Two numbers decide a dining table: how much width each person needs at the edge, and how much floor the table needs around it. Both are well established, and the second one is the one that turns a good table into a room nobody can walk through.
Twenty-four inches per place
The standard allowance is 24 inches of table edge per person. That is enough for a place setting and enough that neighbouring diners are not touching elbows.
22 inches works for a family table, especially with children. 30 inches is what a restaurant allows for a comfortable dinner. Below about 20 the setting itself stops fitting — a dinner plate is 10 to 11 inches and needs glass and cutlery beside it.
These are edge lengths, not areas. A long narrow table seats more people than a square one of the same area, because seating happens at the perimeter.
Ends seat someone only if the table is wide enough
A rectangular table seats people at its ends only when the end is at least one place setting wide. At 36 inches wide that works; at 30 it is cramped and at 28 it does not.
This is also why a long narrow table of 30 inches is efficient per foot of length but caps out at two rows, while going to 40 inches adds two more seats and makes passing dishes across possible.
Width also sets whether two people opposite each other can both have a full setting without the plates meeting in the middle. 36 inches is the usual minimum for that; 42 is comfortable and leaves a centre strip for serving dishes.
Round tables seat more for their footprint, up to a point
A round table's seating is its circumference divided by the place width, so a 60 inch round is 188 inches of edge — nearly eight seats, against six for a 60 by 36 rectangle.
Round tables also have no bad seats and no corner to sit at, which is why they are preferred for conversation.
The limit is reach. Past about 60 inches diameter, nobody can reach the middle, so the centre becomes decoration rather than table. Larger round tables need a lazy Susan to function.
The room is the real constraint
A chair pulled out from the table needs about 24 inches. Someone walking behind that pulled-out chair needs about 36. So the clearance from table edge to wall or to the next piece of furniture wants to be 36 inches minimum, and 42 to 48 if it is also a route through the room.
That means a 72 by 36 table wants a room about 12 by 9 feet at a minimum. The table looks small in the showroom and fills the dining room, every time.
The calculator reports the floor space needed using 36 inches on every side, which is the figure to check against the actual room before buying wood.
Height, apron and knees
Standard dining height is 30 inches to the top, with chair seats at 18. That gives 12 inches between seat and tabletop, which is what knees need.
The apron under the top is what eats that clearance. An apron that hangs 4 inches below a 30 inch top leaves 26 to the underside, and a chair with arms will not go under it.
Counter height is 36 with 24 inch stools; bar height is 42 with 30 inch stools. The 12 inch difference between seat and top holds for all three.