4 sq ft inside. 10 sq ft of run. Per bird. That's the fight on every chicken forum.

Extension-service numbers, rounded up. Not the 2 sq ft prefab that looks cute on Amazon until week six.

Roost bar is the other constraint: 8–10 inches per bird. Nesting boxes: one per 3–4 hens, and they will still queue for the same box. Adults rarely need a heat lamp — damp air kills, not cold.

Use it in 10 minutes. Count standard birds. Multiply. If the coop is the run, double the interior. Bantams: 2 + 8 instead of 4 + 10.

Quick reference by flock size (standard birds)

Flock	Coop sq ft	Run sq ft	Nest boxes	Feed / month
3 hens	12	30	1	23 lb (~1 bag / 2 mo)
6 hens	24	60	2	46 lb (~1 bag / mo)
12 hens	48	120	3–4	91 lb (~2 bags)
25 hens	100	250	7–9	190 lb (~4 bags)

¼ lb layer feed/day each; $22 per 50 lb bag as the default on the calculator. State extension-service space rules.

Worked 6-hen build: 4×6 coop, ~6×10 run, 5 ft of roost, two boxes, one 50 lb bag a month. Heat lamp is for chicks in a brooder, not a dry adult flock.

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