The rule changes with the space
A rule of thumb written for a 12 by 15 room does something else entirely in a 10 by 20 one. These are the 5 shapes worth working through, and each page ends with the list of what the shape rules out.
Under about 130 sq ft, the rules stop being independent. The rug is limited by the walking lane, the walking lane is set by the door, the door decides where the sofa goes, and the sofa decides where the lamp can stand. Change one and you change four.
Target rug 84 by 108 in, so both 8 by 10s are too big
This is the room the rules were written for, and it is also the room where this catalog’s gap is most obvious. The border arithmetic on 144 by 180 in gives a target rug of 108 by 144 in — a 9 by 12. There is no 9 by 12 here.
Target rug 108 by 144 in, and the catalog stops at 96 by 120
An open plan removes the measurement everything else was based on. There is no wall-to-wall figure for the living area because there is no wall, so the border rule has nothing to subtract from.
No wall to measure from, so the rug becomes the boundary
A long narrow room fails the rug rule on one axis and passes on the other. Take 18 in off each side of 120 in and the width target is 84 in. Take 18 in off each end of 240 in and the length target is 204 in. No rug is made at 84 by 204.
Passes the rug rule on width and fails it on length
In a rental the fixed things are fixed: the rod bracket is where it is, the outlets are where they are, and there is already a blind at the window. Sizing here is not about finding the ideal number, it is about finding which catalog size lands closest to a number somebody else chose.
The rod is already hung, so measure that and not the ceiling