Richard, that's the first good defense I've read of the deviation in the Pool Calculator from the formula. THAT is an explanation I can accept. It also points out there is a potential inaccuracy in the FC section. Naturally, neither the PC nor the formula take into account any degradation or breakdown in the chlorine over time. I'm confident your coefficients are correct but where do they come from. BTW, how many grams/mole are there in CL2 and NaOCl?

Madwil, I suppose it's a difference of definition but the VARIABLE "Concentration" must be defined as 4 digits to the right of the decimal point because available VALUES can use any of the 4 places...LC would be expressed as .125, while regular bleach is .0525. Each uses only 3 digits, but 4 are still needed.

Carl