Measure the length and width of a rectangular table and add 24 inches to each measurement. Measure across a round table to get the diameter before adding 24 inches. The addition accounts for the drop or overhang of the tablecloth.
The average overhang, often called the drop, of a tablecloth is 12 inches on each size. Because you want 12 inches on both sides of the table, you add 24 inches to the measurements you take. You need to add 24 to both the length and width of a rectangular table. If the table is 48-inches-by-84-inches, you want a tablecloth at least 72-inches-by-108-inches.
For a square table, you only need to measure one side because all four sides are the same length. If each side is 48 inches, you want a square tablecloth at least 72-inches-by-72-inches. A circular tablecloth only has a diameter measurement, so you only need one measurement straight across the middle of the circle.
While a 12-inch drop is standard, some people prefer a longer drop for a formal look. A 30-inch drop on each side of the table reaches to the floor on standard-height tables. You also have the option of choosing a drop between 12 and 30 inches depending on the look you want. Most tablecloth packages list measurements for the range of table sizes that work with the tablecloth.