I've been making cakes for a few years now and I always just eyeball how large my fondant should be while rolling it out, and most of the time I'm spot on - but I have run into trouble a couple of times resulting in a panic when I realize that my fondant is too short on one side..
I know that the new mat has markings to eliminate guesswork, but I have the old mat and have no plans to upgrade. How do I calculate? My cakes are always a little taller and wider once frosted.
I can use a measuring tape, but I'm embarrassed to say that I'm not sure exactly what to do!
AMeasure across the top and then down the side. Top+side+side will give you how wide and tall your fondant should be. If rolling out for a round your fondant should be at least that width all the way around. I always add a few extra inches to have a skirt to work with.