About this tool
The Pool Shock Calculator is a free, in-browser tool that works out how much chlorine to add to reach breakpoint chlorination — the super-chlorination level that oxidizes chloramines (combined chlorine), the compounds that cause the harsh chlorine smell, red eyes, and cloudy water. It applies the widely used 10× rule: the target free-chlorine level for breakpoint is ten times the combined chlorine present. Everything is computed locally in your browser and nothing is uploaded.
Enter the pool volume, your measured free chlorine (FC) and total chlorine (TC), and the product. Combined chlorine is CC = TC − FC. The breakpoint target FC is 10 × CC, and the dose must raise FC by delta = (10 × CC) − current FC. From there the chemistry matches ordinary dosing: pure chlorine grams = delta × liters / 1000, then divide by the product's available-chlorine fraction (dry) or by density × trade% (liquid). The tool reports the combined chlorine, the target FC, the required increase, and the product amount by weight or volume.
As a worked example, a 10,000-gallon pool with FC = 2 ppm and TC = 3 ppm has CC = 1 ppm, a breakpoint target of 10 ppm, and needs an 8 ppm rise — about 302.8 g of pure chlorine, roughly 2,203 mL of 12.5% liquid. Shock at dusk or after swimmers leave, run the pump, and do not swim until FC falls back to about 1–4 ppm. If CC is zero the tool reports no shock is needed.