Remember you must double the drop measurement as you have to drop down on both sides of the width and both ends of the length.

_______width + ________drop on right +__________ drop on left =______________
_______length + ________drop on top end + __________drop on bottom end = _____________
OR

______Diameter + _______drop of right + _________drop of left = _____________
TIPS:
#1. Adding a few extra inches onto the length of the tablecloth is a good idea. All fabric shrinks when it is washed and it shrinks more in the length than width. By adding the extra 3-4 inches you will be sure that you don't have a tablecloth that is too skimpy on the ends after a few washings.
#2. Most Dining Room Tables are 30" high.
Tablecloths that drop to the floor.
Rounds:
Take the diameter of your table, add 60 inches to that number and you have the correct tablecloth size to go to the floor, i.e. 48" round plus 60" = 108" round tablecloth. (The 60" represents the distance from the floor to the table top - 30 inches, and the table top to the floor on the other side - 30 inches.) Always remember that you have to go up, across and back down!
Rectangles and Squares:
Measure the width of the table and add 60 inches and measure the length of the table and add 60 inches. This is the size you need to go to the floor i.e. Table top size is 36" x 72". Add 36" wide +60 x 72" long + 60= tablecloth size of 96" x 132". I always recommend that you have rounded corners on rectangular or square tablecloths that go to floor, otherwise the points will be too long and drag on the floor.
#2. Rounded corners on full length rectangular or square tablecloths keep the points from dragging on the ground.