How many chickens can fit in a 8x8 coop

I have seen some ads selling 8x8 coops for up to 30 chickens. What is the rule of thumb for square feet per chicken?

I was planning on building a 8x8 coop 2 feet off the ground in the corner of a 16x16 run.

16 hens... run should be twice that big.

18-20 would be doable, but it would be crowded.

I have 8 hens(3 rir, 3 comets, 2 cuckoo marans) and 1 rir roo now and i want to add 9 more hens and 1 roo. How many square feet per chicken in the run?

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About double what you have in the coop. Mine actually have triple the run space, but I have plenty of space...some folks don't.

Right now my 9 chickens stay in a 5x6 coop that sits above a 5x12 run but they are free ranging right now. When i finish my new coop i would like to use this smaller coop as a breeding pen or for broody hens.

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That's not bad. The reason I do mine that way is they have plenty of room to range and basically just sleep and lay in the coop. Other than that, they're almost never in there. SOOOO, that being said...if that's what you're doing too, I see no issue with 20 in an 8x8, but I honestly wouldn't go more than that. For 18 I'd say it would be adequate for those circumstances.