I like this question (being an oceanographer!) so it’s going to be the first one I answer.

I don’t know the answer off the top of my head, but I know a few facts that can give me a rough estimate of the answer:

The radius of the Earth is about 6,400 km, or 6,400,000 metres. The surface area of a sphere is 4 times pi times the radius squared. So assuming the earth is a sphere (it isn’t, but it’s close enough), then the surface area of the earth is about 516,000,000,000,000 square metres!
About 70% of the Earth’s surface is ocean, and the average depth of the ocean is about 4000m. Multiply this all together and we get about 1,400,000,000,000,000,000 cubic metres of water.
This is a huge number, and we haven’t even considered the water that is on land, in the atmosphere on in ice in places like Greenland or Antarctica (though all of these are much smaller than the amount in the oceans).

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