All carabiners come with a kN (kiloNewton) rating, engraved into the spine. If you have carabiners without a kN rating DO NOT use them for climbing!
A kiloNewton is equal to about 225 lbs., which is a force of gravity rating, not static(stationary) weight.
Everything you use for climbing/rescue: rope, webbing, carabiners, anchors and protection is designed to absorb the force (or shock) that’s generated by a fall.
All this equipment has a certain rating of force it can withstand, and that rating is typically referred to as a kN rating.
PS: That rating doesn’t take into account wear and tear on your gear, so always check everything before use, and replace anything with excessive wear.
The greatest strength of a carabiner is in its spine, and is why kN ratings typically offer two different strength ratings. One is if the load is distributed along the spine, and another if the load somehow gets distributed across the gate(the latch that swing open/closed).
Obviously, distributing a load on the gate of the carabiner isn’t safe, and this is evident by the kN rating which will typically be 1/3 of what the spine rating is. For example, the manually locking carabiner in our photo above, is rated at kN 27 along the spine, and kN 8 to 9 across the gate. If you really look at the construction of carabiners you’ll see why they’re rated less along this axis. All that’s holding the gate to the carabiner is a pin where the spring portion of the gate is located. As you can imagine, an aluminum pin of that size can not offer a comparable load rating vs. the spine of the entire carabiner.
Don’t get me wrong, 8 to 9 kN is still almost 2000 lbs. of force that the pin can take, but wouldn’t you feel safer knowing you were protected by 27 kN (6000+ lbs.)?
When shopping for a tactical/climbing/rescue carabiner look along the spine and compare the numbers. Multiply each number by 225 to see how much weight in pounds that part of the carabiner can withstand if you were to fall while using that 'biner.
The reason you want a carabiner to withstand thousands of pounds, when you only weigh ~150lbs is because when you are falling, your weight is increasing with speed. The longer the fall, the more speed your mass picked up, thus the more you will weigh upon impact. Jump on a scale, i guarantee you will the numbers shoot up into the 300, 400, 500 pound range. See why you want a carabiner than can withstand a couple thousand pounds?