You may be calculating square degrees and not metres. It can depend on the crs
depending on the tools you are using.
> Le 11 oct. 2016 à 08:54, Martina Schäfer [via OSGeo.org]
> <ml-node+s1560n5290228...@n6.nabble.com> a écrit :
> I experienced some confusion with calculation of area using the field
> calculator in QGIS version 2.16.3. Since I'm using MapInfo Professional as
> well, I mainly use tab-files that I can open in both programmes, but
> occasionally I save as shapefile since this used to be the default in QGIS.
> When comparing files, I coincidently realized that there was a mismatch in
> calculated area for the shapefile and the tab-file for exactly the same
> polygons! I used the field calculator in the attribute table in both cases,
> but for the shapefile the resulting areas were almost doubled in area
> compared to the tab-file. Any idea why this is happening?
> I also realized similar differences when calculating area in a file where
> projection has been converted from SWEREF99TM (a Swedish national projection)
> to WGS84. There differences occurred in both the tab and shapefile compared
> to the area calculated for the same tab-file in MapInfo. Again I find this
> very confusing!
> I need to rely on the area-calculations thus I really hope someone here can
> explain to me what is happening!
> Thanks in advance,
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