Default imagej scale to measure the area?

I am using ROI to measure the area of multiple images by ROI>measure. I do not change the scale of the images. I wanted to know what is the default value of imagej scale? what is the unit of reported area? I need to convert the reported area to real values for example if I know the reported area is based on the pixels, then I will find how many pixels are in each micron of the real image?

Regardez: analyze-> Set Scale

@Mathew I need to do it automated in macro.