General formulation to calculate  how much bleach is needed to add to a tank or a pipeline to disinfect it to a given chlorine residual using a given disinfectant. Usually someone wants to know how to meet an AWWA Standard (like C651-92, Disinfection of Water Mains). The equation below should be used to estimate the amount of sodium / Calcium hypochlorite (Chlorine Water is 6.00% sodium hypochlorite) needed to disinfect a given quantity of water to a desired chlorine concentration. 

                                                                        req'd residual in ppm
  Valume of hypo  =  volume of water  x  (  ----------------------------- )
                                                                           1,000,000 x hypo %

For example , say you had installed a new 5,000 gallon tank and wanted to make sure that you had at least a 100 ppm solution of chlorine in it.

How much 6.00% Hypochlorite would you need to add  ?

• (5,000 gal x 100 ppm) / (1,000,000 x .06) = 8.33 gallons

How much 12% Hypochlorite solution would you need ?

• (5,000 gal x 100 ppm) / (1,000,000 x .12) = 4.17 gallons

If you use calcium hypochlorite (the white, powder version of chlorine, like HTH pool cleaner), the equation becomes:

                                                                                                              req'd residual in PPM
Wgt. of Calcium Hypo (lbs)= gal of water x 8.33 lb / gal m x   ( ----------------------------- )
                                                                                                               1,000,000 x hypo %

• This is simply the previous equation multiplied by the conversion factor of 8.33 pounds per gallon of water.

Let's assume that we still need to disinfect 5,000 gallons at 100 ppm.

How many pounds of 65% calcium hypochlorite (HTH pool cleaner) are needed?

• (8.33 lbs/ gal. x 5,000 gal x 100 ppm) / (1,000,000 x .65) = 6.4 pounds