Where the doses come from
The chemical calculators scale published rule-of-thumb dosing rates by your pool volume and by the strength of your product. Typical rates used on the pages are about 10 fluid ounces of 12.5 percent liquid chlorine per 10,000 gallons to raise free chlorine by 1 ppm, and about 1 pound of calcium hypochlorite per 10,000 gallons for routine shock, with 2 to 3 pounds for algae or a heavy shock treatment. The salt calculator uses 1 pound of salt per 120,000 gallons for each 1 ppm change. Products differ in strength, so the calculators ask for yours.
Test, dose, retest
A calculated dose is a starting point. Sunlight, bather load, temperature and the condition of your water all change how much you need, so test before you add anything and again after the water has circulated. Pool and spa chemicals can be hazardous: follow the label, never mix different chemicals together and add chemicals to water, not water to chemicals.
Volume comes first
Every dose depends on your water volume, and a wrong volume throws all of them off. If you are not sure of yours, start with the volume calculators. The disclaimer covers the limits of these estimates.