A room at a time
Measure the walls and the ceiling, take the doors and the windows out, and add what is cut off and thrown away: every calculator here does that, and says how much it added. Sheets and boards are counted whole, and a count never rounds down.
The elevation drawn beside the figures is the wall as you entered it, so a wall typed a foot too long shows before the order goes in.
Updated . How we calculate. Found a mistake? Tell us.