Designing Kitchens and Bathrooms - Granite Counter Supports

I am getting 3cm granite installed and part of this installation will be a 18in overhang that runs 76 inches. Ive been told that I need corbels or L brackets. What is the rule of thumb for the maximum spacing between supports and how much can I or should I come in from each end? Also, how much of the 18inches needs to be supported?

If I go with corbels, how much weight do they need to support. I found some that say they are good up to 250 pounds each.

I am also looking for sources for both types of supports. Any suggestions? Thanks in advance for your help.

Granite overhang of up to 12" requires no support. Anything larger requires support brackets. The length of the brackets should be roughly 3" less than the total overhang width and should be placed approximately every 3 or 4 feet. Your granite fabricator should be able to recommend what is best for your type of granite and size overhang. He may recommend installing a steel plate beneath the granite. Many prefer to use the steel plate rather than support brackets or corbels which can get in the way of knees when sitting at the bar.