How much space you need around a dining table
Three numbers get quoted for the gap between a dining table and the wall behind it. They are not alternatives. Each one buys you something different, and knowing which one you need is most of the decision.
24 inches: a chair pulls out
This is the distance from the table edge to the back legs of a chair that someone has pushed back far enough to stand up. Below it, people have to turn sideways and shuffle out, which is the sort of thing you notice every single meal.
It is the right figure for a side that nobody walks down — a table in an alcove, or a side against a window.
36 inches: someone can walk behind
Add another foot and a person can pass behind a seated diner without asking them to shift. This is the usual target for a dining room, and the figure most kitchen designers work to for a main route through a room.
44 inches: serving space
Wide enough to carry a dish past a seated person, or for a chair with arms, which needs more room to push back than a side chair does.
Measure your own chairs instead
All three numbers assume a standard dining chair. Yours may not be one.
Sit in one of your chairs at a table. Push back until you can stand up without twisting. Measure from the table edge to the back legs. That is your number, and it is the one to type into the calculator.
Expect around 24 inches for a plain wooden side chair. Chairs with arms usually want 28 to 32, and so does anything upholstered. The difference is a whole foot across a room once you count both sides.
The number people forget
Clearance is needed on the sides that have chairs. A table pushed against a wall, with seating on three sides, needs nothing behind it at all. That one decision often turns a room that does not work into one that does.
It costs you two seats and the ability to walk right round the table. In a narrow room that is usually the trade to make.
Rectangular tables lose their corners
A rectangular table seats people along its sides, not at its corners. Count each side separately: a 72 by 36 inch table takes three places along each long side and one at each end, so six to eight people depending on how much elbow room you want. Dividing the perimeter by 24 gives you nine, and nine will not fit.
Round tables do not have this problem, which is why they suit square rooms and tight ones.