Appendix 21: Metric and Imperial Metal Pipe Dimensions Equivalents
Australian plumbing and pipe fittings are sized by nominal designation only — DN (mm), the plain metric size as sold, and the imperial fraction are different names for the same nominal size, and none of them is the actual measured dimension of the part. This table gives the standard equivalences plus the actual BSP thread major diameter, which is what you'll get if you put calipers on a fitting.
| DN / NB designation | Metric size as sold (mm) | Imperial nominal | BSP thread designation | Actual thread major Ø (mm) |
| DN8 | 8mm | 1/4" | G 1/4 | 13.16 |
| DN10 | 10mm | 3/8" | G 3/8 | 16.66 |
| DN15 | 15mm | 1/2" | G 1/2 | 20.96 |
| DN20 | 20mm | 3/4" | G 3/4 | 26.44 |
| DN25 | 25mm | 1" | G 1 | 33.25 |
| DN32 | 32mm | 1 1/4" | G 1 1/4 | 41.91 |
| DN40 | 40mm | 1 1/2" | G 1 1/2 | 47.80 |
| DN50 | 50mm | 2" | G 2 | 59.61 |
| DN65 | 65mm | 2 1/2" | G 2 1/2 | 75.18 |
| DN80 | 80mm | 3" | G 3 | 87.88 |
| DN100 | 100mm | 4" | G 4 | 113.03 |
Why the "actual" column matters: the nominal size (e.g. "DN25 / 25mm / 1inch") historically approximated the bore of the original wrought-iron pipe, not any thread or fitting dimension. Manufacturing has moved on since; the label has not. In practice:
- When you buy a fitting, the box or shelf tag will show the '''metric size as sold''' (e.g. "25mm") or the imperial fraction (e.g. "1inch") — these are the same nominal size, freely interchangeable at the trade counter, and both are the number to order by. Neither is the measured thread or bore dimension.
- The BSP thread major diameter (male thread crest-to-crest, or the matching female socket) is noticeably larger than the nominal size — e.g. a socket measuring ~58-59 mm across is a normal DN50/50mm/2" female BSP socket, sized to clear a 59.61 mm male thread crest, not an oversized or non-standard part.
- The actual pipe bore once made up is usually smaller than the nominal label and varies by schedule/class — for steel pipe to AS 1074, DN25 (25mm/1") has a bore around 26-27 mm; for PVC pressure pipe to AS/NZS 1477, 25mm pipe has a bore around 30 mm, because Australian PVC pressure pipe is sized on outside diameter, not nominal bore, unlike threaded metal fittings.
Rule of thumb for this project: when matching threaded metal fittings (valves, Dosatron ports, reducers), go by the nominal label on the box — 25mm and 1" are the same order — and trust the BSP standard to make them mate correctly. Don't try to back-calculate compatibility from a caliper reading against the nominal size in mm. Use the caliper only to identify an unlabelled part, by matching the measured thread diameter against the "Actual thread major Ø" column above.
Note: this table is for BSP threaded metal fittings (brass, stainless, galvanised steel — the needle valve, solenoid, Dosatron ports, tees in this build). Australian PVC pressure pipe (AS/NZS 1477) and metric poly pipe use different sizing conventions (OD-based, not thread-based) and the mm figures are not directly comparable to the columns above.