Use this chart: Fondant – Amounts Needed to Cover a Cake to determine how much rolled fondant is needed when rolled about 3/8- to 1/4-inch thick. Amounts listed do not include decorations. How to Cover a Cake with Fondant
SARAH SAYS: Check out our Rolled Fondant Recipe Tutorial in both vanilla and chocolate.
See also: Amounts Needed for Frosting or Icing a Cake

Cake Size: Size – Inches Use this Rolled Fondant Amount – Ounces (by weight)
Rounds
4 in. high
6″ 18 oz.
8″ 24 oz.
10″ 36 oz.
12″ 48 oz.
14″ 72 oz.
16″ 108 oz.
18″ 140 oz.
Rounds
3 in. high
6″ 14 oz.
8″ 18 oz.
10″ 24 oz.
12″ 36 oz.
14″ 48 oz.
16″ 72 oz.
18″ 108 oz.
Sheets
2 in. high
7 x 11″ 30 oz.
9 x 13″ 40 oz.
11 x 15″ 60 oz.
12 x 18″ 80 oz.
Ovals
4 in. high
7 3/4 x 5 5/8″ 24 oz.
10 3/4 x 7 7/8″ 48 oz.
13 x 9 7/8 “ 48 oz.
16 1/2 x 12 3/8″ 72 oz.
Hearts
4 in. high
6″ 18 oz.
8″ 26 oz.
9″ 32 oz.
10″ 36 oz.
12″ 48 oz.
14″ 72 oz.
15″ 72 oz.
16″ 96 oz.
Petals
4 in. high
6″ 18 oz.
9″ 30 oz.
12″ 48 oz.
15″ 72 oz.
Squares
4 in. high
6″ 24 oz.
8″ 36 oz.
10″ 48 oz.
12″ 72 oz.
14″ 96 oz.
16″ 120 oz.
Hexagons
4 in. high
6″ 18 oz.
9″ 36 oz.
12″ 48 oz.
15″ 84 oz.
Paisley
4 in. high
6 x 9″ 20 oz.
9 x 12-3/4″ 48 oz.
12 x 17″ 72 oz.
Diamond
4 in. high
10-1/4 x 7-2/5″ 24 oz.
15 x 11″ 36 oz.
19-1/4 x 14-1/4″ 60 oz.
Pillow
4 in. high
6-3/4 x 6-3/4″ 16 oz.
10 x 10″ 28 oz.
13-1/4 x 13-1/4″ 48 oz.

from wilton.com