Sea level is the height of the ocean's surface, used as the zero point for measuring how high land is and how deep the sea is. In practice it is an average, because tides, waves and weather move the real surface up and down constantly.
Key takeaways
- Sea level is the reference height from which land elevations and ocean depths are measured.
- On our map 71% of the world's surface lies below sea level, an area of 362 million km².
- Land above sea level covers 147.9 million km².
- Sea level is a global idea, but the real surface varies from place to place and over time.
What sea level means
Sea level is the starting point for almost every height and depth on a map. A mountain has a height because its summit is that far above sea level. A trench has a depth because its floor is that far below it. In a bathymetric grid such as GEBCO_2026, sea level is simply the value zero [2].
The ocean does not sit still, so sea level has to be an average. Tide stations measure the height of the water relative to a fixed point on land, and satellites measure the average height of the whole ocean [1]. Tides, waves, winds and currents all move the surface, and the long-term average is what gets used as the reference.
Two things follow. First, the zero line is a convention, not a natural wall. Second, the sea surface is not perfectly level across the globe: local sea level can differ from the global average because of land movement and other local factors [1].
For a map, none of this affects the picture much. What matters is that everything on the map is measured from the same zero, so land and seabed join up without a gap at the coast.
An example from our data
On our grid, the zero line splits the world into two unequal parts. 71% of the surface, 362 million km², lies below it. The land above it covers 147.9 million km². The highest cell is 8,627 m up, and the deepest is 10,931 m down.
The drain the oceans slider moves the zero line in steps. At the first step, 10 m lower, 2.2 million km² of seabed becomes land. The Facts page gives the other headline numbers, and the oceans page shows how deep each one is on average.
The thing people get wrong
Many people think sea level is a single flat height that is the same everywhere. It is closer to an average of a moving surface. Local levels differ, and land itself rises and sinks, so "height above sea level" depends on which reference a country or survey chose.
Another common idea is that sea level has always been where it is now. It has not, and the Ice Age shows this clearly: see the Last Glacial Maximum. Our map is not a sea level rise tool; it lowers the sea to show the seabed.
Sources
- Is sea level rising?, NOAA Ocean Service.
- GEBCO_2026 Grid, GEBCO.
- Every figure from our own map is worked out from the datasets on the data sources page (checked against GEBCO_2026 grid).