Short answer: plan on roughly 3 to 4 square feet of coop floor per standard-size hen if the birds also have an outdoor run, and closer to 10 square feet per hen if they're shut in full time. Add 8 to 10 square feet of run per bird, 8 to 10 inches of roost per bird, and one nest box for every 4 to 5 hens. These are the ranges extension services commonly recommend; bantams need less, heavy breeds more.
Coop size by flock
| Hens | Coop floor (with a run) | Run | Roost length | Nest boxes |
|---|---|---|---|---|
| 6 | 18–24 sq ft (e.g. 4 × 6 ft) | 48–60 sq ft | 4–5 ft | 2 |
| 12 | 36–48 sq ft (e.g. 6 × 8 ft) | 96–120 sq ft | 8–10 ft | 3 |
| 25 | 75–100 sq ft (e.g. 8 × 12 ft) | 200–250 sq ft | 17–21 ft | 5–6 |
| 40 | 120–160 sq ft (e.g. 10 × 14 ft) | 320–400 sq ft | 27–34 ft | 8–10 |
What an 8 × 12 ft coop really holds
Our Small Poultry House is restored from a 1974 USDA / West Virginia Extension plan sized for 25 to 40 layers in about 96 square feet. That is about 3.8 square feet per bird at 25 hens and 2.4 at 40. The 1970s plan was drawn for laying flocks on a raised roost over a wire droppings pit, which frees floor space. For a backyard flock with outdoor access, 25 hens is the comfortable number; 40 is the working-farm maximum.
Roosts
Give each bird 8 to 10 inches of perch, more for large breeds. Set roosts higher than the nest boxes so hens don't sleep in the nests. Hens kept over a droppings board or pit stay cleaner and are easier to muck out.
Nest boxes
Hens share, so you need far fewer boxes than birds: one per 4 to 5 hens, about 12 × 12 inches for standard breeds, in the darker part of the coop.
Ventilation
Most coop problems are moisture and ammonia, not cold. Vent high, above the roost, so air moves through the coop without a draft across sleeping birds. The original extension plans put windows and vents on the sunny side and the low wall to the prevailing wind.
Summer flocks on range
If the birds live on pasture through the warm months, a light shelter can replace a full coop. The Summer Range Poultry Shelter is a 10 × 10 ft netting-walled gable on skids: shade by day, a dry roost at night, and moved to clean ground every few days.
Frequently asked
How many chickens can fit in a 4 × 8 coop?
32 square feet holds about 8 standard hens with a run, or 3 to 4 kept inside full time.
Can a coop be too big?
In cold climates, yes: a few birds in a large, uninsulated space can't keep it warm. Insulated walls (like the plywood-fiberglass sandwich in the 1974 plan) help.
Do bantams need less room?
Roughly half: about 2 square feet per bird inside, with a run.
Ranges reflect commonly published extension guidance; check local rules and size to your breed and climate.