How it works
Every unit is defined as a number of grams, so converting between any two units goes through grams as a common bridge. The imperial units are tied to metric via the internationally defined exact pound: precisely 453.59237 grams, with the ounce and stone defined from it.
Mass vs. weight
Strictly, these units measure mass, not weight (a distinction that only matters outside Earth’s gravity) — but “weight converter” is the everyday term for exactly this kind of conversion, so that is what this tool is called.
Common uses
Converting a recipe between metric and imperial, checking a shipping weight limit, or converting a body weight reading from a scale that uses a different unit than you think in.