You need a test kit
Nor sure how pee gravel is packed, but you need enough for 64 cubic feet.
You need 13 yrd's of fill
About 6,042 gallons of water.
The best thing to do if you are inexperienced in pool maintenance it to take a sample of water from your pool in a clean bottle from about an arms length in depth to you local pool shop. There they will test the water for free and tel you how much chlorine your pool needs. Take along the dimensions of the pool as well for them to calculate the requirements.
Chlorine has 17 protons.
6
One atom of chlorine per molecule of NaCl.
Chlorine is a meta element. Atomic mass of it is 35.
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The human body is 0.15% Chlorine by weight. an average 160lb. man would have about 4 ounces of chlorine.
16 sq ft*0.5 ft = 8 cubic feet.
Although chlorine is a poisonous gas which can kill people, and chlorine was used for that purpose in WW I, it is virtually impossible that your hot tub will contain so much chlorine as to endanger your health. It's safe.