Is a single packet of crisps enough to make a brocha achrona? I understand they vary in size, but a standard small size packet (e.g. I just had a 28g packet of crisps). How does one know in general how much is a kezayis since a lot of different packets have food that vary in size and shape?

A K’zayis in contemporary terms is usually measured as the volume which would displace 27cc of water. This is the amount of food that would fit in a small match box. The bag you describe would seem to have a kezayis.

Sources:

K’zayis means that the item is the size of an olive.  The Shulchan Aruch states that this shiur is a measurement of cubic volume, and explains that volume is measured by submerging an item in water and measuring the amount of water that it displaces.

The Shulchan Aruch gives the shiur in relation to the size of an average egg. The average egg will displace slightly less then 2 fluid ounces. The Shulchan Aruch states that, in view of some rishonim, a k’zayis is equal to approximately one third of an average egg while other rishonim say the shiur k’zayis is equal to half an egg.

The Mishna Berura and most Poskim rule that with regard to brocha achrona we adopt the most stringent view, not to make a brocha achrona unless one ate an amount equal to ½ an egg.

The shiur k’zayis is defined as an item containing slightly less then one fluid once (more precisely .96 of fl. oz.) It could also be measured as 28.8 cubic centimeters.[from halachos of kezayis].

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