Estimate the mass of water on the earth. The density of water is 1000kgm^-3 . Give answer to 1 significant figure. Here is a approximation you can use: Looking at charts of the oceans, we see that the depth is typically several thousand m (and a serious search tells me the average depth is 4700m) .The radius of the earth is about 6000km .The surface area of a sphere is 4πR².