Dash numbers are the shorthand every fitting drawing speaks and every first-time buyer mistranslates. “-6 tube”, “-8 hose”, a callout that says nothing about threads — the dash is a size code from the SAE 37° flare world, and it decodes with one rule: divide by sixteen. Learn the rule and you can read any drawing without the chart; learn the two traps that ride along with it and you will not order the wrong part.
Key takeaways
- The rule: dash ÷ 16 = nominal size in inches. -6 is 6/16 = 3/8 inch; -8 is 8/16 = 1/2 inch.
- On tube and JIC fittings the dash is the tube OUTSIDE diameter; -8 means 1/2 inch OD tube.
- Dash is not NPS: 1/2 NPS pipe has a 0.840 in OD — the -8 tube beside it is 12.7 mm. Different numbering systems entirely.
- Dash sizing descends from AN/aerospace practice, standardized industrially as SAE J514 37° flare.
- The metric twin is a convention, not an equivalence: -6 pairs with 10 mm in catalogs, but the tubes are 9.53 and 10.0 mm — not interchangeable.
The rule: dash ÷ 16 = inch
Every dash number is the numerator of a sixteenths fraction. -4 is 4/16, a quarter inch. -6 is 6/16, three-eighths. -8 is 8/16, a half inch. -12 is three-quarters, -16 is one inch, and the series runs past -32 for large-bore hose. Our printable dash size chart maps the full -4 to -32 range to fractional inch and millimeters — keep it at the desk; the rule is for the field, when the chart is not. The full mapping, reproduced from that chart:
| Dash | Fractional inch | mm | Typical use |
|---|---|---|---|
| -4 | 1/4 | 6.35 | Small instrument and pilot lines |
| -6 | 3/8 | 9.53 | Workhorse instrument tube size |
| -8 | 1/2 | 12.70 | Instrument and hydraulic mains |
| -10 | 5/8 | 15.88 | Medium hydraulic lines |
| -12 | 3/4 | 19.05 | Process and hydraulic distribution |
| -14 | 7/8 | 22.23 | Less common; larger instrument runs |
| -16 | 1 | 25.40 | Hydraulic supply and return lines |
| -20 | 1-1/4 | 31.75 | Larger hydraulic and process runs |
| -24 | 1-1/2 | 38.10 | Manifold and station feeds |
| -28 | 1-3/4 | 44.45 | Rare; special machinery runs |
| -32 | 2 | 50.80 | Large-bore hose and tube headers |
Full printable version: dash size chart
The metric twin
Catalogs pair the inch dash series with metric tube sizes by convention: -6 sits beside 10 mm, -8 beside 12 mm, -4 beside 6 mm. Treat the pairing as a shopping convenience, not an equivalence. Fitting makers are explicit that the numbers only look close: 3/8 inch is 9.53 mm, not 10; 1/2 inch is 12.7, not 12. The gripping hardware — the sleeve or ferrule — has to match the actual tube, so a -6 fitting on 10 mm tube, or a 10 mm fitting on 3/8 tube, is a leak or a loose joint waiting for its shift. The metric-versus-fractional division of the tube world is worked through in metric vs fractional tube sizes; one more wrinkle for international drawings: metric-based dash callouts drop the millimeter number itself, so a -30 thread is a 30 mm reference, not 30/16 inch.
Where dash numbers come from
The dash is a child of American military aviation: Air Force-Navy (AN) aeronautical standards sized hoses and fittings in sixteenths, the Joint Industries Council carried the practice into industrial hydraulics, and SAE standardized it as the J514 37° flare system — the JIC fitting lineage. That is why dash sizing belongs to the flare-tube world and not to pipe threads, which follow their own charts and conventions. When a drawing mixes the two — a port called “-4 NPT” — read it with suspicion and check the thread itself; the 5-minute bench identification settles it in minutes, and mixing the systems is a documented way to order the wrong part.
Common mix-ups
The expensive confusion is dash versus NPS versus DN, three numbering systems sharing a vocabulary of halves and quarters:
| Callout | What it means | 1/2-inch example |
|---|---|---|
| Dash -8 | Tube OD in sixteenths: 8/16 = 1/2 in = 12.7 mm OD tube | 12.7 mm OD tube |
| NPS 1/2 | Nominal pipe size: name is not a measurement; actual OD fixed at 0.840 in (21.3 mm) | 21.3 mm OD pipe |
| DN 15 | ISO metric pipe name approximating NPS × 25 | 21.3 mm OD pipe, metric name |
Put a “-8 union” and a “1/2 NPT half-coupling” side by side and the pipe is 68% larger in outside diameter — they do not meet, and no adapter improvises it. Two dash nuances worth memorizing: on hose ends the dash usually denotes hose INSIDE diameter, not OD; and the SAE 100R5 hose type is the exception where dash equals a tube OD. Neither changes the tube-fitting rule; both change what you order for hose.
RFQ lines
dash size as drawn— copy the callout exactly: -6, -8.actual tube OD— in inches or mm, whichever the shop measures.system— JIC 37° flare, SAE, or metric tube; the fitting body must match.wall or schedule— dash says diameter, not wall.
FAQ
Is -8 the same as 1/2 inch?
As a tube size, yes: -8 = 8/16 = 1/2 inch, and on a tube fitting that means 1/2 inch tube outside diameter. As a pipe size, emphatically no: 1/2 NPS pipe has a 0.840 in (21.3 mm) OD, fixed by the pipe standard regardless of wall. The dash is a tube-and-hose code; NPS is a pipe code; the two 1/2-inch figures describe different things 68% apart in diameter.
Does -6 mean 6 mm?
No. -6 is 6/16 inch = 3/8 inch = 9.53 mm. Catalogs pair -6 with 10 mm tube by convention, and fitting makers warn the pairing is not an equivalence — the 0.47 mm OD difference is exactly what the gripping hardware cannot forgive. Order the fitting to match the measured tube, not the neighboring column.
What does the dash mean on a hose fitting?
On hose ends the dash commonly denotes the hose INSIDE diameter — -6 hose has a 3/8 inch bore. Tube fittings are the opposite convention: dash equals tube OUTSIDE diameter. One documented exception: SAE 100R5 hose, where the dash number equals a relevant tube OD. When the drawing says hose, think ID; when it says tube, think OD.
Is dash sizing the same as NPT?
No — and the distinction is the source of more wrong orders than any other single confusion. Dash sizing descends from the AN/SAE J514 37° flare tube-fitting system; NPT is the ASME tapered pipe-thread system with its own size names that deliberately do not match any measured diameter. A drawing that mixes them (“-4 NPT”) needs a thread check before it needs a quote.
Where does the dash system come from?
American military aviation: Air Force-Navy aeronautical standards sized lines in sixteenths of an inch, the Joint Industries Council carried the convention into industry, and SAE standardized it as J514 — the 37° flare system most people know as JIC. The dash is the aviation shorthand that stayed when the fittings went industrial. That origin also explains why the system stops at the tube: aviation plumbed with tube, not pipe, and the sixteenths rule was built for tube OD from the first drawing.
