You found a pattern you love, enlarged a pattern, or drew your own design and now you need to buy glass. Before you rush out to buy or order your glass, there are a few things you need to consider to calculate how much glass to purchase.
Glass grain usually runs vertically from top to bottom in a sheet of glass. The grain of glass varies in its visibility. It is important to take the time to determine the grain of the glass and mark the direction of the grain by drawing arrows on the glass. The arrows will help you position the pattern pieces on the glass to insure proper flow of the grain with your finished project. The rule of thumb is to run grain with the longest direction of your window. If your panel is 30 inches tall and 15 inches wide, run the grain vertically. If your panel is 15 inches tall and 30 inches wide, run the grain horizontally. Within nature, floral and animal scenes, run the grain that imitates nature, flowers, and animals
Glass design pattern is found on glass that has a distinct swirl or color striation. You need to determine how you want the striation or swirl to appear in your finished design and mark pattern pieces accordingly.
Location of glass needs to be taken into consideration if you are using a swirl or color striation glass for the background. The background should look like it is continuous within your design, This will require you to place pattern pieces on glass to visually see where the background pieces will fall. This is necessary to make sure the swirls or striation will flow rather that appear haphazardly.
Shape of pattern pieces will play a major role in how much glass you will need to purchase. Pattern pieces that are curved, circular or irregular will require more glass than pieces that are rectangular or square.
To ballpark glass needs, figure out the square inches of your finished design then add an additional 35% to 50%. For a more accurate calculation of how much glass to purchase, cut out all pattern pieces and label each piece with its color and grain direction. Then sort pattern pieces according to the glass you will use. Place all the pattern pieces that you have for say blue glass and lay them out on a sheet of paper that mimics the glass sheet 12 x12. Be sure you are placing them in a manner that will allow you to cut the glass. Do this for each different glass you will need. This will give you an idea of how many sheets of each type of glass you will need to buy. Because not everything goes perfectly, add 25% more for a bad break or other mishap. It is better to have extra glass then to find out you need more and that glass is no longer available or you have to stop working and try to find that glass again or wait for it to be ordered.