This is a lot easier to use in a store than any other formula I've seen used. I will still keep the calculator in my signature up for those that prefer to use it as opposed to memorizing a formula, such as my wife
Formula: [Price] / [#Units] / [Oz per unit] / [% Chlorine]
So if I can get 3 - 121oz jugs of 8.25% for $7.50 it would be:
7.50 / 3 / 121 / 0.0825 = $0.2504
Which is the same as: (7.5 / 3) / (121 * 0.0825) = $0.2504 (the formula most used on this site)
Both formulas produced the same result up to twelve decimal places after doing over 100,000 calculations on random prices/units/unit sizes, and concentrations in Excel.
As with many formulas, you must remember the order in which to enter the numbers. You should start with the price and end with the %chlorine (in decimal form like the example). It does not matter if [#Units] comes second or third and you do not need to enter [#Units] if it is only for one jug.