How many gallons of water are there on Earth?

How many gallons of water is on earth to this day?

• So today I was challenged by my Uncle because he says that we are going to run out of drinkable water on earth before we run out of Oil, I told him that he is just saying that to scare me but he honestly thinks that this is true. So I wanted to do the math for him to prove that we have nothing to worry about. But I got a lot of it finished but I have hit a brick wall, I do not think my math is correct so I would like you guys to help me prove that my math is correct. My variables: Volume of Earth in Cubic meters: 260,000,000,000 cubic miles Volume of (any) water on Earth: 75% of earth's volume, 260,000,000,000 * 0.75 = 195,000,000,000 cubic miles Volume of drinkable water on earth: 1% of Water, 195,000,000,000 * 0.01 = 1,950,000,000 cubic miles Conversion of Cubic Miles to Gallons: 1 Cubic Mile = 1.101117147e+12 Gallons of Drinkable water on Earth: 2.147178437e+21 Conversion of Gallons to Ounces: 1 Gallon = 128 Ounces, Total of 5.359357378752e+32 Ounces Average person consumes: 64 Ounces of Drinking Water per day People on earth: 6.5 Billion Average amount of ounces each person drinks of water per day: 64 * 6,500,000,000 = 416,000,000,000 Ounces of Drinking Water per day for every person on earth How many days earth drinking water would last with this amount: 5.359357378752e+32 / 416,000,000,000 = 1288307062200000000000 I just want some math people to check my work. I am only 15 trying to debate with a 40yo haha, I do not want to screw up. Thanks for any help and it is well appreciated!

• Answer:

  It is not the case that 75% of the earth's volume is water. You are thinking of the statistic that about 75% (closer to 70% really) of the earth's surface is covered by water; there isn't any water on the inside. I'd estimate the volume of water on earth as follows: Circumference of the earth: $c = 4\times 10^7 m$ (almost by definition of the meter) Radius of the earth $r = c/2\pi \approx 7 \times 10^6 m$ Surface area of the earth: $A = \frac{4}{3} \pi r^2 \approx 2 \times 10^{14} m^2$ Area covered by water: $A_W = 0.7 A \approx 1.4\times 10^{14} m^2$ Average depth of the ocean: $D = 2\times 10^3 m$ (wild guess) Total volume of water: $A_W D \approx 3 \times 10^{17} m^3$ This is about $7 \times 10^7 mi^3$ or $8 \times 10^{19}$ gallons, which is about 5 orders of magnitude smaller than your value. It is also worth mentioning that humans consume water for many more things than just drinking (watering crops and livestock, industry, etc), and the rest of life on the planet needs water too. 64 ounces per human per day would not keep the human race alive. On the other hand, it isn't necessarily gone after being used once.