If you were only able to measure the floor area, (length l x width w), because the ceiling is so high, (or whatever), and if the ceiling drops an amount estimated by sight, d, in the direction of the length, you would need to replace l by sqrt (l^2 + d^2) so your newly calculated area would then be
w x sqrt (l^2 + d^2).
However, if you have access to the ceiling you should measure the sloping rectangle directly, which I think was what was meant by the earlier remark, viz. "The area of a rectangle is the same, regardless of the angle that if forms with the vertical".
Regards - Ian
AOL Health