Hallownest Calculator reads your Hollow Knight save file and shows what the game doesn't tell you: what's missing for 112%, item by item, where every grub, Mask Shard and Vessel Fragment you don't have yet is, on the game's own map, and how many Hunter's Journal entries are left. In Chrome and Edge it follows the file while you play, and catches up every time you rest at a bench.
It works without a save too: the charm calculator tells you how each of the Knight's stats changes with each charm (nail damage, hits per boss, soul, healing, spells), and the combat simulator takes your build against 180 enemies and bosses, Godhome included. Every number comes from the wiki, with the game's rounding, and every name from its official translation.
Free, no account and no ads. Your save never leaves your browser: it's read there and kept there.
Reading the save needs the file, and only the computer version (Windows, macOS, Linux) keeps it where you can reach it. On console you can tick your progress by hand.