I am using Edna's buttercream recipe. Calls for 1 cup shortening 4 cups conf sugar, etc. Made a small batch.

I am making a 3 tier cake and am trying to find out about how many batches I am going to need to make? As I dont want to not have enough at icing time. I want to pre-make all the icing.

My cake sizes are 12, 8, and 6.

Does anyone have any idea as to how many batches of icing I need to get this cake frosted! Thanks.

Generally the rule of thumb is one batch of icing using 2# of sugar for every cake mix used.
So------ a 12" round uses 1 1/2 per layer or 3 per tier
8" - 1 1/2 per two layers 1 per tier
6" - 1 per two layers 1 per tier

Total batches of icing each using 2#s of sugar 5 - I would say 8 to 10 of Edna's recipe.

I used 6 batches on my 3 tier in my photos. It was a 14 in 10 in and 6 inch. I used Ednas as well.

I've always used my own formula to calculate this.

If the cake serves 114, I'll need at least 11 or 11.5 lbs.
If the cake serves 156, I'll need at least 15 or 15.5 lbs.
It's roughly 10% of what the cake serves.

Now, keep in mind that everyone uses different cake serving charts, and not everyone does the entire cake in buttercream from roses, borders, ruffles, garlands, stringwork, leaves, decorations in between tiers, around fountains, on boards, and cake tops, etc. Just remember, you'll need plenty of extra leftover to take on your delivery as well incase you need to make any unexpected repairs! (of course, you could get by with fewer lbs. if you're just crumb coating and covering with fondant--if I misunderstood.) Hope this helps!