Analysis by quantity:
If you eat cake after dinner, it is estimated that the average person can eat 3 Liang -4 Liang (about 150 grams to 200 grams), and 15 people can eat about 2,250 grams to 3,000 grams, about 5-6.6 kilograms.
Therefore, in line with the principle of economy, you can order a 5-6 kg cake.
But sometimes if you are worried about not having enough to eat, you can order a bigger one, 14 inch or 15 inch.