"The variability of the strength of glass due to limitations of the manufacturing process means a suitable safety factor must be used when calculating glass thickness. The factor commonly used is 3.8. While not a perfect guarantee, it will remove all risk bar that of damaged or very poor quality glass. The main damage that will cause failures is scratches and chips. Also a point load on the glass surface will cause it to fail. For this reason a soft packer like polystyrene is used under aquariums to stop the point loading of dirt and grit.
Also when manufacturing an aquarium, the joining compound (commonly silicone) must have a minimum thickness (0.5-1mm) to allow for irregularities along the glass edge. When glass is cut it is not flat along its edge unless it has been specially ground.
It is possible to use a lower safety factor if the glass is of excellent quality and has no internal stress. It is at the designer's risk however to lower the safety factor."
How to Maintain an Acceptable Safety Factor With Thinner Glass
If you want to use a thinner glass to keep your costs down and still maintain an acceptable Safety Factor, you can simply design your tank as if it were 2 tanks by installing a front-to-back brace across the top/center of the tank. This effectively turns a 4' long tank into 2 2' long tanks.
To demonstrate this idea, look at the 21" high, 4' wide tank line in the Calculator. It indicates that using 9mm glass will give you a Safety Factor of 2.92. However, if you use a front to back brace, structurally turning the tank into 2 2' wide tanks, it increases the Safety Factor to 4.1, which is a very satisfactory Safety Factor. If the side panels are no more than 2', they will also have a Safety Factor of 4.1.
If you want to have a flat topped tank, you can install (silicone) the brace between the front and back panels, level with the top of the tank. How wide should the brace be? 3" would be a good width, but if you are going to install a glass canopy on the tank, adjust the brace width to accommodate the canopy. If you want more strength in the center of the tank, you can use 2 narrower braces siliconed together.
Determining the Width of the Side Glass Panels
Once you have determined the thickness of the glass you will be using for the front and back panels, calculating the width of the side panels is simple. Double the width of the glass and subtract that number from the front to back measurement of the bottom panel. Example: If the front-to-back measurement of the bottom panel is 18" and you are using 1/4" glass, your side panel width would be 17 1/2".
