Stainless Steel Pipe Fittings: Nipples, Reducers, Elbows and Size Chart
Stainless steel pipe fittings connect pipes to each other, to valves and to instruments, change the direction or size of a line and close pipe ends. To choose the right fitting you only need two things: the connection type (threaded or butt weld) and the size. This guide covers the most common fittings, an inch–DN–mm size chart and practical selection tips.
Threaded vs. Butt Weld Fittings
- Threaded: screwed together and removable. Typically used up to 2" and on lines that need regular maintenance. Sized in inches (e.g. 1/2"); sealed with PTFE tape or liquid thread sealant.
- Butt weld: welded to the pipe and permanent. Preferred for high pressure, vibration, hygienic and large-bore lines. Sized by pipe outside diameter and wall thickness (e.g. Ø33.7 x 2 mm).
Fitting Types
| Fitting | Function | Category |
|---|---|---|
| Nipple (hex, double, single threaded) | Joins two female-threaded parts | Pipe Nipples |
| Reducer, reducing nipple | Changes line size | Reducers |
| Elbow (threaded, street, butt weld) | Changes line direction | Elbows and Tees |
| Tee (threaded, butt weld) | Splits the line three ways | Elbows and Tees |
| Union, socket, hose tail | Removable joint, straight joint, hose connection | Unions and Sockets |
| Plug, cap | Closes a port or pipe end | Plugs and Caps |
| Slip-on and blind flange | Bolted removable joint, line closure | Flanges |
Nipple and Reducer Size Chart (Inch – DN – mm)
The size of a threaded fitting is the nominal inch size of the pipe it fits, not its actual inside or outside diameter. Butt weld fittings are selected directly by pipe outside diameter.
| Inch (BSP) | DN | Pipe OD | Thread OD (approx.) |
|---|---|---|---|
| 1/8" | DN6 | 10.2 mm | 9.7 mm |
| 1/4" | DN8 | 13.5 mm | 13.2 mm |
| 3/8" | DN10 | 17.2 mm | 16.7 mm |
| 1/2" | DN15 | 21.3 mm | 21.0 mm |
| 3/4" | DN20 | 26.9 mm | 26.4 mm |
| 1" | DN25 | 33.7 mm | 33.2 mm |
| 1 1/4" | DN32 | 42.4 mm | 41.9 mm |
| 1 1/2" | DN40 | 48.3 mm | 47.8 mm |
| 2" | DN50 | 60.3 mm | 59.6 mm |
| 2 1/2" | DN65 | 76.1 mm | 75.2 mm |
| 3" | DN80 | 88.9 mm | 87.9 mm |
| 4" | DN100 | 114.3 mm | 113.0 mm |
How to identify a size: measure the outside diameter of a male thread with a caliper and compare it with the last column. A male thread measuring about 26.4 mm is 3/4".
Flange Dimensions (DN / PN10 / PN16)
From DN15 to DN150, PN10 and PN16 flanges share the same connection dimensions. See the full chart on our flanges page.
| DN | OD | Bolt circle | Bolts |
|---|---|---|---|
| DN15 | 95 mm | 65 mm | 4 x M12 |
| DN25 | 115 mm | 85 mm | 4 x M12 |
| DN50 | 165 mm | 125 mm | 4 x M16 |
| DN80 | 200 mm | 160 mm | 8 x M16 |
| DN100 | 220 mm | 180 mm | 8 x M16 |
Why AISI 304 Stainless Steel?
AISI 304 resists corrosion in water, steam, compressed air, food and mild chemical lines and is hygienic. Use AISI 316 for chlorinated water, seawater or aggressive chemicals. Mixing brass or galvanized parts into a stainless line can cause galvanic corrosion.
Sealing and Installation Tips
- Wrap 3–5 turns of PTFE tape against the thread direction, leaving the first thread clear.
- Over-tightening stainless threads can cause galling; keep threads clean and tighten in a controlled way.
- Fit unions before and after equipment that needs servicing (pumps, meters, steam traps).
- On butt weld fittings, match the wall thickness of the fitting to the pipe.
FAQ
How many mm is 1/2 inch?
A 1/2" (DN15) pipe has an outside diameter of 21.3 mm; a 1/2" thread measures about 21 mm.
What is the difference between a nipple and a socket?
A nipple has male threads on both ends and joins two female parts. A socket has female threads on both ends and joins two male pipe ends.
Browse all stainless fittings in our Technical Hardware category, or send us your pipe size and a photo on WhatsApp for help choosing the right parts.