wilton do a chart for party servings, which is bigger then their wedding serving chart. earlene's chart is more generous too, i think its on here somewhere.
xx
i agree.. at first i use eilens.. and realized that the slices were huge...
then i went back to using wiltons... every one in the industry uses wiltons.. no matter what people tell u when they call you to order...
it is basically a 1"x 2" x The height of your cake... most of our cake are 4" high or 5" high, depending on the design... trust me.. its enough cake...
I use the wilton chart to detemrine pricing. It is a nice standard piece of cake. For a kids birthday party, it's an absolutely perfect kid size piece. People hear the words "one inch" and they think "paper thin" .... it's not.
When people buy a cake for a non-wedding event, I will tell them, for example, "a 10" round will serve 25-35, depending on how you cut it. The price is $x". The price is based on 38 servings x $3.50/serving, but I don't get into that with them. Here's how many it serves ... here's the price. Period.
Here's a pic of 1x2x4 pieces of cake: http://www.cakecentral.com/modules.php?name=gallery&file=displayimage&pid=1156785
Here's how to cut cake to achieve Wilton servings: http://cateritsimple.com/_wsn/page10.html
They are welcome to cut the 10" round cake into just 4 pieces if they want to, but they are paying for the number of pieces it's DESIGNED to serve.
As a visual aid when talking to clients, I have adopted bonjovibabe's "mock" cake servings. Hers are made of wood and painted. Mine are cut from a giant yellow car washing sponge, with waterbased craft paint for icing and filling. If people think the 1x2x4 serving sounds small, show them these on dessert plates, but make sure to charge them based on the smallest one. They always want the smaller one. Works every time.
http://www.cakecentral.com/modules.php?name=gallery&file=displayimage&pid=88648
Thanks everyone for letting me tap into your knowledge. I will use the Wilton guide!! 
pi x radius squared x height divided by 8 (1x2x4") gives you the wilton wedding servings for any pan. divide by 12 for party servings.
Or email me for the full chart I already have made up.
Thanks for this KoryAK...I am just now having to refigure tier sizes because the client has changed the number of servings needed and this is coming in very handy. 
Life is so much easier when you understand math!
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