Halacha requires reciting a "Beracha Aharona" after eating the quantity of a "Ke'zayit" of food. This quantity is determined by volume, rather than by weight. Regardless of a food's weight, its consumption requires one to recite a "Beracha Aharona" only if it is large enough to displace 1 fluid ounce of water. This volume is approximately the amount of food that can fill a cube measuring 1.2 cubic inches.
Therefore, one cannot determine the amount of a given food requiring a "Beracha Aharona" by calculating its weight in grams; since the determining factor is volume, the status of any given amount of food depends on the volume of water it displaces.
We list here a number of common snack foods and precisely how much of the food one must eat to become obligated in the recitation of a "Beracha Aharona." These amounts are taken from Rabbi Yisroel Pinchos Bodner's work "Halachos of K'zayis" which is a very useful resource of information concerning this topic:
1) Animal crackers: Seven animal crackers constitute a Ke'zayit, despite the fact that this amount of animal crackers weighs only 14 grams.
2) Butter cookies (standard size): Five cookies constitute a Ke'zayikt despite the fact that they weigh a total of only 15 grams.
3) Large "black and white" cookies: 1/6th of a cookie constitutes a Ke'zayit.
4) Sandwich cookies: 2.5 cookies constitute a Ke'zayit.
5) Standard thin wafers: Two wafers constitute a Ke'zayit.
6) Tea biscuits: Three biscuits constitute a Ke'zayit.
7) Candy corn: 26 pieces constitute a Ke'zayit.
8) Chocolate jelly rings: 3.5 rings constitute a Ke'zayit.
9) Licorice bites: Seventeen "bites" constitute a Ke'zayit. When licorice is sold in "nibs," then thirty such nibs constitute a Ke'zayit.
10) "Snacker crackers": 4 crackers constitute a Ke'zayit.
11) "Tam Tams": Six crackers constitute a Ke'zayit.
12) Flat bread: 1.75 pieces constitute a Ke'zayit.
It should be noted that this basic principle – that the amount of Ke'zayit is determined by volume, and not by weight – affects other areas of Halacha, as well. On the first night of Pesah, for example, Halacha requires eating a Ke'zayit of Masa. Were we to measure a Ke'zayit of Masa by weight, one would be required to eat one or possibly two Masot. Since, however, we measure a Ke'zayit by volume, it is generally assumed that one fulfills his obligation by eating approximately one half of a machine-made Masa.
Likewise, if a person eats 8 Ke'zetim of a "Mezonot" food, such as cake, he must recite Birkat Ha'mazon. Since we measure a Ke'zayit by volume, rather than weight, one who eats a large danish or raisin bagel could conceivably be required to recite Birkat Ha'mazon, if it has a volume of 8 Kezaytim or more (commonly referred to as 8oz.)
