Compression fitting sizes have one golden rule, and every ordering disaster in the category breaks it: a compression fitting is sized by the tube’s outside diameter, never the inside. The ferrule bites the OD; the fitting’s size number is the OD; the whole standard — DIN 2353 and its fractional-inch cousins — is built on OD. When a drawing says just “3/16” and nobody asks of what, the wrong catalog row ships and the fittings take a round trip. Here is how to read the number.
Key takeaways
- Compression fittings size by tube OD, full stop. DIN 2353 names the size after it: 12L means 12 mm OD, light series.
- 3/16 inch is a real tube OD — the catalogs call it dash-3 — but a bare “3/16” on a drawing may come from plumbing-copper nominal sizing, where the ID-era convention hides an OD 1/8 inch larger.
- Tube is ordered as OD × wall; pipe is ordered as nominal size plus schedule. The two grammars do not translate by intuition — 1/2 NPS pipe actually measures 0.840 in OD.
- Same OD, different wall means different ID and different pressure rating — and wall thickness has to suit the fitting or the ferrule cannot grip.
- Five checks before ordering — OD or ID? wall? grade? ends? unit count — kill the classic mistakes.
The golden rule: compression fits the OD
The ferrule seals by biting the tube’s outer surface, so everything follows the outer surface. The size stamped on the body is the tube OD it accepts — 6 mm, 1/4 in, 12 mm — and the standards encode it directly: DIN 2353, the bite-type benchmark, names each size by OD and series, 12L being 12 mm OD in the light series, 12S the same OD in heavy. Fractional catalogs run the same logic from 1/16 in up. There is no ID anywhere in the sizing system because nothing in the joint touches the ID. The metric versus fractional tube sizes post carries the mixed-standard detail.
The 3/16 trap and friends
Three-sixteenths is where the rule earns its keep, because the number lives in two worlds. In the fitting catalog, 3/16 in is a standard tube OD — the sequence 1/16, 1/8, 3/16, 1/4, 5/16 marches upward, and the major fitting systems catalog it as dash-3. In the plumbing-copper world, a bare “3/16” more likely came from the ID-era nominal convention of ASTM B88 tube, where the named size is not any actual dimension — 1/2 nominal copper actually measures 5/8 in OD. So a technician measures a copper line, reads 3/16 off the print, and orders a fitting that fits nothing on the bench. Same number, two worlds, one returned shipment.
The trap has a cousin: same OD, different wall, different ID — ID equals OD minus twice the wall. A 1/2 in OD tube at 0.049 wall has a 0.402 in bore; at 0.065 wall, 0.370. OD is what the fitting grips; wall decides whether it grips at all.
Tube ordering grammar
Tube and pipe are specified in two grammars, and mixing them is the second classic mistake. Tube is ordered as OD × wall — 1/4 in × 0.035, 6 mm × 1 — the dimensions you actually receive. Pipe is ordered as nominal size plus schedule: 1/2 NPS, schedule 40. NPS is a name, not a measurement — a 1/2 NPS pipe measures 0.840 in OD whether the wall is schedule 40 (0.109) or 80 (0.147) — and the schedule encodes the wall, not the bore. The full grammar is in pipe schedule numbers explained and pipe vs tube: what is the difference. One rule: if the number could be taped on the outside of the part, quote OD; if it came from a pipe table, quote NPS and schedule.
A pre-order checklist
Run these five checks before the RFQ goes out:
- OD or ID? Find which dimension the drawing’s number refers to; if it is copper plumbing size, convert to actual OD before opening the catalog.
- Wall thickness? Record it with the OD. Check it against the fitting maker’s wall range for that size — too thin and the ferrule collapses the tube; too thick and it cannot bite.
- Material grade? 316, 316L, 304 — the grade changes pressure rating and compatibilities even at identical dimensions.
- End forms? Compression end on one side, thread on the other — write each end separately, as in dash numbers field guide shorthand if your plant uses it.
- Unit and count? Confirm the units — a 6 mm and a 1/4 in fitting are close enough to argue about, different enough to leak.
RFQ lines
tube OD— the outside diameter, in the units measured: 3/16 in, 6 mm, 12 mm.wall thickness— with the OD, every time; both decide the pressure rating.material grade— 316/316L/304, seamless or welded as applicable.end forms— one line per end: compression size, thread form and size, weld prep.quantity and spares— plus a note if remakes are expected, so ferrules arrive with the fittings.- One line list, one quote: our compression fittings range is sized by OD with the wall range stated per size.
FAQ
Is a 3/16 compression fitting for 3/16 OD or ID?
3/16 OD. Every compression fitting size is the tube’s outside diameter — the catalogs list 3/16 in as a standard fractional OD, dash-3 in the common shorthand. The confusion comes from plumbing copper, where sizes are ID-era nominals: a “3/16” copper figure may belong to a tube whose actual OD is larger. Measure the outside of the tube and order that number.
How do I measure tube OD in the field?
Calipers are the fast answer — close them on the tube and read the OD directly, without squeezing soft plastic. No calipers: wrap a tape or strip of paper around the tube, mark one circumference, measure and divide by π — that gives the OD from the outside. Either beats trusting a faded drawing.
Why does wall thickness matter to a compression fitting?
The ferrule must bite the tube wall hard enough to seal, and the wall must survive the bite. Makers publish a wall range per size: thinner and the ferrule collapses the tube instead of gripping; thicker and it cannot close properly. Wall also sets the pressure rating — OD plus wall is the only honest order.
Is pipe size the same as tube size?
No — two naming systems. Tube is named by actual OD and wall: what you measure is what you get. Pipe is named by nominal size and schedule, a name that stopped matching any real dimension long ago: 1/2 NPS pipe measures 0.840 in OD. Quoting pipe grammar at a tube fitting is how wrong parts get ordered.
What do the DIN numbers like 12L mean?
DIN 2353 sizes are the tube OD in millimeters plus the series letter: 12L is 12 mm OD, light series; 12S the same OD in heavy, with thicker tube wall and higher pressure class. Read the number as millimeters of outside diameter, the letter as the duty, and the metric catalog decodes itself.
