimeters.

Ref PV-0050ASME B36.10MISO 6708

Draft, open for review. Check every value against the governing standard and the manufacturer’s data before use.

What it is

NPS (Nominal Pipe Size) is the inch-based size name used in North America. DN (diamètre nominal) is the metric size name defined in ISO 6708. Both are names, not measurements. Neither one equals the actual inside or outside diameter. For pipe made to ASME B36.10M, an NPS and its matching DN have the same OD. Metric specs often use DN, so a buyer needs to convert names, not measure.

How to read it

Find the NPS or DN.
Read across for the other designation.
Use the OD columns to check a pipe by measurement.

Conversion chart

NPS (in)	DN	OD (in)	OD (mm)
1/8	6	0.405	10.3
1/4	8	0.540	13.7
3/8	10	0.675	17.1
1/2	15	0.840	21.3
3/4	20	1.050	26.7
1	25	1.315	33.4
1-1/4	32	1.660	42.2
1-1/2	40	1.900	48.3
2	50	2.375	60.3
2-1/2	65	2.875	73.0
3	80	3.500	88.9
3-1/2	90	4.000	101.6
4	100	4.500	114.3
5	125	5.563	141.3
6	150	6.625	168.3
8	200	8.625	219.1
10	250	10.750	273.1
12	300	12.750	323.9
14	350	14.000	355.6
16	400	16.000	406.4
18	450	18.000	457.2
20	500	20.000	508.0
24	600	24.000	609.6

The mm values are converted from the inch OD. Some metric standards round them slightly differently.

Key points

A quick rule for NPS 4 and up: DN is about 25 times the NPS. Below NPS 4 the pairs are fixed and must be looked up.
Some metric pipe series use a different OD for the same DN. For example, some European series use a different OD for DN 65 than ASME pipe. Confirm the pipe standard, not just the DN.
Wall thickness is a separate choice. Convert the schedule or wall in millimeters as its own item.
Flanges and fittings must match the pipe standard as well as the DN.

Common mistakes

Treating DN as the inside diameter in millimeters.
Assuming DN 65 pipe from any standard fits an NPS 2-1/2 fitting.
Converting NPS to DN by multiplying by 25.4 for small sizes.
Dropping the pipe standard from the order when using DN.

Governing standards: ASME B36.10M (pipe dimensions); ISO 6708 (DN definition); the specific metric pipe standard named on the order.

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.

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