QGIS » QGIS Application

Using the field calculator from the attribute table, the $area of each feature is shown in the table as float (whether the result asked for is integer or decimal). If the size of the target field is less than 10 decimal places plus decimal point plus the actual maximum number of digits plus at least one to spare, the values are replaced by NULL on save.

The best resolution would be to use internal working space for the float rather than the shape file attribute table space so valid results would be saved as expected i.e. 6 digit results would fit into a 6 width integer field.

Another option might be to not allow the field width to be decreased below the minimum required for results to be saved.