First you need measure your table.
Dining Room and Kitchen Tables
You will need to measure the width and length of your table as well as the drop. The width is the shortest measurement and the length is the longer measurement. To determine the drop measurement, it is easy if you just place a sheet or large towel on your table and allow it to hang to where you like the look. Now, measure this distance from the table top down to the bottom of the fabric to get your drop measurement.
The average drop is between 8-12 inches. Remember to have your chairs in place so that you can see where the fabric will hit the seat with the chair pulled out or pushed in.
Too much fabric in your lap is a distraction and too little fabric in the drop looks like you got the wrong size. Remember that whatever drop you choose you must add this number twice to your measurements as you have to go down on both sides of the table. i.e. 36" wide x 72" long with a 10 inch drop is 36 +20 x 72 + 20. The tablecloth will be 56" x 92".
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 of left =______________
_______length + ________drop on right + __________drop of left = _____________
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
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 n eed 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.
2. Rounded corners on full length rectangular or square tablecloths keep the points from dragging on the ground.