How much air does your coop actually need?

Plan for 5 CFM of airflow per adult standard hen in summer, dropping to about 1 CFM per bird in winter for moisture removal. In square inches of vent area, the cooperative-extension rule of thumb is 144 sq in per 10 sq ft of coop floor, split roughly half low inlets and half high outlets.

There is no clean per-bird square-inch rule because vent area scales with coop floor, not birds alone. The simplest answer: target 1 sq ft (144 sq in) of total vent area for every 10 sq ft of coop floor. The calculator reconciles flock size, floor area, and climate so you do not have to do it twice.

CFM stands for cubic feet per minute, the unit that measures how much air moves through a space. A coop without enough CFM holds humidity, ammonia from droppings, and respiratory irritants. The University of Kentucky small-flock standard is 5 CFM per bird in summer, 1 CFM in winter. Most coops fail the summer number, which is why heat stress and respiratory illness show up in July.

Cold is rarely the killer in a winter coop. Trapped moisture is. Keep the high outlets open above roost height so warm humid air rises out, and reduce low inlets so cold drafts do not blow across the birds. Aim for 1 CFM per bird, not zero. A coop sealed shut against the cold builds humidity, which causes frostbite faster than dry cold alone.

Open everything you can: low inlets near the floor, high outlets at the ridge, and any window vents on the shaded side. The extension-service summer target is 5 CFM per bird, which most passive coops cannot hit without help. Above 85°F, add a small DC fan on the outlet side rather than relying on cross-breeze.

Three signals: a sharp ammonia smell when you walk in, condensation on the ceiling or windows in the morning, and damp bedding that stays damp. Combs that look pale or pasty in winter are also a moisture indicator. If any of those are present, your CFM is below target.

Ammonia rises from wet droppings as bacteria break them down. The smell means moisture is not leaving the coop fast enough. Two fixes, in order: increase high-outlet vent area (you almost certainly need more than you have), then reduce moisture input by changing bedding more often or moving waterers outside.

For 10 standard hens in a typical 4×8 (32 sq ft) coop in a temperate climate, the calculator returns about 460 sq in of total vent area: 230 low and 230 high. That is roughly two 6×12 louvered vents low and two 6×12 high, or one continuous ridge vent plus two soffit vents. Specifics depend on your climate.

Both. Low inlets near the floor pull cool fresh air in. High outlets at the ridge let warm humid air leave. A coop with only one or the other does not move air, it traps it. Split your total vent area roughly 50/50 between low and high.

Passive vents handle most temperate climates if they are sized correctly. Add a fan when summer highs regularly exceed 85°F, when humidity stays above 70%, or when the coop sits in a wind-shadow that kills natural airflow. A small 4-inch DC fan on the outlet side is plenty for flocks under 20.