I have been asked to make a birthday cake for a friend for this Sunday, he said 16-20 people. He wants a layer cake, one layer chocolate, one layer vanilla, with a chocolate frosting.  I have the recipes I wish to use, I just need to know what size to make the cake.  Thanks!

Someone should correct me if I’m wrong, but as I understand it (for one layer cakes):
a 7” cake serves approx. 8 persons
a 8” cake serves approx. 10 persons,
a 9” cake serves approx 12 persons.

If you are making two layers, then probably they should be 8 inches each (which will serve up to 20 persons).

If it was me I’d make a larger one than 8”. For 20 people I’d make at least a 9” cake, possibly 10”, depending on if there is going to be a meal first, or if the party is just dessert and coffee.  I always err on the side of more is better, when it comes to cake.

Thanks for you replies.  I think I am going to go with 1/4 sheet cake, 2 layers.  This is a birthday cake, not a wedding cake, so I think people want a generous slice (or even seconds or leftovers).