Indoor floor space: the four-square-foot starting point
For a standard-sized hen, plan on about four square feet of usable floor space inside the coop per bird. Large, heavy breeds need more elbow room; small bantams manage with less, though they rarely suffer from extra. Usable is the key word: floor eaten up by a feeder hung at bird height, a waterer, or a nest box bank is not space hens can stand, scratch and move in, so measure the floor the flock actually shares. This indoor figure assumes the birds also have daily access outside. A flock confined indoors for long stretches — during severe weather or a disease-control lockdown — needs considerably more, which is worth picturing before you build rather than during the first shut-in week.
Roosting bars and nest boxes
At night the whole flock roosts at once, so perches are measured per bird: allow roughly eight to ten inches of bar per hen, arranged so dominant birds cannot guard the only route up. Wide, flat-ish bars are kinder to feet than narrow round poles, and all bars at one height prevent the nightly contest for the top spot spilling into bullying. Nest boxes need not match bird numbers — hens queue and share, often all favouring one box — so one box for every three or four hens is plenty, sized so a hen can stand, turn and settle, set somewhere darker and quieter than the main traffic of the coop.
The run decides how the flock behaves
Outdoor space is where coop maths becomes flock behaviour. Around eight to ten square feet per hen in the run is a workable minimum; more is visibly better, because hens spend their waking hours out there scratching, dust-bathing and keeping out of each other's way. A cramped run turns to bare mud, concentrates droppings and parasites, and gives a bullied hen nowhere to retreat to, which is when feather pecking starts. If ground is limited, make it interesting instead: perches, a dust bath corner under cover, logs and scattered greens give birds things to do that are not each other.
Why minimums are a floor, not a target
Every figure here is a minimum for a settled flock, and two pressures push real needs upward. The first is flock dynamics: a mixed group with a cockerel, young birds or a known bully needs escape room. The second is the universal law of chicken keeping — flocks grow. A coop built exactly to today's numbers has no slack for the three extra hens that arrive next spring, so building one size up is the cheapest welfare upgrade there is. Ventilation scales with size too: a bigger, airy coop sheds ammonia and moisture that a tight box concentrates.
Once you have settled the dimensions, the plan tends to live in one person's head while someone else builds, cleans or extends. Recording the coop measurements and flock numbers in the shared profile through KinsLe Family Sharing means everyone in the household works from the same plan, and the next extension starts from real numbers.