I'm trying to figure out what size cake my friend needs. She wants a 3-tier round cake and I think she'll be having about 30 people or so.

Yes, that will be enough.

According to the portion guide I have (which is based on a 2" x 1" piece of cake), an 8" round will give 20 portions, a 6" round will do 10, and a 4" round will do about 4 or 5.

HTH

Suzanne x

Thank you!!

4" serves 6
6" serves 12
8" serves 24

Total = 42 servings

Make sure you charge for the actual amount of portions in the cake, not just the amount they 'need'.