Metric compression fittings live in a parallel world to the fractional-inch one: same mechanism, same bite, different numbers — and a hard lesson for anyone who assumes the two worlds nearly-meet. They do not. A 12 mm tube is not a 1/2 inch tube with a rounding error; the ferrule biting it was machined for an exact millimeter diameter, and the 0.7 mm between them is the difference between a seal and a leak. This is the walkthrough for the millimeter system: the sizes, the threads, and the two rules that keep metric lines quotable.
Key takeaways
- The common metric instrument range runs 6, 8, 10, 12, 15, 18, 22 and 25 mm OD — a working subset of maker catalogs that extend the series in both directions.
- Metric and inch fittings are not interchangeable at near-equal sizes: a 12 mm ferrule against 1/2 inch tube leaves roughly a 0.7 mm mismatch the seal cannot close.
- Two thread systems ride on metric fittings: metric fine threads per the ISO 261 ladder (M12×1.5 upward) and G threads on the same bodies.
- DIN 2353 / ISO 8434-1 is the metric-native standard: L series 6–42 mm, S series 6–38 mm, LL 4–12 mm — and its size ladder is why 15 and 22 mm exist at all.
- Write tube as OD × wall plus the tail thread standard, and the metric line quotes without a clarification round.
The metric size pairs
The working metric instrument range is eight sizes: 6, 8, 10, 12, 15, 18, 22 and 25 mm OD, with maker catalogs extending the ladder both ways — one major catalog runs from 2 to 38 mm, so treat the eight as the common core, not the complete list. Walls pair conventionally at 1.0, 1.5 and 2.0 mm, with 12 × 1.5 and 12 × 2 the stock examples every tubing table carries. The ordering grammar mirrors the inch world’s — OD × wall for tube, thread standard and size for the tail — and the two-line RFQ habit that keeps inch lines clean does the same work here. The compression fittings range covers both systems, and the inch-side counterpart of this post is metric vs fractional tube sizes.
Metric vs fractional: not interchangeable
The near-miss trap is the first lesson of the millimeter system. Put a 12 mm tube in a 1/2 inch fitting and the ferrule meets a surface roughly 0.7 mm oversize — the bite geometry cannot close, the seal lands on a gap, and the leak or blow-out risk arrives with pressure. The maker guidance is unambiguous: ferrules and bodies are machined for one exact tube OD, and close does not mean compatible. The same rule runs the other direction — a metric ferrule on inch tubing seats on nothing — and the correct bridge is a purpose-built transition fitting, not optimism. The dash-number system has its own mirror of this trap, and dash numbers on drawings is the inch-side warning in full.
Thread tails: M vs G
Metric fittings carry two thread families on the same bodies, and the RFQ should say which. Metric fine threads follow the ISO 261 ladder — M12×1.5 for 6 and 8 mm tube nuts in the DIN system, stepping up through M14×1.5, M16×1.5 and M18×1.5 as the tube grows — and are the DIN 2353 native tail. G threads, the parallel BSPP form, appear on the same metric bodies where the plant standard is BSP, and maker catalogs list metric-tube fittings with G tails as standard variants. The sealing logic follows the thread: the metric fine and G threads on port studs both seal at the face — per the BSP thread seal logic that parallel threads never seal on the flank — while the nut-to-body joint of the fitting itself is the compression mechanism, the 24° cone bite covered in bite-type fittings explained.
The DIN 2353 size ladder
The reason 15 and 22 mm exist — sizes with no inch near-equivalent at all — is the DIN 2353 size ladder, the metric-native standard now mirrored by ISO 8434-1. Its L series runs 6, 8, 10, 12, 15, 18, 22, 28, 35 and 42 mm; the S series for heavier wall runs 6–38 mm; the LL light series covers 4–12 mm. The ladder is deliberately decoupled from the inch world’s sixteenths, which is why a European drawing quoting 15 mm or 22 mm is quoting native sizes, not conversions — and why the matching ferrule exists only in the metric catalog. The size-walk for DIN-native hardware is the same pressure-chart territory as every bite-type fitting: the DIN 2353 pressure chart carries the L and S series working pressures.
RFQ lines
tube OD × wall— in millimeters, both numbers:12 × 1.5, not12 mm tube.series— DIN L or S where the standard applies, or the maker’s metric series name.tail thread—M12×1.5orG1/4: the thread family is a decision, not a default.system note— metric line, metric ferrules; the words that prevent an inch-fitting substitution.
FAQ
Can I put a metric ferrule on inch tubing?
No. The ferrule is machined to bite one exact millimeter OD, and the nearest inch size differs by four to seven tenths of a millimeter — enough that the bite lands off-center and the seal never closes. Both maker guidance and field failure reports agree the mismatch leaks, and the honest bridge is a purpose-built metric-to-inch transition fitting.
What are the common metric compression fitting sizes?
The common instrument core is 6, 8, 10, 12, 15, 18, 22 and 25 mm OD, with catalogs extending from 2 to 38 mm and beyond. Treat the eight as the working set: below 6 mm is miniature territory, above 25 mm slides toward hydraulic sizing. Walls conventionally run 1.0, 1.5 and 2.0 mm.
Is a 12 mm fitting the same as 1/2 inch?
No — 12 mm is 12.00 mm and 1/2 inch is 12.7 mm, and the 0.7 mm between them is unbridgeable by a ferrule machined for one of them. They are neighboring residents of two different sizing systems, like the dash versus NPS confusion in pipe work. Use a transition fitting where the systems must meet.
That 0.7 mm gap is also why the sizing grammar insists on OD, never ID: the metric ladder names every fitting after the tube’s outside diameter — 12L means 12 mm OD, light series — so the OD picks the catalog row and the wall only has to sit inside the ferrule’s grip range. How OD sizing, wall and the DIN size names interact is laid out in compression fitting sizes: OD vs ID.
What thread is on a metric compression fitting?
Two families: metric fine per the ISO 261 ladder — M12×1.5 and upward in the DIN 2353 system — and G threads, the parallel BSPP form, as a cataloged variant on the same bodies. Both seal at the face on a washer or O-ring; the choice belongs to the plant’s thread standard and belongs on the RFQ.
Why do 15 mm and 22 mm fittings exist?
Because the DIN 2353 / ISO 8434-1 size ladder says so. Its L series runs 6, 8, 10, 12, 15, 18, 22, 28, 35 and 42 mm — deliberately decoupled from the inch world’s sixteenths — so 15 and 22 are native metric sizes, not conversions of anything. A European drawing quoting them is quoting the metric standard on its own terms.
