Why CYA sets the shock level
Cyanuric acid binds most of the chlorine in the water, so the same ppm of free chlorine is far weaker at CYA 80 than at CYA 30. The shock level that reliably kills algae is about 40% of your CYA reading, with a floor of 10 ppm. That is what the calculator targets.
Which product
Liquid chlorine adds nothing but chlorine and a little salt. Cal-hypo also raises calcium hardness, which matters in plaster pools with hard fill water. Dichlor and trichlor add CYA with every dose, so shocking with them keeps pushing your shock level higher. For a green pool, buy liquid.
How to shock
Do it at dusk so sunlight doesn't burn the chlorine off. Pour slowly in front of a return with the pump running, and leave the pump on overnight. Green or cloudy water usually needs the dose repeated each evening until free chlorine holds overnight with less than 1 ppm loss. For routine top-ups, use the chlorine dosing calculator.
After you shock
Swimming again is a reading rather than a wait: see how long after shocking you can swim. If you are shocking often, can you over-shock a pool covers what dichlor and cal-hypo leave behind when you do. Still green a day later? See why a pool stays green after shocking, and for bagged shock, how many bags your pool needs. Opening for the season? See how much chlorine to open a pool, and for routine shocks, how often to shock a pool.
Dose tables by pool size
Written-out amounts for every product: 5,000 gallons, 8,000 gallons, 10,000 gallons, 12,000 gallons, 15,000 gallons, 18,000 gallons, 20,000 gallons, 25,000 gallons, 30,000 gallons.