This planner works from your own measurements, not from a rule of thumb we made up. Give it the room, how much bare floor you want to show around the rug, and (if you like) the furniture the rug should reach under. It lists which common rug sizes fit, and which federal flammability standard each size falls under.
Turn on JavaScript to use the planner. The arithmetic is explained below, so you can also do it by hand.
How the planner works
The largest rug that fits is the room minus your bare-floor margin twice over, once for each side. If you enter a furniture group, the smallest rug that reaches it is the group's footprint plus the overlap you chose, again on both sides. A negative overlap means the rug stops short of the furniture, which is how you would plan for "front legs on" in a living room: measure the group, then enter how many inches the back legs should sit off the rug as a negative number.
The planner tests a list of nominal sizes that US shops commonly list (2×3 up to 12×15 feet). A rug sold as an 8×10 can measure a few inches off, so measure the actual rug before you buy and re-run the numbers.
The bare-floor margin is your choice. Design guides disagree about it, and we have not found a primary source that settles it, so the planner does not pretend there is one right number.
Why the planner shows a flammability standard
Federal rules sort rugs by size. A rug with one side over 6 feet and an area over 24 square feet falls under 16 CFR Part 1630 and must pass the flammability test. A rug with no side over 6 feet and an area of 24 square feet or less falls under 16 CFR Part 1631, and if it fails the test it can still be sold with a permanent FF 2-70 warning label. So a small rug near a fireplace or a space heater is worth checking for that label. Our label checker explains the rules in full.