Why does my fondant tear over the top edge?

Usually because the disc was too small, so it is stretching rather than draping by the time it reaches the side. Roll larger than you think: the disc has to cover the top, both sides and a margin. The other common cause is rolling too thin, below about an eighth of an inch, where fondant has very little tolerance for being pulled.

Do I need buttercream under fondant?

Yes, a thin firm coat. Fondant needs something to stick to and something to smooth over, and bare cake gives it neither: every crumb and dent shows straight through. The coat under fondant is thinner than a finish coat because the fondant is the finish, which is why the calculator reduces the buttercream figure for that option.

How far ahead can I cover a cake in fondant?

A day or two is comfortable, and fondant actually protects the cake underneath by sealing it. Keep it at cool room temperature rather than in the fridge, because condensation on fondant coming out of a cold environment makes it sticky and can dissolve the surface. That is the one thing fondant genuinely does not tolerate.

Can I reroll fondant trimmings?

Once or twice, and then it starts to fight you. Each rolling works more air and more cornflour into it, so it dries and begins to crack at the edges. Knead trimmings back together with well wrapped fresh fondant rather than on their own, and keep everything you are not working with sealed, because it skins over quickly.

Frosting and finishing

Fondant Quantity Calculator

The disc diameter and the weight of fondant a cake needs, at any rolled thickness.

Quick answer

A two layer 8 inch round cake needs a disc about 18.4 inches across rolled to an eighth of an inch, which is roughly 26 oz of fondant. The disc has to reach the cake width plus twice its height plus a margin to hold and trim, and the trimmings are real waste.

Cake size

Layers

Rolled thickness (in)

Result

25.9 oz of fondant

rolled to a 18.4 in disc at 0.125 in thick

Every figure, with what it is based on
Figure	Value	Basis
Fondant needed	25.9 oz (1.62 lb) Buy a little over: trimmings are real waste	Derived
Roll a disc	18.4 in across	Derived
Cake height	3.71 in	Derived
Rolled thickness	0.125 in Thinner tears, thicker looks heavy and tastes of nothing else	Trade convention

disc = cake width + 2 x 3.71 in height + margin, at 0.125 in and 1.35 g per cm3

The disc geometry is derived: it has to reach across the top and all the way down both sides with enough margin to hold and trim. The rolled thickness is a trade convention and the density of rolled fondant is a published figure.

The formula

disc diameter = cake width + 2 x cake height + 2 x margin

The disc geometry is the whole calculation: it must reach across the top and all the way down both sides, with enough margin left to hold the sheet and trim it. That is the cake width plus twice the cake height plus a margin on each side.

Thickness is a trade convention and an eighth of an inch is the usual figure. Roll thinner and it tears over the top edge; roll thicker and the covering looks heavy and tastes of very little else.

Buy over rather than exactly. The corners of a rolled disc are genuinely wasted, and fondant that has been rolled and rerolled several times goes dry and cracks.

Derived, published or convention?

The disc geometry is DERIVED. The density of rolled fondant, about 1.35 g per cubic centimetre, is a PUBLISHED figure. The eighth inch thickness and the margin are TRADE CONVENTION.

Fondant for a two layer cake at an eighth inch thickness
Cake	Cake height (in)	Disc to roll (in)	Fondant (oz)	Fondant (lb)
6 in round	3.71	16.4	20.6	1.29
8 in round	3.71	18.4	25.9	1.62
9 in round	3.71	19.4	28.8	1.8
10 in round	3.71	20.4	31.9	1.99
12 in round	3.71	22.4	38.4	2.4
8 x 8 in square	3.71	18.4	25.9	1.62
10 x 10 in square	3.71	20.4	31.9	1.99

Derived from cake dimensions at a published fondant density. Figures include the trim margin, which is why they read a little above the packet guidance some brands print.

Gear for these numbers

These are rendered from the verified catalog and re-ordered against your numbers once you calculate.

Expert

Satin Ice

Satin Ice Fondant for Cake Decorating, White, 5 lb

$34.99

Five pounds covers roughly three 8 inch cakes with trim to spare, and Satin Ice rolls thinner before tearing than the fondant that comes in a decorating kit.

Best for: covering several cakes, or one large tiered cake

Check price on Amazon

Amazon prices move often, so check before you buy.

Intermediate

Ateco

Ateco 1301 Fondant and Icing Smoother

$7.94

The fondant smoother from a trade supplier rather than an unbranded two pack, with a flat face and a square corner for pressing the bottom edge in.

Best for: finishing a fondant covering

Check price on Amazon

Amazon prices move often, so check before you buy.

Intermediate

OXO

OXO Good Grips Silicone Baking Mat, 11.5 x 16.5 in

$24.95

Cut to the half sheet, and the surface a rolled fondant or a macaron sheet wants, because it releases without the flour that dries fondant out.

Best for: rolling fondant and baking sheet work

Check price on Amazon

Amazon prices move often, so check before you buy.

Questions people ask

Why does my fondant tear over the top edge?: Usually because the disc was too small, so it is stretching rather than draping by the time it reaches the side. Roll larger than you think: the disc has to cover the top, both sides and a margin. The other common cause is rolling too thin, below about an eighth of an inch, where fondant has very little tolerance for being pulled.
Do I need buttercream under fondant?: Yes, a thin firm coat. Fondant needs something to stick to and something to smooth over, and bare cake gives it neither: every crumb and dent shows straight through. The coat under fondant is thinner than a finish coat because the fondant is the finish, which is why the calculator reduces the buttercream figure for that option.
How far ahead can I cover a cake in fondant?: A day or two is comfortable, and fondant actually protects the cake underneath by sealing it. Keep it at cool room temperature rather than in the fridge, because condensation on fondant coming out of a cold environment makes it sticky and can dissolve the surface. That is the one thing fondant genuinely does not tolerate.
Can I reroll fondant trimmings?: Once or twice, and then it starts to fight you. Each rolling works more air and more cornflour into it, so it dries and begins to crack at the edges. Knead trimmings back together with well wrapped fresh fondant rather than on their own, and keep everything you are not working with sealed, because it skins over quickly.

Worked out for you already

A note on what this can and cannot tell you

This calculator does arithmetic on the numbers you give it. It does not know your oven, your recipe or your altitude. Figures are labelled as derived, published or trade convention, and where no honest figure exists the calculator says so instead of producing one. Researched guidance, not professional advice.