A dining table is really a row of places. Each person needs a strip of the edge to rest their elbows and plates on, and the usual guideline is about 60 cm of table edge per person. You can squeeze down to roughly 50 cm if everyone is friendly, and you will want 65–70 cm if the chairs are wide or have arms.
That gives you a quick way to size a rectangular table. A table 120 cm long fits two people along each long side, and a 180 cm table fits three, with room for someone at each end if you want it. For depth, 80–90 cm is the usual range for a six-seater: wide enough for dishes in the middle, narrow enough to pass the salt.
Round tables work a little differently. Chairs fan out around the curve, so a table of 110–120 cm across seats four comfortably, and each extra person needs a noticeably bigger circle. In the planner below we count about 75 cm of edge per person on a round table, which is our own rule of thumb rather than a standard.
Put in your room's measurements and a table size, then drag the table around. The pale green zone is the 90 cm of space that chairs and people need on every side. The seats are drawn pulled out, the way they are when someone is sitting down or standing up.
A chair has to be pulled back before anyone can sit down, and then it has to go somewhere. To get in and out without a shuffle you want about 90 cm between the table edge and the nearest wall or furniture. If people need to walk behind someone who is seated, aim for 100–120 cm. In a really compact room, 75 cm will work, but daily use starts to feel cramped.
The useful part is that this only matters on sides where someone sits. Push one long side against the wall and use a bench on the other, or tuck the table into a corner and seat people on two sides. You lose a few places but gain a lot of floor, which is often the cosiest trade in a small room.
Round tables and tables with a pedestal base are kind to tight spaces, because there are no corners to bump and no legs in the way of the chairs. A drop-leaf or extending table lets you live with a small table on ordinary days and open it for guests.
Most dining tables are about 75–76 cm high and most chair seats sit at about 43–48 cm. That leaves a gap of roughly 28 cm between seat and tabletop, enough for your legs and for your arms to rest naturally. If you mix an old table with newer chairs, or the other way round, check that gap. Thick table aprons and chunky arms can stop chairs from sliding in.
You can test a table size for free. Cut a sheet of paper or lay masking tape on the floor in the exact length and width, then add a line 90 cm out from each seated side. Pull out a real chair and sit down. Does the door still open? Does the radiator or the sideboard get in the way? Walk around it with a tray, as if you were serving dinner.
Measure the doorways, hallway turns and stairs it must travel through, and the width of your window if the table will sit near curtains. Measure the height of any ceiling light if it hangs over the table, and ask an electrician if you want to move a ceiling fitting, since that is mains wiring.
And remember that cosy does not mean a bigger table. A table you already own, a second-hand one, or a smaller table with an extension leaf often leaves more room for what actually makes a dinner feel warm: a lamp at the right height, a few candles used safely, and people sitting close.