The total surface area of Earth is about 510 million km2. Of this, 70.8%,[12] or 361.13 million km2 is below sea level and covered by ocean water. The remaining 29.2%, or 148.94 million km2, not covered by water has terrain that varies greatly from place to place and consists of mountains, deserts, plains, plateaus, and other landforms. If all of Earth's crustal surface were at the same elevation as a smooth sphere, the depth of the resulting world ocean would be 2.7 to 2.8 km. The volume of the oceans reach an estimated volume of 1.332 billion km3. About 97.5% of the water is saline; the remaining 2.5% is fresh water. Most fresh water, about 68.7%, is present as ice in ice caps and glaciers.

It is estimated that one-eighth of Earth's surface is suitable for humans to live on – three-quarters of Earth's surface is covered by oceans, leaving one-quarter as land. Half of that land area is desert (14%), high mountains (27%), or other unsuitable terrains. Less than 75 million km2, is suited for human living.

When 7,5 billion of people will share this area, everyone will theoretically have 10.000 m2. If, or, when the population reach 75 billion heads, the crowded output is 1000 m2 per capita.

100 humans / km² gives every person a space of 1 ha = 10.000 m², could be a square of 100 x 100 m². Today, 2019, this population density is achieved by Spain, France, Turkey and Austria. For Portugal, Poland, Thailand, China, Uganda, Nigeria, Switzerland, the available habitable space is about 7.000 m² to 8.000 m².

Some countries have today the crowded number of 100 humans / km², a number which gives every person a space of 1.000 m². The most of them are small, citystates, like Singapore and Monaco, or islands like Malta and Maldives. But one of these  real overpopulated countries is Bangladesh.
Countries like Taiwan, South Corea and Libanon have more than 500 humans per km², that is real overpopulated too. In Europe, Netherlands has more than 400 people per km², so every Dutch have 2.500 m².

In very approximative terms, the less habituated areas follows two axis one around the Polar Circle in the Norther hemisphere, on other going in South West – North East direction. Two clusters concentrate people, the West Europe-North Central America and the Far East Asia.