How much fondant to cover a cake

You roll out far more than you put on

A fondant covering ends up around 3mm thick on the finished cake. But you cannot roll a sheet 3mm thick and lift it — it tears under its own weight before it reaches the cake, and it will not stretch over the top edge without splitting.

So you roll thicker, drape, smooth, and cut away the skirt at the base. Measured against what actually stays on the cake, the sheet you roll is about 58% more fondant. That overhead is the same at every size, which is why it disappears from most coverage charts and then reappears as a second trip to the shop.

The figures below are what you need to roll, not what ends up on the cake. They are the number that matters when you are buying.

Fondant by tier size, in packs

Why this disagrees with the chart on the box

Published coverage charts tend to sit a little below these numbers, because they assume an efficient roll with minimal waste and often a slightly thinner finish. The 8-inch figure here — 693 g — lands within a couple of percent of the widely quoted 680 g, which is reassuring: two different methods arriving at nearly the same answer.

Where the charts get optimistic is the larger sizes, where the skirt you trim away grows faster than the cake does. At 14 inches you are cutting off and rerolling a genuinely significant amount, and rerolled fondant is never quite as good.

What pushes it up

• Dark or strong colours. Kneading in colour dries the paste, and dry paste cracks, which means re-rolling and losing some.
• Sharp edges. A sharp-edged finish uses more than a soft round edge, because you are effectively covering twice at the top rim and trimming back.
• Square tiers. The corners take more than the geometry suggests — a square tier of the same width as a round one needs noticeably more, and the corners are where tears happen.
• Humidity. In a humid kitchen fondant stays soft and stretches thin, which sounds helpful but usually means it slumps and you start again.