I have some great resealable bags I bought a long time ago somewhere that measure 12 9/16" across and I thought these would be the same thing. While the thickness of the bag itself is fine (2mm) and it's not as fragile as some reviews here state, these bags are on the big side, measuring about 12 13/16". It doesn't sound like much, but it makes a big difference, leaving a 1/4" space above your record, allowing it to slide around, plus it's not as aesthetically pleasing.
A bigger issue, however, is the placement of the adhesive strip, which is too close to the lip of the opening. As a result, when you fold the flap over to snugly close the bag, you have no choice but to fold the other side of the opening (the side with the adhesive) as well. While that happens on lots of resealable bags, in this case, because the adhesive is close to the opening, you'll probably wind up adhering that same side instead of the flap! This is a drag, to say the least.
To be fair, these bags are pretty good for gatefold or double album sets, since they're a bit larger, and the flap issue isn't as prevalent, due to the LP cover taking up more space. I saw that another reviewer complaimed that double-LPs wouldn't fit in these. They fit just fine in the bags I got, so I guess the bag quality is variable.