how much water is there?

Is there a finite amount of water on Earth? How many gallons is it?

There are apx. 326 million cubic miles of water and ice on ther Earth at any one time. That means ther are about 3.589632552591e+20 gallons of water on the Earth. 98% of that is salt water and 90% of the fresh water is tied up in the ice caps. That means there are only 7.179265105183e+17 gallons of fresh water in liquid form.

Jim Petty
An oak tree is just a nut that stood it’s ground

Back of the envelope calculation:

Total surface area of the earth: 4 pi r^2 where the radius is 4,000 miles or 6,440 kilometers making 521,172,188 square kilometers

70% of the earth is covered in water: 364,820,531 square kilometers

Average depth of the oceans is 2 miles, or 3.25 kilometers: 1,174,722,109 cubic kilometers of water or 1.17 trillion cubic meters

There are 0.00378541 cubic meters per gallon which gives: 310,328,896,737,737 gallons or 310 trillion gallons. Give or take.

Obviously, the radius and the percentage covered with water are very round numbers. There’s also water evaporated in the air. But compared to the errors in those round numbers, water vapor is probably insignificant.

1,174,722,109 cubic kilometers=1.17 quintillion * cubic meters (1,174,722,109100010001000) which would translate into 3.103288967377e+20 gallons.

My original figure, 3.589632552591e+20 gallons, was based on the Encyclopaedia Brittanica figure for the volume of water on the Earth. (326 million cubic miles).

Jim Petty
An oak tree is just a nut that stood it’s ground

BUT, the original question was “is there a finite amount of water on the earth?”. Correct me if I’m wrong, but doesn’t the word finite imply a constant, absolute number, and isn’t what we call H2O is really more of a soup, if you will, of hydrogen and hydroxyl ions, which combine and separate over time. This should happen more often with vapor in the air and less often in the ice packs (or ice cubes, for that matter.) In addition, water is created when hydrogen is burned and is destroyed in certain electrochemical reactions.

Profound question, and it goes to prove the point that any question is profound if you just confound it enough.

      • Gas in the planet’s upper atmosphere is evaporating off into space to the sun so it was previously assumed that the Earth’s water was slowly but surely being depleted. Then, a while back a special satellite (I forget the name) detected large balls of ice from space that melted in the upper atmosphere of Earth, never reaching the ground. Their consistency is similar to hard-packed snow and no other method was capable of detecting them. It is being debated how many fall to our planet how often.
  • Eventually everything will be on, or in the sun. - MC

No, there is an actual infinite amount of water on earth. In fact, being infinite, the amount of water on the earth exceeds the mass of the entire galaxy!

Of course there’s a finite amount of water! Since the amount of matter in the universe is finite, and earth’s water is by definition a subset of that matter, how could it possibly not also be finite?

peas on earth

Finite doesn’t mean constant, no. Finite means not infinite. There is definately a finite amount of water on the earth, and although this amount changes constantly, it could not possibly be infinite.

peas on earth

On a related note: The Star Trek: Voyager episode “Thirty Days” had the crew finding a 600 km radius ball of water in space with a ecosystem and civilization. It was revealed that this water was siphonned off of a planet centuries before.

Upon hearing that, I wondered if their math was screwy (which it often is). Surprisingly, that figure was accurate if the planet was the size of Earth and its oceans were 2 km average depth.

cornflake begged us:

Glad to oblige, you. I’ll correct you. You are wrong. The word fixed implies a constant, absolute number.