Space is counted per bird, not per acre
Chickens are the one animal in this category whose land needs come in square feet. Inside the coop, plan on around four square feet of floor per standard hen, with roosting bar space for every bird and a nest box shared between three or four layers. Bantams need less; the large heritage breeds need a little more. The coop figure matters most on the worst nights of the year, when the whole flock is shut in together from dusk to dawn and crowding turns quickly into pecking and stress. Outside, a fixed run should give each hen at least ten square feet, and that is a welfare minimum rather than a comfort target: at that density the ground will be bare earth within weeks whatever you do, which is fine for the hens as long as the run stays drained, dry underfoot in patches, and interesting enough to scratch through.
Free range changes the maths completely
The numbers above keep hens healthy; keeping the land itself healthy takes far more room. A hen ranges, scratches and manures a wide area, and grass only survives the treatment when each bird has something in the region of twenty-five times her run minimum to wander over — which is why genuinely free-ranging flocks live on smallholding-scale plots or rotate around a large garden in sections. Most backyard keepers land on the practical middle: a secure run for everyday use, moved or rested in sections if the plot allows, with supervised ranging time on the wider garden when someone is home. That combination gives six hens a good life on a plot no bigger than a tennis court, run included, without turning the whole garden into a hen yard.
The limits that are not about grass
For many would-be keepers the binding constraint is not space at all. Local ordinances frequently cap the number of hens, forbid roosters, or set the distance a coop must sit from a boundary or a neighbour's house, and those rules decide the flock before the acreage does. Predators set the other condition: the run's footprint has to be roofed or netted where hawks hunt, and dug-proof at the edges where foxes patrol, so the usable area is the area you can genuinely secure. Shade belongs in the plan too — hens overheat faster than they chill, and a run that is all midday sun in summer needs trees, a shelter panel or part of the coop's shadow inside its boundary.
A workable backyard layout
Put together, a starter flock of five or six hens needs a coop of modest footprint, a run you can walk into for cleaning, and whatever extra ranging ground the garden can lend on rotation. Site the whole setup on the best-drained ground you have, because mud is the backyard flock's standing enemy and no amount of area compensates for a run that floods. Leave yourself room to expand the run sideways rather than the flock upward; chicken keeping is famously additive. While you sketch the layout, put a follow-up question to KinsLe's Kimmy; she answers using the KinsLe Answers library.