QGIS Buffer Wrong Size on EPSG:4326 — Euclidean vs Geodesic Fix and Projection Guide
TL;DR: If your QGIS buffer is the wrong size, your CRS is EPSG:4326 (degrees) — switch to a projected CRS (e.g. a local UTM zone) before buffering, or enable the Geodesic Distance option in the QGIS 3.28+ Buffer dialog. The QGIS Buffer tool has one requirement that is easy to miss: the distance you enter is in the unit of the layer’s CRS, not the unit of your project CRS, and not any intuitive real-world unit you might have in mind. ...