How the shock level is chosen
Shock means raising free chlorine far above normal so that algae and chloramines are oxidised. The calculator takes the highest of three rules:
- SLAM level: 40 % of your CYA, rounded (12 ppm at 30 ppm CYA, 20 ppm at 50).
- Breakpoint: 10 times the combined chlorine above your current free chlorine, so 1 ppm of combined chlorine needs 10 ppm.
- Minimum: 10 ppm above where you are now.
dose = (shock level − free chlorine now) × factor per ppm × gallons ÷ 10,000
A 15,000-gallon pool at 2 ppm free chlorine, 1 ppm combined and 30 ppm CYA shocks to 12 ppm, a 10 ppm raise: 30 oz of 65 % cal-hypo (two 1 lb bags) or 160 fl oz of 12.5 % liquid chlorine. One 1 lb bag of 65 % cal-hypo raises 10,000 gallons by about 8 ppm, and equals about 86 fl oz of 12.5 % liquid chlorine.
How to shock safely
Shock in the evening with the pump running. Dissolve cal-hypo in a bucket of pool water first and pour it near a return; liquid chlorine goes in directly. Do not mix products and never add shock to acid. Dichlor shock adds stabilizer, so it is a poor choice for a SLAM. Keep the level up until the water is clear, the combined chlorine is at or near 0 and the chlorine loss overnight is under 1 ppm; then swim again once free chlorine is below 5 ppm. For routine dosing use the pool chlorine calculator or the liquid chlorine calculator.
SLAM level (40 % of CYA), 10 × combined chlorine breakpoint and dose factors: Trouble Free Pool, Pool School "Shock levels" and "Pool Math"; product labels. Read October 18, 2026.
Frequently asked questions
How much shock to add to a 15,000 gallon pool?
To raise free chlorine by 10 ppm: 30 oz of 65 % cal-hypo (two 1 lb bags) or 160 fl oz of 12.5 % liquid chlorine (1.25 gallons). Shocking to the level your stabilizer needs (40 % of CYA) is the safer rule than one bag.
How much shock should you use in a pool?
Enough to bring free chlorine to the shock level: 40 % of your CYA for a SLAM, at least 10 ppm for a green pool, or 10 times the combined chlorine to clear chloramines. The calculator takes all three inputs and gives the highest.
How much does 1 pound of shock raise chlorine levels?
One pound of 65 % cal-hypo raises free chlorine about 8 ppm in 10,000 gallons, 4 ppm in 20,000 and 2.7 ppm in 30,000. A 73 % product raises it about 9 ppm in 10,000 gallons.
How many gallons of liquid chlorine equal one bag of shock?
One 1 lb bag of 65 % cal-hypo equals about 86 fl oz (0.67 gallon) of 12.5 % liquid chlorine, for the same chlorine boost. It is about 1.4 gallons of 6 % bleach.
How long after shocking can you swim?
Wait until free chlorine falls to 5 ppm or less, usually 8 to 24 hours. After a heavy SLAM wait until the level holds overnight without dropping more than 1 ppm. Test it; the clock is only a guide.