What do the numbers on a carabiner mean?

Pick up any climbing or work carabiner and there’s a block of small text on the spine: three numbers with kN after them, a standard reference, a letter, a CE mark and a string of digits nobody reads. The layout takes about ten seconds to decode once you know what each part is for.

The part worth slowing down for is what kind of number those three figures are, and which of the three describes the situation you’re actually in when something goes wrong. It is very often not the big one.

What are the three kN figures on a carabiner?

The three numbers are strengths measured in three specific pulling directions: major axis with the gate closed and locked, minor axis across the gate, and major axis with the gate open. DMM sets the group out in its carabiner instructions as “major axis gate closed, cross gate (minor axis, gate closed), major axis gate open,” printed beside three pictograms that show each pull (DMM, explanation of markings).

A kilonewton is a unit of force, not weight, and the numbers get large fast. CMC’s summary of the NFPA hardware requirements converts 27 kN to 6,069 lbf and 40 kN to 8,992 lbf (CMC, NFPA 1983 hardware). The major-axis figure is always the largest of the three, which is why it’s the one people quote and the one printed largest.

Are the numbers breaking strengths or safe working loads?

They are breaking strengths from a slow pull on a new sample in a test fixture. DMM’s marking key labels the trio “MBS (Minimum breaking strength)”. AustriAlpin, an Austrian maker that publishes its own explainer, describes the marking as the “breaking load … when the load is applied lengthwise (major axis), sideways (minor axis) and along the major axis when the gate is open,” and adds that the printed value is “rounded down to the next round number” (AustriAlpin, carabiner know-how). A carabiner stamped 22 kN may well have broken at 23-point-something. Same rounding logic as nominal versus actual rope diameter.

Nothing on the spine is a working load, and the gap between the two is covered in our guide to what MBS means on a rope. Worth knowing that not every stamped figure is even the same kind of number: SATRA’s summary of ANSI/ASSE Z359.12-2019 describes its connector tests as holding a load “for 1 minute without failure,” so an ANSI carabiner is being asked to survive 16 kN on the minor axis, not to break there (SATRA, ANSI Z359.12).

What does the letter next to the standard mean?

The letter is a connector class, and it decides which set of minimum strengths the carabiner had to pass. SATRA lists seven classes under EN 12275:2013 — B basic, H HMS, K klettersteig, A specific anchor, T termination, Q screwed closure and X oval (SATRA, EN 12275:2013). Class K is the one to look for on a via ferrata set.

Manufacturers do not all describe these letters the same way, and it’s worth saying so plainly. Edelrid publishes a class table that calls T a “Directional carabiner” (Edelrid, EN 12275); SATRA calls the same letter a termination connector. The BMC’s older guidance on UIAA 121 uses D for “Directional” and lists no T at all (UIAA/BMC, Standard 121). DMM’s manual lists only B, H, K, T and X; AustriAlpin lists only H, X and K, noting that “If none of these types are indicated, it is a type-B carabiner – ‘B’ for Basis.”

Shape does not settle it either. Kong’s steel oval is marked EN 12275/B rather than X (Kong, Oval Steel Straight Gate), and DMM’s Rhino carries two letters at once — “EN362:2004 B EN12275:2013 B/H” (DMM, Rhino).

The reason all of this is attributed to manufacturers rather than to the standard is that the standard is not free to read. BS EN 12275:2013 sells as a secure PDF for $294.64 at one reseller (Accuris, BS EN 12275:2013). The same wall sits in front of climbing helmet standards.

What does each standard actually require?

Each standard sets its own floor, and the floors are far apart.

Marking	Major axis, gate closed	Minor axis	Gate open
EN 12275 type B (basic)	20 kN	7 kN	7 kN
EN 12275 type H (HMS)	20 kN	7 kN	6 kN
EN 12275 type K (klettersteig)	25 kN	7 kN	8 kN
EN 12275 type X (oval)	18 kN	7 kN	5 kN
EN 12275 type Q (screwlink)	25 kN	10 kN	no requirement
ANSI/ASSE Z359.12-2019	22.2 kN (5,000 lbf)	16 kN (3,600 lbf)	16 kN, gate face and side
NFPA 1983 Technical Use	27 kN (6,069 lbf)	7 kN	7 kN
NFPA 1983 General Use	40 kN (8,992 lbf)	11 kN	11 kN

EN figures are SATRA’s; NFPA figures are CMC’s and are 3-sigma minimum breaking strengths; ANSI figures are one-minute hold tests, not breaking loads. One disagreement to flag: SATRA gives type K a 25 kN major axis, while Edelrid’s own table gives 20 kN for the same class. We have not read the standard text, so we can’t say which is current — treat 20 kN as the conservative reading.

Why is the gate-open number the one that matters?

Because a gate can be held open by the exact thing the carabiner is clipped to. Petzl states that a carabiner with the gate open has “only 30 % of the major axis strength (e.g. 7 kN instead of 27 kN on the major axis for the Am’D),” and that minor-axis loading leaves about 35 percent, 8 kN against 27 (Petzl, dangerous carabiner loading). The same page puts a carabiner loaded over an edge at 6 kN instead of 23 kN on Petzl’s Spirit SL, and one loaded on the gate side at 7 kN instead of 27 kN on the William.

The gate does not have to look open. Petzl calls the locking sleeve “the weakest part of your carabiner” and notes that European standards ask for only 1 kN of sleeve strength under external pressure, “a value easily reached in the field,” where ANSI Z359.12 asks for 16 kN. A sleeve resting against rock, a rope pressed across the gate, or a nose caught on a bolt hanger all produce the open case. Black Diamond tested that last one: a carabiner rated 20 kN closed failed at “less than 2 kN (500lbf or 227 kg)” when nose-hooked, under 10 percent of its rating (Black Diamond QC Lab, nose-hooked carabiners).

Published figures across the roster show how consistent the ratio is. Petzl’s Am’D is 27 / 8 / 7 kN (Petzl, Am’D), DMM’s Rhino 27 / 9 / 7, Edelrid’s HMS Strike Twist 22 / 8 / 7 (Edelrid, HMS Strike Twist), Kong’s steel oval 24 / 8 / 8. Gate-open strength lands somewhere between a quarter and two-fifths of the major axis in every case. There are exceptions worth spotting: Kong’s Ferrata Express Sleeve publishes 30 / 10 / 12 kN, so its lowest number is the minor axis, not the gate (Kong, Ferrata Express Sleeve).

Does any stamped number cover a cross-loaded carabiner?

No — each of the three is a single pull along one line, so a carabiner loaded in two or three directions at once is outside all of them. Black Diamond pulled a VaporLock with a 21 kN CE rating: a clean sling baseline came in at 21.8 kN, and the same setup with the sling forced to the far side of the basket broke at 10.63 kN. Quad-axis loading cost roughly 25 percent. The lab’s own summary reads: “Is the VaporLock weaker away from the spine? Yes—in this case about half as strong” (Black Diamond QC Lab, off-axis loading). Petzl puts cantilever loading at “less than 30 % of the major axis strength.”

What else is stamped on a carabiner?

Everything that isn’t a kN figure is traceability or scope. DMM’s key decodes the rest: manufacturer name and country of origin, “CExxxx” for the notified body, “YRDAYXXXX#” for year and day of manufacture plus an individual serial number, a book pictogram meaning read the instructions, an RFID reference, the standard and class, and a product code.

In North America the marking changed recently. Roco Rescue reports that NFPA grouped 1983 with 1858 and 1670 into NFPA 2500, so gear now reads “NFPA 2500 (1983) 2022 ED” instead of “NFPA 1983 (2017 ED),” followed by a G, T or E for General, Technical or Escape use; manufacturers stopped selling 2017-edition gear in spring 2023 (Roco Rescue, new NFPA markings). One connector often carries several: Petzl lists the Am’D as “CE EN 12275, UIAA, CE EN 362, EAC, NFPA 2500 Technical Use.”

The short version

The three kN figures are breaking strengths from three separate lab pulls — major axis closed, minor axis, major axis open — and none of them is a working load. The major-axis number is the one printed largest and the one least likely to describe a real accident, because gate-open strength runs roughly a quarter to two-fifths of it and a gate is easy to hold open with a bolt hanger, a rope or a sleeve that never fully closed. The letter beside EN 12275 tells you which class the carabiner was tested as, though manufacturers describe those letters inconsistently and the standard itself costs several hundred dollars to read. A carabiner loaded across two or three axes at once is outside all three certified cases, and tested strengths in that state fall to about half, or far less if the nose is hooked. Read the smallest number on the spine as the one that matters, and rig so it never gets a turn.