The maximum number of individuals that can be supported sustainably by the available resources and services of that environment is known as its ‘carrying capacity’. Living within the limits of an environment depends on three factors: 

1. the amount of resources available in the ecosystem,
2. the size of the population, and

3. the amount of resources each individual is consuming.

    

Example:

Consider a few people who are surviving in a lifeboat after a shipwreck. Their survival depends on how much food and water they have, how much each person eats and drinks each day, and how many days they are afloat. If the lifeboat made it to an island, how long the people survived would depend upon the food and water supply on the island and how wisely they used it. A small desert island will support far fewer people than a large continent with abundant water and good soil for growing crops.

Resource utilization as per carrying capacity:

Utilizing resources within the carrying capacity means using them no faster than they are replenished by the environment, i.e., using the 'interest' income of the natural capital. A community that is living off the interest of its community capital is living within the carrying capacity. This community undergoes sustainable development.

A community that is degrading or destroying the ecosystem on which it depends is using up its community capital and is living unsustainably.

So, in the context of sustainability, carrying capacity is the size of the population that can be supported indefinitely upon the available resources and services of supporting natural, social, human, and built capital.