Choosing the table base to use with your table top should be taken seriously as it can cause you nightmares if not done correctly. If you choose too small of a base, the table can be tippy and spill drinks or even worse injure someone. If you choose a base that is too large, it could make it difficult for the customer to sit comfortably, or even worse the legs can stick out and trip someone. There is a difference in base sizes used for standard table height and Bar Height. Basically, you would use 1 size bigger table base under a bar height table top, but I will note more information on that below. If you want information about cross bases, see my previous article.
Some of the more common sizes of Round Table Bases are the 18" round, 22" round and 30" round.
The 18" Round Table Base will work for a 24" x 24" top to a 30" x 30" top (also 24" to 36" Round Tops – I personally would want a larger one under a 36" round though), that would mean that the base is 6" - 12" under the top. If used on a 24" top, it will mean that you can not push the chairs all the way in, but otherwise it will be comfortable for the customer. And under a 30" top, they will have plenty of room and this base is heavy so it will be sturdy.
The 22" Round Table Base will work for a 30" X 30" to a 36" X 36" (also 30" to 42" Round Tops – once again, I would not use it under the 42" top.). That will give 8" - 14" of clearance from the base to the side of the table. These bases are bottom heavy, so they will be less tippy than cross bases.
The 30" Round Table Base will work for a 42" X 42" to a 48" X 48" (also 42" to 72" Round). That will give 12" or more of clearance from the disc base to the side of the table. You can can use smaller bases under round tops as there is no corners that stick out for people to lean on and tip the table.
For rectangular tops that do not fit in the formula above, you can always use 2 round bases with one on each end of the table tops to be sure that they are stable.
For Bar Height Table Bases, as I noted above, you will basically want to go up 1 size of base if possible. For example, you can use an 18" Round Table Base with a 30x30 top, but if you go bar height, you would probably want to move to a 22" Round Table Base. There is still clearance under it and it will be much sturdier.
The choice of what base to use can be a difficult decision in some cases, but others are simple. Make sure to do your research as to what size of base that you want. If you do not have any experience in this, sneak a tape measure into a restaurant and measure their tops and bases and make the decision for yourself if the top is too tippy or the legs stick out too far.