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If you're picking a tablecloth for an occasion, consider the nature of that occasion. Depending on the event type, there should be different levels of drop from the edge of the table to the bottom of the tablecloth.  For a formal occasion, such as a wedding, you want a longer drop. Have about 15 inches from the edge of the table to the bottom of the tablecloth.  For a casual occasion, like a low-key birthday party, you do not need as long of a drop. Aim for a 6-to-8 inch drop from the edge of the table to the bottom of the tablecloth. There is a simple calculation you can use to determine the measurements of your tablecloth. To start, you'll need to know the measurements of your table.  For a rectangular or square-shaped table, use a measuring tape to measure your table's length and width.  For a round table, you'll need to measure the diameter of your table. A diameter is the straight line going through the center of a circle. If you have a rectangular table, you'll need to know your desired drop. Remember, this depends on the type of event you're planning.  To start, add twice the desired drop to both the length and width of the table. For example, say your table is 40 inches wide and 70 inches long. You want a 6-inch drop. You would add 12 to 40, to get 52. You would then add 12 to 70, to get 82.  The resulting values are the dimensions of the tablecloth you should aim to purchase. In our above example, you should look for a tablecloth that is 52 X 82 inches. Calculating the dimensions of a round table is somewhat simpler. To figure out your length, you simply add twice your desired drop by your table's diameter. Say you want a 9-inch drop and your table has a diameter of 60 inches. You would add 18 to 60, getting 78. When purchasing a tablecloth, select a 78-inch round tablecloth.
Consider the occasion. Measure your table. Calculate the proper size for a square or rectangle table. Figure out the size for a round table.