I have a question. I have a bride who is wanting a cake for roughly 150 people and she wants every other tier to be an octagon and the others to be round. I told her she would need at least 5 tiers and the bottom one would be octagon. My problem is that I can't find a serving chart for octagon shaped cakes. What sizes would I need to make and will 5 tiers be enough? Thank you in advance.

According to a chart in a VERY old Wilton book, here's what hexagon cakes yield:

Wedding cake-size slices --
6" cake = 6 servings
9" cake = 22 servings
12" cake = 50 servings
15" cake = 66 servings

Party cake-size slices --
6" = 6 servings
9" = 12 servings
12" = 20 servings
15" = 48 servings

I don't know how the 6" can serve the same in both categories, but that's what it says. HTH!!!

Do you think then that an octagon and a hexagon yield the same amount of servings?

I'm sorry, Jamie! I somehow read your post wrong and my mind immediately went to hexagons.
So, I have no idea how to answer your question. I hope someone else helps you!

Let me see if I can explain this so it makes sense (it does in my head but may not when typed out lol). You use Wilton's wedding serving size of a 1x2x4 inch piece. Assuming that all of your tiers are the same height at 4 inches, you can ignore the 4 inch part in your calculations. So to find the number of servings in a particular size and shape, you need to determine the area of the top of the cake (ex: area of circle for round tiers, area of square for square tiers, area of octagon for octagon tiers). I'm not sure if you'd be using an octagon shaped pan or cutting them yourself from square pans (in which case I would think you should include charging for servings in the corners you have to cut off to get an octagon) but go to this link http://rechneronline.de/pi/octagon.php and it has a calculator for the area of an octagon.

Plug in the length of the diagonal (measuring either from opposite corners for the long diagonal or from flat side to flat side for the short diagonal; I'm not sure which measurement pans would use but if cutting square cakes you'd use the short diagonal) and it will give you a number for the area. Divide the area by 2 (for the 1x2 piece bc we can ignore the 4 inch height) and that gives you the number of servings. Now this number isn't going to account for oddly shaped pieces so you may want to round up a little.

You can plug in different sizes for the octagon tiers and figure out how to get the correct number of servings but you'll have to play around with the size combinations because the corners of the octagons are going to be wider. For example if you put an octagon tier that measures 10 inches from flat side to flat side on a 12 inch round tier the octagon is going to measure 11.892 inches corner to corner so the corners are going to be almost even with the edge of the 12 inch round tier it's sitting on. If you have different sized pans I'd stack different combos to see how they fit together or cut paper in different octagon and round sizes if you don't have the pans.

I hope that makes sense. If not let me know and I'll see if I can clarify better.

that does make sense, thank you. I will be cutting squares to get the correct shape... I wish I had the money for pans but I can't find any that are a reasonable price.

I made one for my son's wedding and the bottom tier was round. It was hexagon and round.

http://cakecentral.com/gallery/1866296/benjamin-and-katies-cake

Jamie, I'd cut rounds down instead. I think you'll have less wasted cake.

I'm math impaired. I would cut out octagon shapes with newspaper and draw a grid to see how many each size serves.

I was thinking that with the squares all I would need to cut off would be the corners.... that would give 8 sides

True, but a round is a closer shape to an octagon and you would lose less cake that way. Up to you

If cutting from squares you can estimate that the number of servings for the octagon is about 82% of the square you're cutting from.