Hi - I've been asked to make a cake to serve about 30 people - at the moment I have no further details on what's required but wa wondering what size cake could cater for that number.  i could perhaps do a tiered cake - I have variety of round cake tins, and just an 8" square.  Can anyone point me in the right direction ??  Many thanks, Helen

Helen, depending on how the server cuts the cake, you can get 8 servings from a round 8"x4" cake, or 30 servings. 

In the USA (don't know if you are in the USA or another country), the standard serving is 1" wide x 2" deep x 4" tall.  This is what most custom bakeries use as a size when quoting a price per serving.  However.... YOU, the baker, need to KNOW how to get the appropriate number of servings from a cake BEFORE you sell one.  It is harder than it looks to get the recommended number of servings on the Wilton chart.

Wilton charts:

http://www.wilton.com/wedding-cake-cutting-guide.html  

http://www.wilton.com/cms-baking-serving-guide.html  

According to the chart above, you should be able to serve 32 people with a 9" x 4" round cake.

Forgot to say, according to the chart, an 8" x 4" high square should also serve 32.

thank you for that - I'm in the UK and from a chubby family so we probably have larger servings!!

Ah Ha!  Found a long 'tutorial' that I wrote a couple of years back--hope this is helpful.  (Some of the info above is repeated--sorry 'bout that.)

HOW TO CUT A WEDDING CAKE:

It takes practice to actually GET the number of recommended servings from a cutting chart.  Customers will NOT know how to get the recommended number of servings for which they have paid. The Wilton Serving Guide is the un-official serving/price-per-serving guide for the USA.

Wilton Cake Baking and Serving Guide

http://www.wilton.com/cakes/making-cakes/baking-serving-guide.cfm

Cake decorators should learn the skills and logistics involved in cutting, plating and serving.    Practice by making cakes to cut and plate at home  [You can give the cut pieces to neighbors/family//wrap in plastic wrap and freeze for future desserts.]  Time yourself when you do this on a small round cake, a large round cake, and a square cake.  You'll be amazed at the mess and how long it takes. 

When you are practicing, pretend there are 30-100 people impatiently standing in line for a piece of cake and watching you.  Many  times, cakes are cut where they are placed, instead of being removed to a kitchen. Additionally, IF the cake is stacked, you have to allow time to unstack it, and make room for the tiers on the cutting table, and remove the dowels/support plates/pillars. 

Will YOU be cutting and plating the cake slices?    Will a professional at the venue with cake/cutting/plating experience be cutting the cake and charging for the service?    Will an aunt or cousin or mom of the bride be cutting and serving?    Depending on who will be cutting the cake, the same size cake could end up with servings for 25 to 100 people.

Here are two superb resources that can be utilized to help get the recommended number of servings per cake.  You may provide your customer with both a copy the the Indydebi instructions and a fake cake slice in the appropriate size. [HUGE thanks to both Indydebi and Schwammrs]

HOW TO CUT A WEDDING CAKE    (I use a pancake type spatula and/or gloved hands instead of the cake comb)

http://cateritsimple.blogspot.com/search/label/cake%20comb   

SAMPLE CAKE SERVING SIZES—MADE OF PAPER OR CARDSTOCK (or large sponge)

http://www.cakecentral.com/forum/t/689515/cake-serving-size-examples-made-of-paper-or-cardstock/#post_7660753

Hi Helen! For 30 servings I like to make a tiered cake using  6" & 8" rounds.  Both 2 layers so each tier is about 4" tall.