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From all the documentation I've been able to find, using $area in the field calculator should return the area in the layer's projection's units. I have a layer in UTM (the project is in the same UTM CRS, which uses meters for its units) but $area is giving me acres. I have double checked this in ArcMap, I used the Calculate Geometry feature, selected acres and arrived at the same value.

It is handy because I was planning on converting to acres myself, but I want to make sure I get consistent results down the road. I do have acres selected as the unit to be used by QGIS' measure tool. Does that selection effect the output of $area now?

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Since QGIS version 2.14, the '$area', '$length' and '$perimeter' options are calculated based on the unit settings defined under 'Project > Project Properties' (see the changelog as well as the description in the image below). enter image description here

When you change the units in 'Measure Tool', it updates the values in 'Project Properties' and hence you are getting the output in acres. So, if you consistently want the results in a particular unit, you can update that in either Measure Tool or Project Properties.

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