The chart
5×8: bedrooms (peek layout), small seating nooks, home offices. 6×9: apartment living rooms with front-legs layout, full beds. 8×10: the workhorse — most living rooms (front-legs), queen bedrooms (peek-plus), dining sets to 6 ft. 9×12: the designed look — all-legs living rooms, king bedrooms, dining sets to 8 ft. 10×14: great rooms and long tables.
Why standard sizes matter
Custom rugs cost 2–4× standard and take weeks; the standard ladder covers most rooms if the furniture plan flexes. The calculators map your room to the closest standard — often the right answer is moving the sofa six inches rather than commissioning a rug.
The two-size test
When torn between sizes, the bigger size wins more often than not — undersized rugs are the most common rug mistake by a wide margin, and the border rule (12–18" to walls) still leaves the larger rug working. The exception: hallways and dining rooms, where the subtraction rules rule.