The right dose, not a guess
Pool chemistry goes wrong when chlorine is added by eye. Too little and the water is unsafe, too much and it is harsh and wasteful. The correct dose is a simple mass-balance calculation, and this tool runs it exactly for whatever product you own. Enter the pool volume, your current and target free chlorine, and the product strength, and it returns the precise weight to add.
How the maths works
To raise free chlorine by a given number of parts per million you need a proportional mass of pure chlorine — the increase times the pool volume in litres, divided by a thousand. Because real products are only partly available chlorine, the tool then divides by the product strength you select. That is why it asks whether you are using liquid chlorine, cal-hypo, dichlor, trichlor or a custom percentage: the same target needs very different amounts depending on the bottle. Nothing is looked up or invented; it is exact chemistry from your inputs.
Add safely and re-test
The result is a starting dose, not a licence to pour. Add chlorine in stages with the pump running so it circulates, then re-test with your kit before swimming. Approaching the target gradually protects both the swimmers and the pool surfaces, and it means the next dose is always based on a fresh reading rather than an assumption. Used that way, the calculator takes the guesswork out of keeping the water clear and safe.