FitterMaster Pipe size inch to mm: what is the real outside diameter of each NPS?
A 2 inch pipe is not 50.8 mm. NPS is a name, and 2" pipe has a 2.375" outside diameter, which is 60.3 mm; the table below gives the OD in mm for every size from 1/8 to 24 that FitterMaster's Pipe Data carries.
Typing 2 into a unit converter gives 50.8 mm. A caliper held on a 2" Sch 40 nipple reads 60.3. The converter is right about inches and wrong about pipe.
Up to NPS 12 the nominal size is only a label that stuck. From 14 up the label and the OD are the same number, so 14" pipe measures 14.000" and 355.6 mm.
Outside diameter of every NPS, in inches and mm
| NPS | OD (in) | OD (mm) |
|---|---|---|
| 1/8" | 0.405" | 10.3 |
| 1/4" | 0.540" | 13.7 |
| 3/8" | 0.675" | 17.1 |
| 1/2" | 0.840" | 21.3 |
| 3/4" | 1.050" | 26.7 |
| 1" | 1.315" | 33.4 |
| 1-1/4" | 1.660" | 42.2 |
| 1-1/2" | 1.900" | 48.3 |
| 2" | 2.375" | 60.3 |
| 2-1/2" | 2.875" | 73.0 |
| 3" | 3.500" | 88.9 |
| 3-1/2" | 4.000" | 101.6 |
| 4" | 4.500" | 114.3 |
| 5" | 5.563" | 141.3 |
| 6" | 6.625" | 168.3 |
| 8" | 8.625" | 219.1 |
| 10" | 10.750" | 273.1 |
| 12" | 12.750" | 323.8 |
| 14" | 14.000" | 355.6 |
| 16" | 16.000" | 406.4 |
| 18" | 18.000" | 457.2 |
| 20" | 20.000" | 508.0 |
| 22" | 22.000" | 558.8 |
| 24" | 24.000" | 609.6 |
The OD stays put whatever the schedule. A heavier wall eats into the bore instead, so the pipe still goes into the same fitting. One screen holds every OD; IDs take a chart per schedule.
Reading it in the app
- Open Pipe Data under Reference. The free schedules, STD and Sch 40, already show NPS, OD and ID in inches to three decimals.
- For mm, go to Settings, then Units, and set Measurement System to Metric. This is a Pro setting; without Pro the screen shows Metric is a Pro feature. and tapping Metric opens the upgrade screen.
- Back in Pipe Data the same rows show mm with one decimal, and the weight column switches to kg/m.
You will not find 22" in Sch 40. It is not made in that schedule, so the app leaves the row out rather than invent a wall. STD does have a 22" line.
The free shortcut, and its limits
Without Pro there is Tape Math: type a value under Decimal inches and read Millimetres. It works, but it only accepts two decimals and rounds the answer to the nearest 0.5 mm. Typing 2.38 for the 2" pipe gives 60.5 mm, a fifth of a millimetre over the real 60.3. For cutting that does not matter. For checking a clamp or a sleeve against a drawing it does, which is when the metric setting is worth having.
Who should bother
For one number on one size, a printed chart beats any app, so installing this just for that is not worth it. It is worth keeping for the days you also need the ID or the weight per foot from the same row, or the fitting takeouts, in a pipe rack with no signal.
Questions and answers
What is the outside diameter of 2 inch pipe in mm?
Why is 1 inch pipe not 25.4 mm?
Is NPS the same as DN?
Can I see the inch to mm pipe table for free?
Does the OD change between schedule 40 and 80?
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