In a nutshell: if you’re looking for the exact match between “DN 50”, 2″, “60.3” or “26 × 34”, you’re in the right place!
We untangle every unit here, using the official dimensions of standard EN 10241 together with BSPT thread lengths (ISO 7-1).
Quick-memo size table:
1. Why this article?
Whether you’re a professional plumber or a keen DIYer, you often have to juggle three systems:
- the inch (
″) — an Anglo-Saxon legacy; - the DN (Nominal Diameter) — a commercial “code” defined by ISO;
- the true diameter in millimetres.
Add the difference between welded and seamless pipe and the confusion is complete! This guide helps:
- Installers who need to quote correctly,
- Buyers who must order the right “26 × 34 mm galvanized” item,
- Makers restoring vintage steel heating circuits.
2. Quick reminder of the units
- The inch (
″) dates back to the industrial revolution: 1″ = 25.4 mm, but in plumbing it refers to the historical inside diameter. - DN (Nominal Diameter): rounded values (8, 10, 15 …) that do not equal the real diameter — they’re only codes.
- Outside Ø (OD) / Inside Ø (ID): the only dimensions you can measure with callipers. OD is fixed by EN 10241; the minimum ID depends on wall thickness (2.3 → 4 mm for the medium series).
3. Master conversion table (DN ↔ inches ↔ mm)
UX tip: the table is sortable; click a header to reorder.
| DN | Inch | “mm” size (FR) | OD (mm) | Min ID* (mm) | Male thread R L₂ (mm) |
|---|---|---|---|---|---|
| 8 | ¼″ | 8 × 13 | 13.5 | 8.9 | 8.4 |
| 10 | ⅜″ | 12 × 17 | 17.2 | 12.6 | 9.9 |
| 15 | ½″ | 15 × 21 | 21.3 | 16.7 | 11.4 |
| 20 | ¾″ | 20 × 27 | 26.9 | 22.3 | 13.0 |
| 25 | 1″ | 26 × 34 | 33.7 | 28.5 | 16.3 |
| 32 | 1¼″ | 33 × 42 | 42.4 | 37.2 | 17.6 |
| 40 | 1½″ | 40 × 49 | 48.3 | 42.5 | 19.2 |
| 50 | 2″ | 50 × 60 | 60.3 | 54.5 | 21.4 |
| 65 | 2½″ | 66 × 76 | 76.1 | 69.7 | 25.7 |
| 80 | 3″ | 80 × 90 | 88.9 | 81.7 | 26.4 |
| 100 | 4″ | 102 × 114 | 114.3 | 106.3 | 28.9 |
*Min ID calculated with the minimum wall thickness of EN 10241 (2.3 → 4 mm).
4. How to read a “26 × 34 mm” designation?
Memory rule: in French notations “15 × 21”, “20 × 27”, “26 × 34”…
- the first number ≈ historical inside diameter;
- the second number ≈ outside diameter of the male BSPT thread.
| Value | Meaning | EN 10241 figure |
|---|---|---|
| 26 mm | Historical ID (before wall thickness) | Min ID ≈ 28.5 mm for DN 25 |
| 34 mm | Male BSPT thread OD | True OD = 33.7 mm |
🧩 Conclusion: “26 × 34 mm” ⇢ DN 25 / 1″
“26” and “34” are rounded values used on-site for quick identification. For calculations or pressure-drop sizing, always rely on the exact figures in the standard.
5. BSPT thread (R): the backbone of steel plumbing
| Inch size | DN | Taper | Angle | Thread length L₂ (mm) |
|---|---|---|---|---|
| ¼″ | 8 | 1:16 | 55° | 8.4 |
| ⅜″ | 10 | 9.9 | ||
| ½″ | 15 | 11.4 | ||
| ¾″ | 20 | 13.0 | ||
| 1″ | 25 | 16.3 | ||
| 1¼″ | 32 | 17.6 | ||
| 1½″ | 40 | 19.2 | ||
| 2″ | 50 | 21.4 | ||
| 2½″ | 65 | 25.7 | ||
| 3″ | 80 | 26.4 | ||
| 4″ | 100 | 28.9 |
Need ready-threaded lengths? Visit our pre-cut galvanized steel pipes ↗.
*Extract from ISO 7-1. Use hemp & compound or PTFE tape for sealing.
For complete dimensional checks and leak-free assembly, read our essential EN 10241 guide ↗.
6. Welded vs seamless pipe – what difference?
Welded pipe: a steel strip is rolled then longitudinally welded (ERW). The seam is visible under a light bench or microscope.
- Pros : affordable, widely available, easy to cut straight.
- Cons : pressure rating roughly 20 % lower at the same DN; re-threading beyond the first third is not recommended.
Seamless pipe: produced hot-finished (HFS) or by extrusion.
- Pros : higher pressure and bending strength; no heat-affected zone.
- Cons : 15–25 % more expensive; slight ovality often needs correction before threading.
How to tell them apart ? Bevel one end: a slight bead or layered look means welded. On galvanized tube, the weld line is often shinier.
7. Typical applications and max flow rate
Reference: cold water at 15 °C, head loss ≤ 250 Pa / m.
| DN | Inch | Max flow (L / min) | Typical uses |
|---|---|---|---|
| 15 | ½″ | 35 | Domestic fixtures, 8-bar compressed air |
| 20 | ¾″ | 60 | Wall-hung boiler loops, DHW recirculation |
| 25 | 1″ | 100 | Fire-hose reels, steam ring mains |
| 32 | 1¼″ | 180 | Primary manifold in plant room |
| 40 | 1½″ | 280 | Process air, light fuel oil |
| 50 | 2″ | 450 | Industrial cooling circuits |
8. FAQ – conversions & technical answers
¼″ — DN 8 — 8 × 13 mm
- ¼″ equals what size in mm ?
- ¼″ equals 8 × 13 mm.
- ¼″ equals what DN ?
- ¼″ equals DN 8.
- 8 × 13 mm equals what size in inches ?
- 8 × 13 mm equals ¼″.
- 8 × 13 mm equals what DN ?
- 8 × 13 mm equals DN 8.
- DN 8 equals what size in mm ?
- DN 8 equals 8 × 13 mm.
- DN 8 equals what size in inches ?
- DN 8 equals ¼″.
⅜″ — DN 10 — 12 × 17 mm
- ⅜″ equals what size in mm ?
- ⅜″ equals 12 × 17 mm.
- ⅜″ equals what DN ?
- ⅜″ equals DN 10.
- 12 × 17 mm equals what size in inches ?
- 12 × 17 mm equals ⅜″.
- 12 × 17 mm equals what DN ?
- 12 × 17 mm equals DN 10.
- DN 10 equals what size in mm ?
- DN 10 equals 12 × 17 mm.
- DN 10 equals what size in inches ?
- DN 10 equals ⅜″.
½″ — DN 15 — 15 × 21 mm
- ½″ equals what size in mm ?
- ½″ equals 15 × 21 mm.
- ½″ equals what DN ?
- ½″ equals DN 15.
- 15 × 21 mm equals what size in inches ?
- 15 × 21 mm equals ½″.
- 15 × 21 mm equals what DN ?
- 15 × 21 mm equals DN 15.
- DN 15 equals what size in mm ?
- DN 15 equals 15 × 21 mm.
- DN 15 equals what size in inches ?
- DN 15 equals ½″.
¾″ — DN 20 — 20 × 27 mm
- ¾″ equals what size in mm ?
- ¾″ equals 20 × 27 mm.
- ¾″ equals what DN ?
- ¾″ equals DN 20.
- 20 × 27 mm equals what size in inches ?
- 20 × 27 mm equals ¾″.
- 20 × 27 mm equals what DN ?
- 20 × 27 mm equals DN 20.
- DN 20 equals what size in mm ?
- DN 20 equals 20 × 27 mm.
- DN 20 equals what size in inches ?
- DN 20 equals ¾″.
1″ — DN 25 — 26 × 34 mm
- 1″ equals what size in mm ?
- 1″ equals 26 × 34 mm.
- 1″ equals what DN ?
- 1″ equals DN 25.
- 26 × 34 mm equals what size in inches ?
- 26 × 34 mm equals 1″.
- 26 × 34 mm equals what DN ?
- 26 × 34 mm equals DN 25.
- DN 25 equals what size in mm ?
- DN 25 equals 26 × 34 mm.
- DN 25 equals what size in inches ?
- DN 25 equals 1″.
1¼″ — DN 32 — 33 × 42 mm
- 1¼″ equals what size in mm ?
- 1¼″ equals 33 × 42 mm.
- 1¼″ equals what DN ?
- 1¼″ equals DN 32.
- 33 × 42 mm equals what size in inches ?
- 33 × 42 mm equals 1¼″.
- 33 × 42 mm equals what DN ?
- 33 × 42 mm equals DN 32.
- DN 32 equals what size in mm ?
- DN 32 equals 33 × 42 mm.
- DN 32 equals what size in inches ?
- DN 32 equals 1¼″.
1½″ — DN 40 — 40 × 49 mm
- 1½″ equals what size in mm ?
- 1½″ equals 40 × 49 mm.
- 1½″ equals what DN ?
- 1½″ equals DN 40.
- 40 × 49 mm equals what size in inches ?
- 40 × 49 mm equals 1½″.
- 40 × 49 mm equals what DN ?
- 40 × 49 mm equals DN 40.
- DN 40 equals what size in mm ?
- DN 40 equals 40 × 49 mm.
- DN 40 equals what size in inches ?
- DN 40 equals 1½″.
2″ — DN 50 — 50 × 60 mm
- 2″ equals what size in mm ?
- 2″ equals 50 × 60 mm.
- 2″ equals what DN ?
- 2″ equals DN 50.
- 50 × 60 mm equals what size in inches ?
- 50 × 60 mm equals 2″.
- 50 × 60 mm equals what DN ?
- 50 × 60 mm equals DN 50.
- DN 50 equals what size in mm ?
- DN 50 equals 50 × 60 mm.
- DN 50 equals what size in inches ?
- DN 50 equals 2″.
2½″ — DN 65 — 66 × 76 mm
- 2½″ equals what size in mm ?
- 2½″ equals 66 × 76 mm.
- 2½″ equals what DN ?
- 2½″ equals DN 65.
- 66 × 76 mm equals what size in inches ?
- 66 × 76 mm equals 2½″.
- 66 × 76 mm equals what DN ?
- 66 × 76 mm equals DN 65.
- DN 65 equals what size in mm ?
- DN 65 equals 66 × 76 mm.
- DN 65 equals what size in inches ?
- DN 65 equals 2½″.
3″ — DN 80 — 80 × 90 mm
- 3″ equals what size in mm ?
- 3″ equals 80 × 90 mm.
- 3″ equals what DN ?
- 3″ equals DN 80.
- 80 × 90 mm equals what size in inches ?
- 80 × 90 mm equals 3″.
- 80 × 90 mm equals what DN ?
- 80 × 90 mm equals DN 80.
- DN 80 equals what size in mm ?
- DN 80 equals 80 × 90 mm.
- DN 80 equals what size in inches ?
- DN 80 equals 3″.
4″ — DN 100 — 100 × 114 mm
- 4″ equals what size in mm ?
- 4″ equals 100 × 114 mm.
- 4″ equals what DN ?
- 4″ equals DN 100.
- 100 × 114 mm equals what size in inches ?
- 100 × 114 mm equals 4″.
- 100 × 114 mm equals what DN ?
- 100 × 114 mm equals DN 100.
- DN 100 equals what size in mm ?
- DN 100 equals 100 × 114 mm.
- DN 100 equals what size in inches ?
- DN 100 equals 4″.
Technical questions
- What is the minimum wall thickness for DN 25 pipe ?
- 3.2 mm (EN 10241 medium series).
- Is there a BSP thread for DN 65 ?
- Yes : taper thread R 2½″ ISO 7-1.
- What pressure can a welded DN 50 pipe handle ?
- PN 25 (≈ 25 bar) at 120 °C in EN 10255 M.
- How do I tell welded from seamless pipe ?
- The welded type shows a longitudinal seam; seamless is uniform.
- DN 50 – what’s the max water flow ?
- ≈ 450 L/min at 2 m/s velocity.
- Hemp or PTFE for sealing ?
- Hemp + paste for diameters > 1″; PTFE tape for quick jobs ≤ 1″.
- Can I thread an existing galvanized pipe ?
- Yes, but spray cold-galv on the bare steel afterwards.
- Torque for a ¾″ BSPT joint ?
- Around 90 N·m (adjust with lubrication).
- Remove galvanizing before welding ?
- Yes – strip about 30 mm to avoid toxic fumes.
- Thread depth for a DN 40 housing ?
- ≈ 19 mm (L₂ ISO 7-1).
9. Best installation practices
- Torque: start at 70 % of the max value, then tighten another quarter-turn if needed.
- Cut tolerances: ± 1 mm for DN ≤ 40, ± 2 mm above.
- Inspection: plug gauge, dye-penetrant or borescope for DN ≥ 80.
10. Conclusion & next steps
You now master DN-inch terminology and sound assembly practice. To go further :
- Order ready-to-install lengths in our pre-cut galvanized-pipe range ↗.
- Read the essential EN 10241 / ISO 7-1 guide ↗ for full torque tables.