walls of length x that will produce room of length y?

[Bearpecs]

I want to do what the user guide instructed and make walls before clicking to create rooms. But if I make 4 walls of 20' each, the resulting room is not 20x20 but smaller. (How much smaller depends on the thickness of the wall.) Is there a formula I can use to create walls that will give a room that's 20x20?
thanks

[hansmex]

(start wall thickness ./. 2) + wall length + (end wall thickness ./. 2)


./. = divided by

H

[UbuntuBirdy]

Or the short way (Hans explained it absolutely correct) desired wall length plus wall thickness.
For a 20x20 room your walls need to be 20 + (wall thickness) long.
Why? Because the mesurepoint is always at the center of the wall (thickness).
Choosing an even value for the wall thickness makes the math easier...

[dorin]

Maybe this mini tip could be useful.