Chicken Coop Size Calculator: Floor Space, Run, Roost and Nest Boxes
Space is the one thing you cannot fix later without rebuilding. Enter your flock and bird size to get the four numbers that matter, or work backwards from a coop you already own.
The number to get right before you buy anything
Almost every problem people bring to a backyard flock forum in year one — feather-picking, bullying, foul-smelling bedding, hens that stop laying — traces back to the same root cause, and it is not feed or breed. It is space.
Enter the flock you want and the size of bird you are keeping. The last output works the other way round: type in the coop you already own and it tells you how many birds it honestly holds.
| Example | Coop floor area | Coop floor in m² | Outdoor run area | Roost bar length | Nest boxes | Vent opening (about 1/10 of floor) | Birds your existing coop holds |
|---|---|---|---|---|---|---|---|
| 6 standard hens, daytime run | 18.0 sq ft | 1.67 m² | 48.0 sq ft | 54 in | 2 | 1.8 sq ft | 10 |
| 4 standard hens, daytime run | 12.0 sq ft | 1.11 m² | 32.0 sq ft | 36 in | 1 | 1.2 sq ft | 10 |
| 6 heavy breeds, hard winter | 30.0 sq ft | 2.79 m² | 60.0 sq ft | 72 in | 2 | 3.0 sq ft | 6 |
| 12 bantams, daytime run | 24.0 sq ft | 2.23 m² | 72.0 sq ft | 72 in | 3 | 2.4 sq ft | 16 |
| 10 standard hens, daytime run | 30.0 sq ft | 2.79 m² | 80.0 sq ft | 90 in | 3 | 3.0 sq ft | 16 |
| Bird size | Coop floor, with daytime run (sq ft) | Coop floor, confined (sq ft) | Outdoor run (sq ft) | Roost bar (in) | Birds per nest box (birds) |
|---|---|---|---|---|---|
| bantam | 2 | 3 | 6 | 6 | 5 |
| standard | 3 | 4 | 8 | 9 | 4 |
| heavy | 4 | 5 | 10 | 12 | 4 |
Figures are the mid-point of the ranges published by US and UK extension and welfare guidance for small backyard flocks. They are recommendations, not regulations — local rules and certification schemes may require more. Where sources disagree (coop floor for standard hens is given as anywhere from 2 to 4 sq ft), this table takes the more generous end, because crowding is the single most common cause of feather-picking in small flocks.
Assumptions and limits
- These are minimums for healthy birds, not targets. Every keeper who has rebuilt a coop built it bigger; nobody rebuilds smaller.
- Coop floor area excludes nest boxes — hens do not live in them, they lay in them. Boxes hung outside the footprint do not count towards the floor.
- The run figure assumes birds are shut in the coop overnight and have the run available through the day. Genuinely free-ranging birds need less run, but that is a predator decision, not a space one.
- Vent area is the open, permanently unobstructed area — not the size of a hatch you close in winter. Ammonia and damp cause more winter losses in small flocks than cold does.
- Local ordinances, tenancy rules and certification schemes may set their own minimums. Check yours before building.
Why coop area and run area are different problems
Coop floor area governs the hours birds are shut in: overnight, in bad weather, and during the stretch in late autumn when it is dark before you get home. Run area governs the hours they are awake and active. Birds tolerate being packed tightly while asleep on a roost far better than they tolerate it while awake, which is why the run figure per bird is roughly double the coop figure.
This is also why the confinement question changes the answer. A coop sized for "shut in at night" becomes badly overcrowded if a fortnight of snow means the birds are inside for days at a time.
Roost length and nest boxes are counted differently
Roost bar length is measured per bird because hens sleep in a row, shoulder to shoulder, and the birds at the ends get pushed off if the bar is short. Give the full length even though they will crowd onto part of it in cold weather — the point is that the space exists when they want it.
Nest boxes work the opposite way. Hens do not need one each; they will queue for whichever box is currently fashionable and ignore the rest. Four hens per box is enough, and building more does not stop the queueing.
Ventilation without a draught
The vent figure — roughly a tenth of the floor area — is the most commonly ignored number here, and the most costly in winter. Chickens produce a remarkable amount of moisture through their droppings and their breath. In a sealed coop that moisture condenses on combs and wattles, and frostbite in a small flock is nearly always a damp problem rather than a cold one.
The distinction that matters is between ventilation and draught. Vents belong high, above roosting height, so damp air rises and leaves without blowing across sleeping birds.