I am planning to make my nieces wedding cake in Novemeber. She wants a four tier wedding cake. They will be 18, 14, 10.(must feed 240 people) but not sure how high they should be. Is 6" to high for each tier....Please help.

if you're doing rounds, 4" high is enough to yield 243 servings (8 cubic inch servings).

http://shinymetalobjects.net/cake/calculator/cake_calculator_byVolume.cgi

Your serving slice of cake is: 1" x 2" x 4", which is 8 cubic inches.

10" round:  5" x 5" x π x 4"   = 314 cubic inches
14" round:  7" x 7" x π x 4"   = 615.44 cubic inches
18" round:  9" x 9" x π x 4"   = 1017.36 cubic inches

total volume: 1946.8 cubic inches
1946.8 / 8 = 243.35 total servings.

Thanks Metria that is a lot of help. I am doing rounds and they want to serve the cake for desert, so I want to make sure there is enough and I didn't want the cake to look dumpy (short). I always see these big beautiful cakes posted on CC but never knew how high each tier is. Thanks again

If you make your tiers 6", they might not fit on a standard dessert plate. I think people go as high as 5", and sometimes they do a double-barrel or double-tier, same size rounds, stacked on top of each other (8"). But those are really two-tiers in one, so you still get a 4" high piece of cake.

Here's another thread for you.

https://cakecentral.com/modules.php?name=Forums&file=viewtopic&p=6316686&sid=7ea34067468f9ea8c39202be1bc4ba01

Thanks, metria, for the detailed math! I love that stuff.