How the numbers are worked out
The calculator starts with a per-bird floor allowance, then adjusts it for how your birds actually live. Extension services publish a range rather than a single number, so we use the comfortable end of that range for backyard flocks:
| Breed size | Coop with a run | Roost per bird |
|---|---|---|
| Bantam | 2 sq ft | 7 in |
| Standard | 4 sq ft | 10 in |
| Heavy | 5 sq ft | 12 in |
If the coop is the birds' only space, Oregon State recommends 8–10 sq ft per bird, so the calculator multiplies the base allowance by 2.25 (9 sq ft for a standard hen). Birds that free-range all day mostly use the coop to sleep and lay, so the allowance drops by 20%. A snowy climate adds 25%, because hens that are shut in for a week get bored and start pecking each other.
Ventilation uses the common rule of 1 sq ft of opening per 10 sq ft of floor (1 per 5 in hot climates). Put vents up near the roof, above roost height, so moist air leaves without a draft blowing across sleeping birds.
Quick chart for common flock sizes (standard breeds, with a run)
| Hens | Coop floor | Example build | Roost | Nest boxes |
|---|---|---|---|---|
| 3 | 12 sq ft | 3 × 4 ft | 2.5 ft | 1 |
| 4 | 16 sq ft | 4 × 4 ft | 3.3 ft | 1 |
| 6 | 24 sq ft | 4 × 6 ft | 5 ft | 2 |
| 8 | 32 sq ft | 4 × 8 ft | 6.7 ft | 2 |
| 10 | 40 sq ft | 5 × 8 ft | 8.3 ft | 3 |
| 12 | 48 sq ft | 6 × 8 ft | 10 ft | 3 |
Before you buy a prefab coop
Box-store coops are often labeled "fits 6–10 chickens" while offering 12–15 sq ft of enclosed floor. Measure the inside of the house itself, not the run underneath it or the nest box bump-out, and compare that number with the result above. If you are between two sizes, build the larger one. Nobody regrets the extra room after a stormy week.
Gear that helps
- Pre-built coop kits (compare the real floor area with your result)
- 1/2 in hardware cloth for vents and windows
- External nest box kits or plastic nest liners
- Roost bars: a 2×4 laid flat is the classic choice
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Frequently asked questions
How many square feet does a chicken need?
About 4 sq ft of coop floor per standard hen when there is also an outdoor run, and 8–10 sq ft when the coop is the only space. Bantams need roughly half.
How many nest boxes for 6 hens?
Two. One box per four hens is plenty, since hens take turns and often all choose the same favorite box anyway.
How high should the roost be?
Typically 18–36 in off the floor and higher than the nest boxes, so hens don't sleep in the nests.
Sources
- Oregon State University Extension, EC 1644: Backyard chicken coop design
- Penn State Extension: Management Requirements for Laying Flocks
Last reviewed October 2026. These are planning estimates for hobby flocks. For sick birds, contact a poultry vet or your state extension office.