Find your shock target FC and exactly how much chlorine to add to reach it. The shock level is set by your CYA — not a fixed number.
CYA (cyanuric acid) stabilizes chlorine against UV degradation — but it also reduces chlorine's ability to oxidize contaminants and kill pathogens. As CYA rises, you need a higher FC to get the same sanitation effect. The shock level (also called the breakpoint chlorination level) scales with CYA to account for this.
The formula is straightforward: Shock target FC = CYA × 40%. A pool with CYA 50 ppm needs FC at 20 ppm to shock. A pool with CYA 80 ppm needs FC at 32 ppm. Using a fixed shock target like "10 ppm" or "one bag of shock" ignores your CYA entirely — which is why many shocks fail.
After adding chlorine, you need to hold FC at or above the shock target — continuously — until combined chlorine (CC) drops to 0.5 ppm or less and the water clears. If FC falls below the shock target before CC reaches zero, you may need to add more chlorine and start the clock again. Test every few hours. Run the pump 24/7. Brush the walls daily.
UV from sunlight destroys unprotected chlorine quickly. Even with CYA, high FC levels are vulnerable to UV degradation. Add chlorine in the evening so it has the full night to work before sunlight can degrade it.
Liquid chlorine (sodium hypochlorite) is the best choice for shocking because it adds no calcium, no cyanurate, and no stabilizer — just chlorine. Cal-hypo works if your calcium hardness is low, but it raises calcium with every dose. Trichlor is not suitable for shocking: each dose adds CYA at the same time you're trying to raise FC, and you'd need an impractical amount to reach shock levels.
PoolChem Tracker monitors CC, tells you when your FC has slipped below the shock target, and calculates how much to add to get back on track — automatically.
