How to calculate polygon areas and perimeters using QGIS?

Open field calculator, select the new column and type the following expression

• For area: $area
• For perimeter: $perimeter

Make sure your layer is in a projected coordinate system (not lat/long ) and in correct units. ( i.e. if your layer's projection has 'meters' as units, your area will be square meters ) Note that reprojecting layers on-the-fly does not change the units for area/perimeter calculation, so you have to first reproject using Save as ... with the correct target CRS.

You also can use fTools: Vector menu -> Geometry Tools -> Export/Add geometry columns

It updates your layer automatically by adding AREA and PERIMETER columns.