Long story short, the latest installment in the "I'm a sucker" saga is I am making a wedding cake for my sister.

Obviously, it's for a very small group. My plan is a 6-inch red velvet tier, filled with cream cheese frosting, an 8-inch white almond sour cream tier filled with raspberry jam, and a 10-inch dummy tier. Cakes will be covered with American buttercream and the plan for decorations is a buttercream flower cascade. Maybe a pulled sugar topper, maybe just a cluster of buttercream flowers. Depends on how much time I have and how ambitious I'm feeling.

I'm wondering how much buttercream frosting all of these will require. I have supplies to make two batches. But after looking at how far the batch I made today for cupcakes went, I'm not sure whether that will be enough. For one batch, I use 1 lb butter, 2 lbs confectioner's sugar, vanilla extract, and then add cream to adjust the consistency until I'm happy with it.

Would any of you wonderful, more experienced people be able to give me a rough idea of how much frosting it will take to cover that much cake?

I would make at least double that amount.  I always make a big batch of frosting because it inevitably requires more than I anticipated.  If you are not planning to use any of the buttercream between the layers, however, just frosting the 6" and 8" tiers and making flowers, 1-1/2 batches might be enough.

You can freeze any left over buttercream for later use.

Thanks Sandra. I'm planning to also cover the dummy tier in the same frosting. I figured if I tried to use a shortening frosting or something on that tier to cut costs, it would be difficult to match the shades of white. I'll probably make two full batches, and buy some extra butter and sugar to have on hand just in case I run out. Freezing the extra is a good idea. It would be nice to have on hand if I decide to make spur of the moment cupcakes or something.

I'm incredibly bad at estimating. I always either run out of frosting or have way too much. I have yet to find a happy medium for any size cake.

Thanks Sandra. I'm planning to also cover the dummy tier in the same frosting. I figured if I tried to use a shortening frosting or something on that tier to cut costs, it would be difficult to match the shades of white. I'll probably make two full batches, and buy some extra butter and sugar to have on hand just in case I run out. Freezing the extra is a good idea. It would be nice to have on hand if I decide to make spur of the moment cupcakes or something.

I'm incredibly bad at estimating. I always either run out of frosting or have way too much. I have yet to find a happy medium for any size cake.

I am the same at estimating the exact amount.  That's why I always just make a big batch to start with.  It freezes perfectly and it makes me so happy when I need to frost a cake and already have the frosting made, lol.  I did forget about the big dummy tier!  Definitely you will need at least a double batch, maybe more.

 to estimate how much icing needed is to figure one batch using 2#s of sugar per cake mix used.   I  your case figure the dummy cake the same as if you baked it. So you’d use 2 for a 10” tier; & another 2 for the 8&6 together if you used the WASC recipe:)   Sooo that adds up to 4 batches , but I think you can get by w/3   As Sandra said you can always fz any leftover        BYW:  that would be enough to fill, frost, & decorate  

Wow, 4 batches? Definitely more than I anticipated needing! Guess I'll need to buy some more butter and sugar. Even if there's a chance I can get away with 3, I'll probably just make 4 batches. I hate the feeling of running out of frosting after I've put away the mixer and gotten started decorating. It probably also pays to have extra so I can mix generous amounts of each color for the flowers

Yep that’s the rule of thumb I have always used.   Was taught that way back when I started decorating in the 1980s