This gives you the amount to completely cover an area:

Length times Width will give you the area of a surface.
Example: if the phone was 2 by 4 Inches, then it would have 8 square inches of surface.

Stone Size and Number Required to Cover
One Square Inch
5SS - 253
6SS - 205
7SS - 170
9SS - 121
10SS - 105
12SS - 91
16SS - 51
20SS - 37
30SS - 20
34SS - 16
40SS - 11
42SS - 10
48SS - 7

1 Square Inch = 645.16 Square Millimeters
How To Calculate the Area of a Circle
1. Divide the diameter by 2 to find the radius.
2. Square the radius, that is, multiply it by itself. For example, the radius is 4 mm, then the square of that would be 4 x 4 which is 16mm.
3. Multiply the answer from step 2 (radius squared) by pi (3.1416). This is the area.