Guide · 7 min read
Hydraulic fitting pressure ratings, and what they actually mean
A pressure rating is not a property of a thread. It belongs to a specific part, in a specific material, assembled a specific way, and it is only meaningful next to the conditions it was derived under.
That is why this site publishes a rating only where two independent publishers give the same figure under conditions they both state, and never prints one without those conditions beside it. It is also why five of the ten standards here carry no rating at all.
Working, proof and burst
Three numbers get quoted and they are not interchangeable. Minimum burst is the pressure at which the joint is expected to fail. Proof is a test pressure the part must survive without permanent deformation or leakage. Working pressure is what the joint is rated for in service.
The SAE tables this site cites tie the three together at a 4 to 1 design factor: on the face seal table one publisher prints working, proof and minimum burst side by side as 6,000, 12,000 and 24,000 psi for the smaller sizes. Proof is twice working; minimum burst is four times it.
So a figure quoted as a burst pressure is four times the number you should actually design to. Quoting one for the other is a common and expensive mistake.
| Size | Value | Note |
|---|---|---|
| -04 ORFS working | 6,000 psi | Steel, 4 to 1 design factor against minimum burst, per SAE J1453. |
| -04 ORFS proof | 12,000 psi | Twice the working figure, from the same published table. |
| -04 ORFS minimum burst | 24,000 psi | Four times the working figure. Not a design number. |
The same size, two different ratings
An SAE O-ring boss stud comes in two forms. A straight, non-adjustable stud screws in and stops. An adjustable stud has a lock nut and a washer so an elbow or tee can be oriented, and it is rated lower - at -05 the two catalogues both give 5,000 psi straight against 4,000 psi adjustable, and the gap widens at the big end.
An SAE O-ring boss stud comes in two forms. A straight, non-adjustable stud screws in and stops. An adjustable stud has a lock nut and a washer so an elbow or tee can be oriented, and it is rated lower: at -05, in steel at the same 4 to 1 design factor against minimum burst, the two catalogues both give 5,000 psi straight against 4,000 psi adjustable, and the gap widens at the big end.
The face seal table does the same thing at -16 and -24, on the same basis: 6,000 psi straight against 5,000 adjustable, and 4,000 against 3,000. Both publishers agree on both pairs.
The weakest end sets the rating
Both pressure catalogues state the same rule: where a fitting has two different ends, or is a jump size, the lower rating of the two governs the whole part.
This matters most on adapters that carry a pipe thread. In steel, at the same 4 to 1 design factor against minimum burst, a -16 straight thread end is rated 3,000 psi and the 1 in NPTF end on the same body is rated 2,000, so the adapter is a 2,000 psi part. A circuit designed around the straight-thread number is over-pressured at the pipe end.
Metric ratings work differently again
DIN 2353 fittings are rated per series and per tube size rather than per thread, and the published figures come with conditions that change them substantially.
The source this site relies on states them explicitly: the nominal pressure is calculated for STATIC working conditions at a safety factor of 4 for a cutting-ring connection. For fittings with a metal-to-metal seal, and for banjo fittings, the safety factor drops to somewhere between 1.5 and 2.5 - which means the same tube on the same thread with a different seal arrangement is a materially different part.
Temperature moves it too. The same source puts carbon steel at minus 40 to plus 120 degrees C and notes that above 100 degrees C the maximum working pressure may need reducing.
The spread across manufacturers is wide. For the light series at 6 to 15 mm the ISO 8434-1 cutting-ring figure is 250 bar, while producers publish anywhere from 160 to 400 bar for the same sizes. Where two publishers disagreed inside that spread, this site prints both figures.
| Size | Value | Note |
|---|---|---|
| 12L metric | 250 to 315 bar | Cutting ring, carbon or stainless steel, static, safety factor 4. |
| 10S metric | 630 bar | Same M18 x 1.5 thread as the 12L, heavy series, both publishers agree. |
| 30S metric | 250 to 400 bar | The one row where the two publishers disagree. Both figures are printed. |
Where this site publishes nothing
SAE 45 degree flare, BSPP, BSPT, JIS 30 degree and Komatsu 30 degree carry no working pressure here. That is a deliberate omission rather than an oversight: no two independent publishers in the source register give the same figure for them under conditions they both state.
Two of the ten pipe sizes are missing a figure for the same reason. The two catalogues agree exactly on NPTF working pressure from 1/2 in upward and differ by a whole pipe size at 1/4 and 3/8, so neither of those two carries a number.
A generic typical figure would be easy to write and would be worse than nothing. Somebody would design to it.
Frequently asked questions
What does 4 to 1 mean?
The minimum burst pressure of the part is at least four times its rated working pressure. It is a design factor, not a safety margin you are entitled to use: pressure spikes, impulse cycling and temperature all eat into it.
Can I use the rating for a steel fitting on a brass one?
No. Every rating published here states steel, and brass and stainless parts carry different figures from the same manufacturers. Read the rating for the material you are actually fitting.
Why does my supplier quote a higher number than this site?
Probably because they are quoting their own uprated proprietary part, or a burst rather than a working figure. Both are legitimate; neither is the generic figure two independent catalogues agree on. Use the number for the part you are buying, from the people selling it.
Does the hose rating matter more than the fitting rating?
The assembly is rated at its lowest-rated component, whichever that is. A 6,000 psi fitting on a 3,000 psi hose is a 3,000 psi assembly.
Keep going
Related sizes, tools and guides
Every dimension quoted in this guide is published on the size page it links to, with the sources it was cross-checked against. Working pressures are published only where two independent sources give the same figure under conditions they both state, and are always shown with those conditions. A pressure rating belongs to a specific part in a specific material: confirm it, and the assembly torque, against the manufacturer specification for the exact part before a system is charged. See the full source register and the stated gaps.