If the Earth were a smooth spheroid, how deep would the ocean be?

Question

At the moment there are deep seas and high mountains. But imagine that the land elevation of the Earth is equal everywhere. How deep would the ocean be in that case?

Answer 1

This is a fascinating thought experiment that helps us understand the true scale of Earth's water relative to its topography. If we were to "smooth out" the entire Earth—effectively grinding down the mountains and filling in the deep ocean trenches until the lithosphere was a perfectly smooth spheroid—the entire planet would be covered by a global ocean.

To find the depth, we look at the total volume of water in the world's oceans, which is approximately 1.335 billion cubic kilometers. We then distribute this volume over the total surface area of the Earth, which is roughly 510 million square kilometers.

By simple division (Depth=Volume/Area), we find that the global ocean would be approximately 2.6 to 2.7 kilometers deep (about 8,600 to 9,000 feet).

In this scenario:

1. No Landmass: There would be no dry land anywhere on the planet.
2. Hypsographic Curve: This calculation highlights that our current "dry" land exists only because of tectonic processes that create significant vertical relief.
3. Scale: While 2.7 km sounds deep, compared to Earth's radius of 6,371 km, this layer of water is incredibly thin—thinner than the skin of an apple proportional to its size.