What percent of the earth's surface is covered by water?

About 71 % of the is covered by water.

The surface area of the Earth is ##5.1 × 10^8 "km"^2##.

The water segment has an area of ##3.6 × 10^8 "km"^2##.

So the percent of Earth's surface that is covered by water is

##(3.6 × 10^8 color(red)(cancel(color(black)("km"^2))))/(5.1 × 10^8 color(red)(cancel(color(black)("km"^2)))) × 100 % = 71 %##

If you could form all of the into a sphere, it would have a diameter of 1385 km.

That's a lot smaller than Earth itself.