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For standard Oreo cookies, how many would be a kezayis by volume? I have heard maybe 2.5 or maybe 3.

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Too bad the great Double Stuf experiment measured only weight and not volume. (Otoh....) – msh210 Feb 28 '14 at 0:52
maybe helpful, a kezayis chart: zootorah.com/RationalistJudaism/MatzahMarorChart.pdf – josh waxman Feb 28 '14 at 1:31
1 Oreo is bigger than any olive I've ever seen. – Double AA Feb 28 '14 at 4:35
There is this book, but alas the cookie section does not have Oreos, or normal sized cookies, in the Amazon "look inside" amazon.com/Halachos-Kzayis-Yisroel-Pinchos-Bodner/dp/1583304894/… – josh waxman Feb 28 '14 at 4:42
PTIJ: How many standard olives is a kezayis? – Double AA Feb 28 '14 at 6:14

According to the site projectsbyjen.org, the dimensions of an Oreo are 1.75 inches in width, and .314 inches in height. Since there's, surprisingly, not a lot of easy to find info on Oreo dimensions, let's go with this. A k'zayis (according to berachot.org, quoting Rav Bodner's "Halachos of K'zayis") is 28.8 cm³ (the most stringent view (I think), equal to half the volume of an egg.)

Math time!
1.75 in = 4.445 cm, and .314 in = .79756 cm.
The formula for volume of a cylinder is radius² × π × height.

(4.445 ÷ 2)² = 4.93950625 cm (radius²)  
4.93950625 × π = 15.5179165474 cm² (area of cookie face)  
15.5179165474 × .79756 = 12.3764695215³ (volume of cookie)  
28.8³ ÷ 12.3764695215³ = 2.32699639828³ (k'zayis divided by volume of cookie)

So! Approximately 2.3 cookies in a k'zayis, though because of empty space under the ridge, and the grooves in the cookie's design, maybe move it a little closer to 3.

Looks like you heard right. :)

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...according to the most stringent (=largest?) view. Can you include calculations for the more mainstream views? – Double AA Feb 28 '14 at 6:00
@DoubleAA Basically, yes. Mishna Berura and "most poskim." – HodofHod Feb 28 '14 at 6:02
He only quotes them regarding bracha achrona. The question just asked about how many are a kezayis. Maybe the OP took a shevua to not eat a kezayis of oreo? How much oreo can he eat outside a sukkah? How many mint-creme oreos he can eat for karpas? – Double AA Feb 28 '14 at 6:04
Should we exclude the creme from the height? – Seymore Cohen Feb 28 '14 at 6:07
@DoubleAA Actually, I'm so stringent that I don't eat Oreos. Except special runs. :D And then I make sure to eat at least 30. Cause I'm so stringent. – HodofHod Feb 28 '14 at 6:26

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