Fire Glass Calculator Let AuthenTEAK help, it is actually very simple to find out precisely how much fireplace glass product is required for your fireplace. You will need a tape measure and a calculator. You can also use the following fire pit glass calculators to give you an estimate of how much glass you will need. We recommend 2" - 4" of glass depth for most fireplaces. For rectangular and square fireplaces, use the same front width as back width. All measurements should be made in inches. You may change the glass depth from the recommended to fit your needs. For American Fireglass Lite Stones and the entire line of Lava Rock including the Tumbled Lava Rock divide the number below by 2. The same weight of lava rock fills more space than fireglass because of its density.You can also view the American Fire Glass Calculator to calculate all sizes. - Measure the opening width of your fireplace - Then measure the depth of the fireplace, measure from the front of the firebox to the back wall - Now multiply the width (inches) x depth (inches) - Multiply the total x 4 - Divide total by 30 Total number represents how many lbs. of fire glass you will need. Divide the number by 2 to calculate lbs. of lava rock you will need.