From what I can tell, fully free range chickens need about 5,000 square feet per bird on fairly productive ground without predators. They will still need some supplemental feed because without it the birds will be impossible to control and manage. I'm curious if others think this figure is in the ball park? BYC has all sorts of rule-of-thumb numbers and specificaions to go by.but not one for free-range carring capacity.
Kind of hard to say because if they are fully free range, they wouldnt have a fence - at all. I would say that if you wanted to attempt to emulate a free range scenario with a fenced area that the chickens can not exit, 5000sqft per bird would probably be close to what you need, but as big as possible is probably the way to think of it in that situation.
I think in a real free range system the chickens wouldn't technically use that much space.
I like to think of it more in terms of how many chickens you can have on the land based out of a singular coop station without over stressing your land. Depending heavily on the local vegetation density, variety of plant types, terrain, and agricultural zone- I would say that, in my location, I could have up to 250-300 chickens on an acre, but it's not like I could put 1200 chickens on a fenced 4 acre plot and expect there to be any vegetation left at chicken height after a few months of that.
Even if you had 1000 chickens based from one chicken house and able to free range unlimited amounts of space and range unlimited distances, they would completely ruin the ground around the coop to a certain radial distance and they would probably ruin a few big spots beyond that as well, because they are, well, chickens so of course they flock together, and have cliques, and dust in certain areas every day, roost in the same spot every night, etc.
This is the argument for mobile coops, chickens tractors, blah blah blah.
But, you can see, it gets complicated when talking about free range numbers.