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Hi
I am looking to buy a pair of La Sportiva Olympus mons. However I am still unsure what size to get even after trying on several different sizes. My usual size for mountaineering boots is 44.5. For the Olympus mons I was thinking about either 46.5 or 47. The 46.5 fits quite well and my heel does not move much, however I am not sure how much feet swell at altitude and how this may affect the fit? With size 47 my feet have a more room (so there is more reserve in case my feet swell), however my heel moves quite a lot.

Does anyone have any experience with how much reserve is needed when feet swell at altitude?

I don't know whether feet swell significantly more at altitude than at sea level; personally I never noticed any difference. Then again, I've never been high enough that I would need the Olympus Mons, my highest peaks are all in the Andes.

Even at sea level, my feet swell a little when walking, which is totally normal. Therefore, when shopping for new boots, it's best to walk a bit before trying them on. If, with somewhat tired feet, the boots feel good and have a snug fit, without any pressure points, I'm happy. Of course I try the boots while wearing socks similar to what I'll be wearing later on. I don't want much movement around my heel then. After that, only a couple of days in the mountains will tell me how good the boots really are.

Though you don't ask about it, I feel I should add that I also pay attention to wiggling room for my toes. I need a little space for that, otherwise I may end up with blisters between my toes (especially if it's too narrow), or my toes suffer a lot when descending (when the boots are too short).

I have a pair of Scarpa Phantom 8000 which I used successfully for Aconcagua last year but they are a size too big and I get blisters by the end of the day. I wear a size 42 and these are a size 44.
Do you think I can make up for the size with better heat molded liners like a pair of intuition liners or its not going to work and I should get a new boot.

I'm not aware of feet "swelling at altitude". There might be some/few people who experience swelling, but as a norm, there is not anything like that high up (24,000 and up). Dehydration is what contributes to swelling, but that should be handled by drinking enough liquids..

Of course, there are plenty of stories of people buying too small boots and getting frost bite because their toes were hitting the front of the boot...

So, if you buy boots just a bit bigger than your normal single mountain boots (say 1 size up) you will be most likely fine. That would also allow you to add another pair of socks in very cold conditions.

I only have experience with plastic Koflachs (Alaska three times, Andes twice) and One Sport Everests (two Himalayan 8,000m trips). But I don't think the technology is any different these days. As someone mentioned above, too big a boot is not the answer either. Just a little bigger is ok, 2-3 sizes? I don't think so.

Thank you very much for this thread in the forum, it provides very useful information for those of us who are starting in this exciting world of climbing. Thank you very much and greetings!