Our question “How many chickens does it take to make an egg?” was inspired by the successful replacement of egg-based mayonnaise with plant-based mayonnaise in general dining at the University of California, Santa Barbara, in order to increase animal welfare. Our indicator of improved animal welfare due to decreased egg consumption was the reduction in number of chickens in the stressful and unhealthy conditions of the US egg industry. To measure this we calculated the ratio of chickens to eggs and (...) found it takes 6.3 chickens to make 1000 eggs. This equals 158 eggs per chicken, less than half the amount of eggs per laying hen because of mortality from hatching to entering the laying flock, including the disposal of male chicks. In addition, greenhouse gas emissions, irrigation water, reactive nitrogen, and land use would be reduced 43–98% from that of eggs. While the impact of plant-based mayonnaise was relatively small, we also estimated the substitution of eggs with tofu, which had a much greater impact: substituting 50% of eggs with tofu in first-year student breakfasts on all UC campuses would reduce the number of chickens in the egg industry by 9245. If this substitution was made by the US population, the welfare and environmental benefits would be 29 thousand times greater. Reducing egg consumption would greatly improve chicken welfare even if welfare certified eggs are replaced, since the requirements of the most commonly used chicken welfare certification programs do relatively little to reduce chicken suffering. (shrink)