It gets the DPI settings from scanned image to know real one inch on paper equal how many pixels on image. The DPI settings come from scanner and recorded in image file. For example, an image may be of 1,000 × 1,000 pixels. If it is labeled as 250 DPI, that is an instruction to Whoitam Image Measurement to measure it at a size of 4 × 4 inches. So, Whoitam Image Measurement can determine real one inch on scanned image by DPI. If you put real one inch of paper equal what is in your unit in real world as calibration, it will show the area or length in real world units (feet, mile, kilometer, decimal etc). So, just scan and open in Whoitam Image Measurement, because most of the image editors destroy the original DPI settings from scanner. If your image is not scanned then you have to put manual calibration and in this case see our “Measuring length and Angles in Degrees in an image using Whoitam Image Measurement” post.