A rug that is too small is the most common reason a living room feels slightly off. The furniture floats in one place, the rug floats in another, and nothing seems connected. A rug is meant to tie the seating together, so the first rule is simple: it should be big enough for the furniture to sit on it, not just next to it.
The good news is that this needs no taste, only a tape measure. Below is a small planner. Put in your own room and sofa, then drag the sliders and watch the checks change.
These are common interior-design guidelines rather than laws, but they work for most rooms.
Rugs are made in a handful of common sizes, which is why the planner rounds to them: roughly 120 × 170, 140 × 200, 160 × 230, 200 × 290 (or 300), and 240 × 340 cm. If the size you want falls between two, go for the larger one when it still leaves the bare floor border, and the smaller one when it does not.
If the room is tiny, a rug that only holds the coffee table and the front sofa legs is better than one that fights the walls. In an open-plan space, a rug can mark out the sitting zone on its own, so let it be smaller than the room allows and keep the zone clear. Round tables and odd layouts want a rug shaped to match, not a rectangle forced in.
Renting? A rug is one of the cheapest ways to make a rental feel yours, and a non-slip pad underneath keeps corners from curling and the rug from sliding on smooth floors. Already own a rug that is a bit small? Try it under just the front legs first, or move it to a spot where it works, such as a reading corner. Cosy does not have to mean buying new.
A cheap trick: lay painter's tape or old sheets on the floor in the size you are considering, and live with it for a day. If it looks too small, it is.