The Optimum population for a region or country is a level of (human) population which is ecologically sustainable . It is less than the carrying capacity (the amount before a country would theoretically sink under the weight of its population), so the amount of resources available can support the country's population needs indefinitely, which maximizes the overall welfare (happiness, standard of living) of the population. There are different views on the appropriate measure of welfare, and this is an area of political disagreement.

• If the population is below its optimum, it can increase with a net social benefit without damaging the natural resources needed for the population.

• If it is above the optimum, the negative effects of overcrowding (pollution, crime, social dysfunction, etc) and environmental damage reduces welfare or there are too few resources to maintain the population at its current level.

To reach optimum population, a country must be able to measure human welfare and environmental impacts, as well as having policies in place to influence the fertility rate and net migration. This implies agreement on welfare, environmental issues, accurate projections of immigration, age distribution and changes in lifespan among other factors. Reaching optimum population is an important objective in the wider project of creating a sustainable society.

The UK based think tank Optimum Population Trust have calculated the optimum population of nearly 150 countries.

Achieving Optimum Population

Although no country has yet formally adopted an optimum population target, many recognize the problems caused by population growth and overpopulation. Most of the effort has been focussed on reducing population growth. There are several policy areas that influence this:
Social and Cultural
Changing people's views and attitudes on religion to adjust it into a modern fashion, changing social attitudes, such as giving women more rights and thought in starting a large family than following tradition.
Economic
Increasing career opportunities will have peoples' minds set on education, career prospects, and maintaining their job, such that the immediate impulse to start a family might be delayed.
Medical and Scientific
Increasing the amount of contraception in LEDC (Less Economically Developed Country) educating adults and children about sexual education, on how to use contraception and the risks involved.
Political
Improving education to direct people into a career, this will have people concentrate on getting a stable job rather than planning ahead on starting a family.

Increasing emigration, and/or reducing immigration, within certain areas (especially those in which the residents have an atypically-high impact on the environment), is viewed by some as a key factor in stabilizing population and preserving natural resources.