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What share of Earth's surface is covered by ocean?
Roughly 71% of the surface is water, almost all of it ocean. That is why Earth looks blue from space.
Earth's surface covers about 510 million square kilometers, and the ocean fills roughly 361 million of them, which gives the 71% figure. Land is not spread evenly. The Southern Hemisphere is about four-fifths ocean, while the Northern Hemisphere is closer to three-fifths, and even the hemisphere centered near western France, which holds the most land, is about half water.
Measured by volume, the ocean holds about 96.5% of all water on Earth, according to USGS estimates, with most of the rest locked in ice caps, glaciers and groundwater. Freshwater lakes and rivers account for a tiny sliver of the total. The average ocean depth is near 3,700 meters, and the deepest known point, the Challenger Deep in the Mariana Trench, is a little under 11,000 meters down.
Despite its size, the ocean is a thin film in planetary terms. It makes up only a few hundredths of one percent of Earth's mass, so a model globe with a coat of paint is a better picture than a deep bowl. Much of the seabed remains poorly mapped at high resolution, and the 71% figure is itself an approximation that varies slightly with how coastlines, ice shelves and inland seas are counted.