Calculating Distance

I need help! I have a picture of a map and i need the user to be able to mark a distance using a line, which ive done, then i need the program to calculate this distance in miles or km depending on the scale of the map and output this into a text box. I know i have to use the pythagoras formula, i have put that into the mouse up event and a figure is being outputted but not in miles/km, im assuming what it is is twips or pixels. Please help if you can.