The measurement tool is used to measure distances and areas in a document. In order to convert the pixels of a document into another unit a resolution of 300 dpi is used. On working in a picture the resolution of the background layer is used which can be modified by using "Layer/Layer Properties" (see chapter 7.5.48).
By clicking and dragging the following information are displayed:
All values are shown using the unit chosen in the Tool Settings.
In order to calculate an area, there has to be a lasso are a vector layer. If you want to measure the area of a vector layer, ist has to be the active layer. By clicking on Calc Area the area of the lasso or vector layer is calculated.
Keep in mind, that all measurements are only approximations.