Ok, I am doing a 12/10/8/6, covered in fondant. Using the wilton fondant mat that has the measurements on it, should I roll my fondant out to 14 or 16in round? My cake is 4 inches tall.
I am letting the b/c dry as we speak before I attempt to cover with fondant. I have only done 1 fully covered fondant cake and it was tiny compared to this one...
I'm a bit nervous.

If the cake is 12 inches across and 4 inches tall, then you need it to be at LEAST 20 inches across (and a little extra... no less than 22+, I'd say)

I think that you need it to be at least 20" across (12" for the diameter of the cake + 4" for one side + 4" for other side = 20").

Just a guess as I don't do fondant, but maybe someone else has a better way to explain it.

For a 4" tall 12" round cake you will need at least 20" round circle of Fondant. 4" up one side + 12" across the top + 4" down the other side.

I second the above. At least 22" in diameter.

Hi there. for the 12", roll to the 20" mark (so a bit bigger than the mat, because if I remember, it only goes to 18"). for the 10", go to 18", for the 8" go to the 16", for the 6", go to the 14". Here is why. Your cake measures whatever across the top, right? But then there are 4" on either side. So the total amount of surface area that needs to be covered is the diameter of the cake PLUS the height of the cake on either side.

ALSO, Wilton gives the weight of the fondant needed to roll out for each sized cake. I find that they have NEVER been wrong.

HTH

Yep... formula is usually this: up one side, over the top, down the other side with a tape measure --on your 12" cake: (twelve plus eight) for a total of (20)... anything past that is insurance that you have full coverage with plenty left to smooth and trim.

16" would be enough for the 6" tier