That depends on the table. If the glass piece is being used as a table cover or a protective cover then 1/4″ thickness is sufficient. If the glass piece is being set upon a stand or a pedestal to form the actual table surface, however, then depending on the size you may choose to use a thicker glass.
In these cases if the glass table to be used is larger than 24″ in dimension or is to support heavy objects using any range of thickness larger than 1/4″ is recommended.
When it comes to choosing a thickness between 3/8″, 1/2″ or 1/4″ it is mostly a matter of preference. Obviously thicker glass has a more substantial look and feel but the weight of the glass top becomes a major consideration after a certain size. Thicker glass can become very heavy as the size increases.
3/16 – This is a good thickness for patio tables, small tables, inserts, and as glass protective table covers.
1/4 – This is a good thickness for glass tabletops and glass protective table covers.
3/8 – This is a good thickness for heavy, unsupported table tops where the glass is the only table top.
1/2 – This is the strongest, most attractive thickness available, ideal for heavy, unsupported table tops where the glass is the only table top.