General rule of thumb is 4 sqft per bird in coop, 10 sqft of run per bird (you can do less, I have used anywhere from 8-10 sqft total for birds depending on their size). Even at those first larger numbers they're still turn it barren. It's been a while since I looked into it but I want to say for them to not destroy it the requirement is something like 200 sqft of pasture?I don't know how run space is calculated, but I figure it's big enough if the grass can keep up with the population of chickens foraging and scratching. If they turn it into a barren wasteland... Too small!
Edit: I currently have three rescued hens in a 16x24 area, which works out to 128 sqft per bird and it is barren dirt. Just as an FYI
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