Renting wins whenever the trip is shorter than the item's break-even window, and buying wins the moment you plan a second trip. For an 800-fill down jacket at $420 to buy and $12 a day to rent, break-even lands at 35 days on the hill — almost nobody reaches that in one season, so renting is the rational choice for a first expedition.
The picture flips for anything that touches your skin or your feet. Base layers, socks and boots should be bought and broken in at home: a rented boot that does not match your foot shape will end a trek faster than altitude will.
Technical hardware — crampons, ice axe, harness — is the strongest rental case of all. It is expensive, heavy to fly with, and only used for a handful of days on a summit push. Our smart gear widget defaults these to rent for exactly that reason.
Rule of thumb: buy the soft layers, the boots and the sleeping bag liner. Rent the insulation, the shelter and the hardware. Our calculator does this comparison for every item on your specific trek so you can see the split before you commit.