If you need to calculate the area of an odd-shaped room, simply follow these steps:
If you plan to paint the trim a different colour, calculate these areas separately. Simply multiply the length by width of each trim then add your totals together.
A skirting board area is its height multiplied by its length (which is generally the same as the perimeter of the room).
Estimating time brings in a new set of variables. In 1 hour, you're likely to cover about 7 square metres of an ordinary surface, or about 5 square metres of bare wood or plaster.
You'll probably be able to paint that 4m x 5m room with one coat in 5–6 hours. If you apply 2 coats, you must allow time for the first coat to dry. Allow 4 hours for water-based paint and overnight for oil-based paint (check the paint can label). You can safely estimate 2–3 days to paint a room and clean up afterwards.