Calculate exact shock dosages based on your CYA and FC levels. Reach breakpoint chlorination to kill algae and clear cloudy pool water. Using the correct amount of shock is critical for destroying combined chlorines and restoring your pool's clarity and safety.
Enter your pool volume, current Free Chlorine (FC), current Cyanuric Acid (CYA), and, if you have it, your Combined Chlorine (CC) reading. Shocking means raising FC sharply to destroy chloramines, bacteria and algae, and the dose that actually works depends heavily on CYA, because stabilizer blunts chlorine's strength. The calculator uses your CYA to set a true shock target rather than a fixed number: a pool at 40 ppm CYA needs an FC near 16 ppm to shock, while one at 80 ppm CYA needs about 31 ppm. It then computes how much chlorine raises FC from current to that target. Pick your shock chemical too: unstabilized liquid chlorine (sodium hypochlorite, typically 10-12.5%) or cal-hypo (calcium hypochlorite, about 65-73%) add no CYA, while dichlor adds stabilizer and is best avoided for routine shocking.
Method: liquid chlorine at 12.5% adds about 1 ppm FC per 10.7 fl oz per 10,000 gallons; cal-hypo at 73% adds about 1 ppm per 2 oz weight per 10,000 gallons. The tool first finds the needed rise, then converts using your product's strength. Worked example: a 20,000 gal (75,700 L) pool with 40 ppm CYA reads FC 2 ppm; the shock target is 16 ppm, a 14 ppm rise. With 12.5% liquid chlorine: 14 ppm x (20,000 / 10,000) x 10.7 fl oz = about 300 fl oz, roughly 2.3 gallons (8.9 liters). For SLAM-style algae clearing you hold that shock level, retesting and re-dosing until FC loss overnight is minimal, CC reads below 0.5 ppm, and the water is clear.
Accuracy matters because under-shocking wastes chemicals without crossing breakpoint, letting chloramines and algae persist, while the right dose protects swimmers; CDC Healthy Swimming notes that proper free chlorine and pH are what actually inactivate germs. Shock in the evening so sunlight does not burn off the chlorine before it works, run the pump for several hours, and never pre-mix different shock types in one bucket. With cal-hypo, watch that it raises calcium hardness and can cloud soft-water or vinyl pools; with liquid chlorine, watch for a temporary pH rise. Add slowly near a return with the pump on, wearing gloves and eye protection, and store oxidizers away from acids. Do not swim until FC drifts back below about 5 ppm and water is clear; retest FC, CC and pH before reopening, and recheck CYA if you used a stabilized product.
Shocking (super-chlorination) means raising Free Chlorine to roughly 10 times the combined chlorine — breakpoint chlorination — to burn off chloramines, kill algae, and clear bacteria. It is needed when the pool smells strongly of chlorine, after heavy use or a storm, or whenever combined chlorine climbs above about 0.5 ppm.
Shock after heavy bather load, after a rainstorm, when you notice a strong chloramine smell, at the first sign of algae, or when combined chlorine exceeds about 0.5 ppm. Many owners also shock once a week in peak season as preventive maintenance to keep chlorine demand from building up.
Always shock at night or in the evening. This prevents the sun's UV rays from destroying the shock before it has a chance to work on the algae and bacteria.
Calcium hypochlorite (cal-hypo, 65-78% available chlorine) is the common, low-cost shock but adds calcium and raises pH. Dichlor (56-62% available chlorine) is pH-neutral and dissolves fast but adds CYA, so repeated use can over-stabilize the water; pick cal-hypo when CYA is already adequate and dichlor when you need a gentle, fast-dissolving option.
You should wait until the Free Chlorine level has dropped back down to the safe range (usually below 5 ppm) and the water is crystal clear.