How much phone or computer storage do I actually need?

Your needs depend mostly on how much media you keep locally. For a phone, 128GB is the tight-but-workable minimum for a light user, but 256GB is the comfortable sweet spot for most people, because high-resolution photos and especially 4K video devour space fast, and a few years of memories plus apps and downloaded shows add up quickly. Heavy shooters and those who keep everything offline should consider 512GB. For a laptop, 256GB fills up faster than people expect once the operating system, apps, and files accumulate, so 512GB is the sensible baseline for most, with 1TB for those working with large files, video, photography, games, or big software.

There is a real cost tradeoff worth understanding. Manufacturers charge steep premiums for storage upgrades, so a higher tier can add a lot to the price. The alternative is cloud storage, iCloud, Google, OneDrive, Dropbox, which lets you buy a smaller device and offload photos and files to servers you access over the internet, but you pay a monthly subscription forever and depend on a connection. The math often favors buying enough local storage upfront on a phone (which you cannot usually upgrade later) while leaning on cloud for overflow and backup. On computers, especially desktops, you can often add storage later cheaply. The one thing worth doing regardless of capacity is backing up, because storage full is annoying, but data lost to a dead device is heartbreaking.