I have been asked to make a 12" round Rose Swirl Cake- 4 layers of cake and 3 layers of filling, as one of the optional designs. 

I have never done a 12" cake before. I have done rose swirl cakes, it takes a lot of frosting!

Can anyone help me figure out just how much frosting would be needed so that I can figure out a price?

Details: 12" round,  2 of the 3 layers will have frosting in the filling.

I hope you can help!

If you are doing 2 layers of rose swirls on the side of the cake, they need to be 2" round - assuming your cake will be 4" which is what it sounds like.

A 12" cake has a circumference of 38". 38"/2" roses = 19 roses. Multiply by 2 layers = 38 roses.

Now you just have to make a few roses that size and get the average weight. Let's say they are 40g each. 38 roses x 40g = 1520g frosting for the roses.

I use around 400g of frosting per layer in a 12", so 2 layers would be 800g. New total is 2320g for filling and frosting.

Add another 300g for a crumb coat, and another 200g for the little stars that go between the roses, and you have 2820g, which is just over 100oz, or 6.3lb.

You may want to make a little extra, just in case.

And if you are doing roses on the top you need to work out how many fit in a 12" round, plus the little stars for in between.

My mind is spinning- this is such a great help.  Yes, roses on the top, too. 

Thank you so much! I appreciate your time to share this with me.

No problem - I was bored and had a calculator handy. Good luck!