An item of furniture is rarely used by only one individual, most furniture must accommodate the variations of a wide range of end users. The "average" person is a mythical creature. When measurements are taken from a target population for a particular design, a mid-point (termed the 50th "percentile") divides users into two groups - one above and one below the "average."
One must consider human movement when measuring interior design space. People want to move around comfortably. Leave room for normal activities in your plan. Diners want to move back their chairs to get in and out without banging the hutch behind them. From a couch, you should be able to reach the coffee table without standing.
Measure furniture and other objects. Leave an extra inch or two. Experiment with placing. Make an arrangement that harmonizes attractiveness, ease and space. The room should not feel empty or cluttered. Art, pictures and mirrors should fall directly at eye level. To hang evenly, mark the place with a pencil or finger. Use tape, pencil or a handy chalk string to mark where the nails or screws should go. Use calculations for unstructured decoration. You do not want to hang 3-foot curtains on a 6-foot window or put a bookshelf over a heating vent. Calculations can show you where you need more storage and the best angle for your bed. They help to determine where to add new features and discover solutions like pot hanging racks.