qgis - Field Calculator returning incorrect length and area values

I can get accurate lengths of line features and areas of polygon features using the "Measuring Tool" in a QGIS project. However, when I attempt to generate length or area data in Attribute Tables using the "Field Calculator" I get values that are outrageously incorrect (too large by a factor of 1000 or more). A surprising aspect is that the values that pop up as "Output Preview" in the Field Calculator appear to be accurate/correct values. Has anyone else encountered this ? Can anybody tell me where I am going wrong ? Thanks