serve 40? I usually go by the Wilton party chart...but a customer wants a 3-tiered, square shaped cakes to serve 40 & I can't seem to come up with a close enough combination

4" x 6" x 8" squares... your party chart shows 32 servings for the 6" & 8", add a 4" for the additional 8 servings that are 1" x 2" (x 4" height).

How am I going to get 8 servings out of a 4" cake? It's still early in the morning (& I am not a morning person!)...but wouldn't the cake servings have to be 1/2" by 2"? I don't think I can get 8 servings out of a 4" square.

Think of how you would divide up the square - 2 Colima, 4 rows = 8 slices. Although small cakes are a 'challenge' to decorate - rounds are bad enough! So maybe better to encourage them towards something with a bigger top tier.
xx

They may just have to purchase more cake than they need to get the look that they want. Sometimes it just works out that way.

The 8 inch serves 32, the 6 inch serves 18 and let's just say the 4 inch serves 4- that is 54 servings-- and not that far off from the number of servings that they want- and they get the number of tiers they are looking for.

Just make sure you charge them for 54 servings.

Will they consider a fake tier?

I have a wedding cake this weekend and the couple needs to serve 30 people but wanted a 3 tier cake. Their cake is a 10" real cake tier, 8" dummy cake tier, and a 6" real cake tier (6" will be saved for 1 yr anniversary).

I like doing fake tiers because the cake isn't as heavy to carry and support is much easier.

like another poster mentioned. they may have to just buy more cake to get look they are wanting. i personally have a minimum cake size. i will not do a cake smaller than 6" because it sucks to decorate. what would happen if they want a 3 tier cake and only feed 25 servings? same thing, it is not possible to do that small of a cake.

I would do the 4-6-8 combo. I personally love doing 4" cakes. Maybe I'm weird but the top tier of my own wedding cake was a 4" and I found it insanely easy.

Plus if you do fondant, just piece the fondant together and it will be super smooth.

Also, a 4" square is 16" in area so you can get 8 slices of the 1"x2" wedding slice out of it. (8x2=16)

I'd tell them they either need to do 2 tiers or order more cake than they need. Or do a dummy tier, but make sure to charge for that.