Heres an illustration that will get you thinking…
The ball on the left (shown to scale) shows all the air in the atmosphere (5140 trillion tonnes of it) all gathered into one place and condensed to sea level density.
The ball on the right (shown to scale) shows all the water in the world (1.4087 billion cubic kilometres of it). Thats all the sea water, ice from the caps, lakes, rivers, ground water, clouds etc.
via The Thinking Blog