No: water does not make up 70 per cent of the Earth!

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

04 January 2026

We learn in school that approximately 70 per cent of the Earth’s surface is covered by water. Like many scientific facts, this one is often misunderstood: some people think that it means 70pc of the Earth is made of water. But is this the case?

To find out, we need to pieces of information: the surface area of the water bodies and their average depth. The surface area is easy to get. We know that the radius of the Earth is about 6,400km and so, we can calculate its total surface area using the formula learned in school (4 times pi r-squared). The answer comes to slightly over 500 million square kilometres.

Now, 70pc of this is covered by water, so, the surface area of the water comes to about 360 million square km. With this number at hand, we can work out the volume if we know the average depth of these water bodies. The deepest part of the oceans is the Mariana Trench which is about 11,000m (11km) deep. The shallowest is, of course, any beach; that is, zero metres!

It would be a mistake to simply add the deepest to the shallowest and divide by two to get the average. The reason is that the profile of the seabed drops down quickly as we go into the sea and then levels out more gradually. We therefore need measurements from many points in the ocean in order to get a better average. This comes to about 3,700m (3.7km).

When we multiply 360 million sq.km by 3.7km, we find that the volume of water on earth is about 1.3 billion cubic kilometres. I must emphasise: these are not litres! Indeed, I leave it to the curious reader to convert that number into litres. The result is mind-bending…

The present question is how this compares to the total volume of the planet. Again, we start from the radius of Earth (6,400km) and apply the volume of a sphere formular from school (4-thirds pi r-cubed). The answer comes to approximately one trillion cubic kilometres. Ponder on that for a moment…

The planet is one trillion cubic km and it was 1.3 billion cubic km of water. In other words: 1,000 billion cubic km, of which only 1.3 billion cubic km is water. A simple division of the two numbers reveals that the water is just 0.13 per cent of the volume of the planet. What about by mass? Well, that’s a story for another day. Meanwhile, I know some of you will go to ChatGPT to fact-check my calculations: Don’t!