Clear the room of furniture and obstacles
Measure the internal length of the room from wall to wall
Measure the internal width of the room from wall to wall
Multiply length by width to get the area of the room
Repeat for each separate room or usable space
Add the areas of all measured spaces together
Exclude wall thickness, balconies, shafts, and common areas
Use the same unit for all measurements
Convert units if needed before adding totals
Verify measurements for irregular-shaped spaces by dividing them into smaller rectangles or triangles and summing the areas