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, 
> Martina 
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