You would use square metres (m2) to find the area of a door.
To find the area of a book cover in metric units, the most appropriate unit to use would be centimeters. The formula for finding the area of a book cover would be: length x width.
A square nanometer.
Square meters, generally.
Square centimetres (cm2).
To find the area of the door, get its length and breadth measurements then multiply them.
You would find millimetres on a ruler using metric.
cm for a reg. hb pencil it's about 1 cm
If it's a standard book, the standard units would probably be cm2.
You would find centimeters, millimeters and meters on a metric ruler or tape measure.
One can find a metric to imperial conversion at a neighborhood library. A bookstore would be another source to find a metric to imperial conversion table.
You would use meters.