One of my clients recently asked whether QGIS has a tool for quickly calculating area breakdowns within a shape you draw yourself. It doesn’t, at least not out of the box.

So I built a plugin that does: QuickShapeStatistics

You can install it directly through the QGIS Plugin Manager (make sure “Show also experimental plugins” is enabled).
https://plugins.qgis.org/plugins/quick_shape_statistics/

More details HERE.