Free chlorine, ppm and how pool dosing math works
Understand what a ppm of free chlorine really means, how product strength changes the dose, and how to reach a target level without guesswork.
What free chlorine and ppm actually measure
Free chlorine is the portion of chlorine in your water that is still available to sanitize, as opposed to combined chlorine that has already reacted with contaminants. It is measured in parts per million, and in water one ppm is the same as one milligram of chlorine dissolved in one liter. Most pools aim for a free chlorine reading somewhere between 1 and 3 ppm for everyday swimming. Because the unit is tied to liters, knowing your volume in liters is the key to turning a target ppm into a real weight of product.
Turning a ppm target into grams of product
The pure chlorine you need equals the ppm rise multiplied by your volume in liters, expressed in milligrams, then divided by 1000 to get grams. That figure assumes 100 percent pure chlorine, which no bagged product is. To find the actual product mass you divide by the available chlorine fraction, so a 65 percent cal-hypo dose is the pure figure divided by 0.65. This is exactly the sequence the calculator runs, which is why a weaker product always shows a heavier dose.
Why product strength changes everything
Two bags labeled as pool chlorine can behave very differently because their available chlorine differs. Calcium hypochlorite typically lists around 65 percent, dichlor near 56 percent, and trichlor tablets close to 90 percent. Trichlor and dichlor also add cyanuric acid stabilizer, while cal-hypo adds calcium hardness, so the choice affects more than dose size. Always read the strength printed on your container and enter that number rather than assuming a default.
From the calculated dose to a safe result
The number the tool gives is a starting dose measured to a fresh test, not a promise of a final reading. Sunlight, swimmer load and a low stabilizer level all consume chlorine, so real pools often need a touch more than the theoretical figure. Pre-dissolve granular products in a bucket of water, add them with the pump running, and give the water a full turnover before retesting. If the reading is still low, enter the shortfall as a new ppm rise and dose again.