JIC Metric Conversion Chart (Inch to mm)
JIC fittings are an inch system. This chart gives every size in millimetres and shows how to tell JIC from metric DIN and BSP fittings that measure almost the same.
Last updated October 30, 2026. Reference values; verify against manufacturer specifications.
JIC fittings are an inch system: tube sizes are fractions of an inch and threads are UN/UNF. When you work with metric calipers, metric tube, or equipment that mixes JIC with metric DIN or BSP fittings, it helps to have every JIC size in millimetres and to know which metric and BSP threads are close enough to cause confusion.
JIC size chart in millimetres
All conversions use exactly 25.4 mm per inch. Thread pitch in millimetres is 25.4 divided by the threads per inch. Female thread ID values are approximate, as published catalogs vary slightly.
| Dash | Tube OD (mm) | Thread | Male thread OD (mm) | Thread pitch (mm, = 25.4 / TPI) | Female thread ID, approx. (mm) |
|---|---|---|---|---|---|
| -2 | 3.18 | 5/16-24 UNF | 7.94 | 1.058 | 6.9 |
| -3 | 4.76 | 3/8-24 UNF | 9.53 | 1.058 | 8.6 |
| -4 | 6.35 | 7/16-20 UNF | 11.11 | 1.270 | 9.9 |
| -5 | 7.94 | 1/2-20 UNF | 12.70 | 1.270 | 11.4 |
| -6 | 9.53 | 9/16-18 UNF | 14.29 | 1.411 | 13.0 |
| -8 | 12.70 | 3/4-16 UNF | 19.05 | 1.587 | 17.5 |
| -10 | 15.88 | 7/8-14 UNF | 22.23 | 1.814 | 20.6 |
| -12 | 19.05 | 1-1/16-12 UN | 26.99 | 2.117 | 24.9 |
| -14 | 22.23 | 1-3/16-12 UN | 30.16 | 2.117 | 27.9 |
| -16 | 25.40 | 1-5/16-12 UN | 33.34 | 2.117 | 31.2 |
| -20 | 31.75 | 1-5/8-12 UN | 41.28 | 2.117 | 39.1 |
| -24 | 38.10 | 1-7/8-12 UN | 47.63 | 2.117 | 45.5 |
| -32 | 50.80 | 2-1/2-12 UN | 63.50 | 2.117 | 61.5 |
The inch version of the same data, with TPI and fractional sizes, is in the main JIC fitting size chart.
Quick inch-to-mm conversions for JIC
- Tube OD in mm = dash number ÷ 16 × 25.4. Example: -8 = 8 ÷ 16 × 25.4 = 12.70 mm.
- Thread pitch in mm = 25.4 ÷ TPI. Example: 16 TPI = 1.588 mm.
- Thread OD in mm = fractional thread size × 25.4. Example: 9/16 in = 14.29 mm.
JIC pitches convert to 1.058 mm (24 TPI), 1.270 mm (20 TPI), 1.411 mm (18 TPI), 1.588 mm (16 TPI), 1.814 mm (14 TPI) and 2.117 mm (12 TPI). None of them is exactly a standard metric pitch, which is what makes a pitch gauge so useful for telling inch and metric threads apart.
JIC tube vs metric tube
JIC tube sizes are inch sizes. Several are close to metric tube sizes but are not the same:
- -4 is 6.35 mm, close to 6 mm metric tube.
- -6 is 9.53 mm, close to 10 mm metric tube.
- -8 is 12.70 mm, close to 12 mm metric tube.
- -12 is 19.05 mm, close to 18 or 20 mm metric tube.
- -16 is 25.40 mm, close to 25 mm metric tube.
A difference of a few tenths of a millimetre matters for flaring, sleeves and compression rings. Do not substitute metric tube for inch tube, or the reverse, in a JIC connection.
How to tell JIC from metric DIN fittings
The most common metric hydraulic tube fitting is the DIN 24° cone fitting, standardized as DIN 2353 and ISO 8434-1. It is very different from JIC once you know what to look for:
- Seat angle: DIN uses a 24° cone (often with a cutting ring or an O-ring in the cone). JIC uses a 37° flare.
- Tube end: DIN tube is cut square and held by a cutting ring or ferrule. JIC tube is flared to 37°.
- Threads: DIN uses metric threads such as M14x1.5, M16x1.5 or M22x1.5. JIC uses inch UNF/UN threads.
- Series: DIN parts are marked by tube OD in millimetres and a series letter, for example 12L (light) or 12S (heavy). The same tube size uses different threads in L and S series.
Other metric connections you may meet include DIN 60° cone hose ends and metric O-ring boss ports (ISO 6149). If the pitch gauge reads metric, the fitting is not JIC.
How to tell JIC from BSP fittings
BSP (British Standard Pipe) is inch-based but very different from JIC:
- Thread form: BSP uses a 55° Whitworth thread form; JIC uses the 60° UN form. A thread gauge for the wrong form will not seat cleanly.
- Parallel or tapered: BSPP (parallel, also marked G) threads are straight. BSPT (tapered) threads narrow toward the end, like NPT.
- Seal: BSPP usually seals with a bonded washer or O-ring, or with a 60° cone on hose swivels. JIC seals on a 37° flare.
- Sizes are nominal: a 1/4 BSP thread measures about 13 mm across, not 1/4 in. The size name refers to an old pipe bore, not to the thread diameter.
- Pitches: BSPP uses 28, 19, 14 and 11 TPI. 19 and 11 TPI never appear on JIC threads.
Metric and BSP look-alikes
These pairs are close enough in diameter or pitch to be confused with calipers alone. Always confirm with a pitch gauge and a seat gauge.
| JIC size | JIC thread (OD mm / pitch mm) | Look-alike | Look-alike (OD mm / pitch) | How to tell |
|---|---|---|---|---|
| -6 | 9/16-18 (14.29 / 1.411) | Metric M14x1.5 | 14.0 / 1.5 | Pitch is very close; check over the full gauge length, then check the seat (37° vs 24°). |
| -8 | 3/4-16 (19.05 / 1.588) | BSPP 1/2 (G 1/2) | 20.96 / 14 TPI | BSPP is about 2 mm larger with a coarser pitch. |
| -10 | 7/8-14 (22.23 / 1.814) | BSPP 5/8 (G 5/8) | 22.91 / 14 TPI | Same TPI; BSPP is about 0.7 mm larger and uses a 55° thread form. |
| -10 | 7/8-14 (22.23 / 1.814) | Metric M22x1.5 | 22.0 / 1.5 | Diameter is close; pitch is clearly finer on the metric thread. |
| -12 | 1-1/16-12 (26.99 / 2.117) | BSPP 3/4 (G 3/4) | 26.44 / 14 TPI | Close diameter; BSPP has 14 TPI, JIC has 12. |
| -16 | 1-5/16-12 (33.34 / 2.117) | BSPP 1 (G 1) | 33.25 / 11 TPI | Almost identical diameter; count the threads (12 vs 11 TPI). |
| -20 | 1-5/8-12 (41.28 / 2.117) | BSPP 1-1/4 (G 1-1/4) | 41.91 / 11 TPI | Count the threads (12 vs 11 TPI). |
| -24 | 1-7/8-12 (47.63 / 2.117) | BSPP 1-1/2 (G 1-1/2) | 47.80 / 11 TPI | Almost identical diameter; count the threads (12 vs 11 TPI). |
Japanese and other 30° fittings
JIS fittings (common on Japanese equipment) use a 30° seat and, in many sizes, BSP-style parallel threads. Some Japanese machines use 30° seats with metric threads. A 30° reading on the seat gauge rules out JIC even if the thread diameter seems close.
A simple identification order
- Measure the thread OD in both inches and millimetres.
- Try both the inch and the metric pitch gauges and use the one that fits along the full leaf.
- Check whether the thread is parallel or tapered.
- Check the seat angle: 37° (JIC/AN), 45° (SAE flare), 24° (DIN/ISO), 30° (JIS or BSP cone).
- Match the results to the right chart.
Step-by-step measuring technique is covered in how to measure JIC fittings.
References
JIC values are cross-checked against published SAE J514 thread charts from hydraulic manufacturers and distributors. BSPP diameters follow ISO 228-1 basic sizes, and DIN descriptions follow published DIN 2353 / ISO 8434-1 guides. Always verify against the fitting manufacturer's specifications.
Frequently asked questions
Is JIC metric or imperial?
JIC is an inch (imperial) system. Tube sizes are fractions of an inch and threads are UN/UNF inch threads. Metric values are conversions.
What is a -8 JIC in mm?
A -8 JIC fitting is for 12.70 mm (1/2 in) tube and has a 3/4-16 UNF thread, which measures 19.05 mm with a 1.588 mm pitch.
Can JIC connect to a metric DIN fitting?
Not directly. DIN fittings use metric threads and a 24° cone, while JIC uses inch threads and a 37° flare. Use an adapter made for both.
Is BSP the same as JIC?
No. BSP uses a 55° thread form, different pitches and different seals. Some sizes have similar diameters, so check the pitch and seat.
What thread pitch is JIC in mm?
JIC pitches convert to 1.058 mm (24 TPI), 1.270 mm (20 TPI), 1.411 mm (18 TPI), 1.588 mm (16 TPI), 1.814 mm (14 TPI) and 2.117 mm (12 TPI).