The most simple expression nowadays is this one for
x | lon coordinate, if the layer is not already in the desired CRS:
x(transform($geometry, layer_property(@layer_name, 'crs'),'EPSG:4326'))
y(transform($geometry, layer_property(@layer_name, 'crs'),'EPSG:4326'))
y | lat coordinate. You can use it for example in QGIS field calculator or anywhere else, where expressions are accepted. Simply replace
EPSG:4326 by the EPSG-Code of the CRS you wish to display coordinates in.
EPSG:4326 is the code for WGS 84, so it will return latitude and longitude.
transform() function transforms every known CRS to the specified destination CRS (here
EPSG:4326 WGS 84). You can also type in every other known EPSG Code here instead.
layer_property() function automatically detects the source layers CRS by
'crs' as well as the current layer name by
y() return the coordinates of the current
If the layer already is in the CRS you wish to display coordinates in, the accepted answer still is the shortest expression, by using