Like this:
1. Find your pool factor: Divide 120,000 by the gallons in your pool and round: 120,000/13,000 ~= 9.23 ==> So . . . your PF = 9
2. Use the following values: 3% bleach @ 8.4 lbs/gal; 6% bleach 8.6 lbs/gal; 8.25% bleach 8.8 lbs/gal
3. Calculate chlorine ppm per gallon for YOUR pool: => FC ppm = PF x lbs bleach x % => 9 x 8.8 x 0.0825 = 6.5 ppm.
So, you get about 6.5 ppm per gallon of 8.25% or 1.5 ppm per quart.
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