As Pat suggests, the "4 sq ft. Rule of Thumb" we love to tout so much is a direct offshoot of the fledgeling commercial business. It is, in reality, a fairly demanding compromise for the home chickeneer who keeps standard size breeds.
It arose sometime after WWII, near as my research indicates. I have yet to determine who was the first to suggest it in the small flock.
But what is often overlooked with such numbers is that they are best reserved for interior coop space, where nesting and roosting are the only activities that SHOULD take place.
Outside, where chickens belong, only a glutton for punishment will subject adult chickens to such space allowances.
But things get sticky in the harsh northern winter. What one does then is open to discussion. Give them too little space and then cram them into it during winters frigid embrace... and what happens? Again, a good point for discussion.
How many unseen ills result from such intensive practice - and how many don't even realize it because they followed "The Rule Of Thumb?"
In the old days, if you lived in Maine, you were admonished to cull your flock down to the best birds, when going into winter.
Many people don't like to be so "mean" or even deliberate as to cull for such a purpose. Which, of course, takes us back to the point of conjecture....
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If you want a mentor on this matter, you can do none better than Pat. Stick to her like glue. If you crave numbers to go by, use hers.
That's all I'll add for now - -