In the length() function QGIS uses the CRS unit of the layer to calculate the 
result. When I use for example a layer in a local CRS like EPSG:31370 or 
EPSG:5439 and a layer in EPSG:3857 (that all use meters as unit) the results 
are very different (+40% difference). Changing the project CRS makes no 
difference. When I check the length using the Measure Tool the results are very 
similar to the local CRS.

How can I measure the length of a line and use it as a label independent from 
the layer CRS and get results similar to the results of the Measure Tool?

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