A person who ate a Ke'zayit of food is required to recite a "Beracha Aharona" after eating. This quantity is determined by volume, and not by weight; a volume of food capable of displacing 1 oz. of water constitutes a Ke'zayit. Therefore, one cannot determine whether a certain amount constitutes a Ke'zayit by weighing it; one must ascertain the volume of the food in order to determine its status vis-à-vis "Beracha Aharona."
We list here a number of common foods and the amount of each that requires one to recite a "Beracha Aharona." This list is taken from Rabbi Yisroel Pinchos Bodner's work "Halachos of K'zayis."
1) 1/6th of an apple
2) A bit less than half a banana
3) ¼ cup of blueberries
4) 3 baby carrots
5) 4 cherries
6) 30 chickpeas
7) 2.5 sections of an orange
8) 3 sections of a tangerine
9) 1/3 cup of raisins, or approximately 92 raisins
10) 1 2/3 slices of American cheese
11) ¼ cup of cheerios, or 60 cheerios
12) 1/3 cup of cornflakes, or 70 flakes
13) One-half of an egg
14) A bit more than 1 granola bar (it should be emphasized that over granola bars one recites "Bore Peri Ha'adama" before eating and "Bore Nefashot" after eating)
15) 1/3 cup of rice
16) 1/3 of a rice cake (over a rice cake one recites "Bore Peri Ha'adama" before eating and "Bore Nefashot" after eating)
17) 25 almonds
18) 12 "Dipsy Doodles," or 2/3 of a small bag
19) 8 onion rings
20) 30 peanuts
21) 1/3 cup of popcorn
22) 5 pretzel nuggets
23) ¾ of a standard "Dutch" hard pretzel
