Calculate volumes is an easy task on QGIS 3 with the use of SAGA GIS tools under the Processing toolbox. Raster volumes can be calculated above or below a base level and two other specific calculation, input rasters need to be in UTM. These type of calculation is useful for earth movement, reservoir design, risk analysis and other spatial analysis.
The tutorial presented on this post shows the complete procedure to calculate the raster volume above a base level on QGIS3.
Tool location: Processing > SAGA > Raster Volume
You can recreate the tutorial with the input data given in this link.