For commercial painters, painting to doors is priced by measuring 'flat', each side of the door that is to be painted. So if you have a door 900mm x 2000mm and you are painting both sides, the measure will be: 2 x(0.9m x 2m)=3.8m2
Irrespective of whether the door is flush, panelled or glazed, they are all measured flat in m2.
Of course you separate your measures according to the door type, so flush doors are measured separately to panelled or glazed doors. Also, your measure for glazed doors needs to be separated according to pane sizes. Separate your measure for large panes doors, from medium or small panes doors.
All the doors on the job can be measured in this way.
You then apply the appropriate £/m2 rate which will be different according to the type of door.
Why?
Because the amount of time and materials for each door type are different, so your all-in rates need to be different.
If you're not sure what rates to use, purchase Quick Paint Rate which is a painting calculator software package created by painting estimators panddonline. Quick Paint rate gives you the power to calculate painting and wallpapering rates to any overall daily painter charge and your own material prices.