If you're thinking of adding glass shelves to your family room's built-in bookcases or your dining room's china hutch, you'll need to know how thick the glass must be to bear the weight you plan to put on it, to prevent breaking. Fortunately, glass strength is well established, so finding out how thick your shelving needs to be is as simple as taking a few measurements and looking up the capacity in a chart.
Measure the width you need the glass to be by measuring the distance between the supports on either side. If you have a bookcase or something similar, the width of the opening of the case is what you want. It may be a span of 3 feet, for example. The shorter the span, the more weight the shelf can carry, or the thinner glass can be to bear the same weight.
Measure the depth of the glass you need. This will be constrained by the depth of your bookshelf, but a typical one for a shelf is 8 inches.
Find the square footage of your shelf by multiplying the width times the depth, in feet. For example, 3 feet times 2/3 of a foot (8 inches) is 2 square feet.
Weigh on a scale the contents you plan to put on the shelf. For example, you may want to place several decorative objects on the shelf, totaling 10 lbs.
Divide the weight from the previous step, by the total square footage you calculated in Step 3. For example, 10 pounds divided by 2 square feet is 5 pounds per square foot.
Click on the "Permissible Loads for Glass Shelves" in the references section, and refer to the chart in the document. First, at the top of the chart find the distance in feet—1 through 5—between supports that corresponds to your shelf's span. If your span between supports falls between these—18 inches, for example—then round up to the next higher number of feet—or 2 feet. Then, trace down that column until you reach a number that's greater than or equal to the pounds per square feet of load you calculated in Step 5. For example, if your shelf is 3 feet wide and needs to carry 5 pounds per square foot, start at the column with "3" at the top, and trace down until you hit the second row, which reads "5". Look to the left margin to see that your glass shelf needs to be 1/4 inch thick.