Generally, solid marble countertops are between half an inch and two inches thick. They are built for durability, and must be able to support their own weight in addition to the weight of the load they carry.
Thickness
The thickness of a marble countertop depends on the size of each piece. Generally, thickness is kept in proportion with the size; larger pieces are thicker than smaller ones. The additional thickness gives the piece the rigidity and structural integrity it needs to support its own weight. For example, a marble countertop for a relatively small bathroom sink can be between 24 and 36 square inches. A countertop of this size will probably be between half an inch and three-fourths of an inch thick. On the other hand, a countertop for an entire kitchen counter will usually measure 84 square inches or more, with a thickness of an inch and three-fourths to two inches. These have to be sturdier than counters for bathroom counters, because they usually support the weight of sinks, trash compactors, and other in-counter kitchen appliances.
Weight
Logically, it follows that the larger and thicker the countertop, the heavier it is. Therefore, a countertop measuring 24 square inches with a thickness of three-fourths of an inch will probably weigh 25 to 30 pounds, depending on the grade of the marble used. On the other hand, a countertop of 96 square inches that is two inches thick will probably weigh more than 100 pounds. Installing a large piece usually requires more than three people, while smaller models can be installed without the help of a professional.
How thickness and weight are determined
Determining the dimensions of a marble countertop is a precise science. Making one that is too thin and light will probably end up with the piece breaking within a few days. On the other hand, a marble countertop that is too thick is certainly more durable, but may be too expensive for the average home.
Most marble countertop fabricators and manufacturers have skilled masons and technicians as part of their staff. These people use industry-approved formulas to determine the dimensions of a piece that is both sturdy and reasonably priced. Formulas for deriving thickness and weight vary between fabricators, but a general rule of thumb is that for every 32 square inches of countertop surface area, an additional inch of thickness must be added to maintain the structural balance.
Thickness and weight of marble tiles and sheathing
Because solid marble countertops can be very expensive, most people prefer to buy marble tiles and have their counters covered with these instead. These usually measure four by four inches square, and are about three-eighths of an inch thick. These weigh about a pound and a half each.
Individually, these tiles are much lighter than solid marble countertops, but not many projects will require just one of these. Large surfaces may require as many as 20 tiles, and that translates into more than 30 pounds of weight.
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Marble Countertops 
