ImageMeter provides several different ways to enter measurements into an image:
If you want to measure dimensions in a photo, you first have to understand that this is impossible without further information. Just consider two similarly looking photos. One is showing a real house and the other one a well-made model of the house. Both photos will look exactly the same, so it is clearly impossible to get absolute measurements from the photo alone.
In order to be able to measure absolute lengths in a photo, you have to provide at least one known reference size. ImageMeter can then use this given size to compute more dimensions in the image.
There are different tools in ImageMeter to provide this reference size. Depending on how the image is taken, you should choose the tool most suitable to the task.