How this calculator works
Enter a target chosen from the product instructions. The stock label can state available chlorine or sodium hypochlorite, and either mass per volume or mass per mass. The latter requires stock density. Sodium hypochlorite is converted to equivalent available chlorine using molar masses. This is a concentration calculation, not a recommendation for disinfecting strength or contact time. Use a current product label and never mix bleach with acids, ammonia or other cleaners. Prepare to the final volume; subtraction assumes additive volumes.
Formula
Cstock Vstock = Ctarget Vfinal. For % w/v available chlorine, Cstock(mg/L) = 10000 x stock percentage. For % w/w also multiply by density(g/mL). NaOCl to Cl2 mass factor = M(Cl2)/M(NaOCl).
Worked example
A 5% w/v available-chlorine stock is 50000 mg/L. Making 1 L at 500 mg/L needs 10 mL stock and water to 1 L. A 10% stock needs 20 mL to make 2 L at 1000 mg/L.
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Frequently asked questions
Is a 5% NaOCl label exactly 5% available chlorine?
No. Their mass bases differ, so select the label chemical basis explicitly.
Can I use an old bleach concentration?
Bleach loses strength during storage. The arithmetic assumes the entered stock strength is current.
Does this select a safe target?
No. The target and contact time must come from the applicable product instructions.