Shock after sunset with the pump running; retest before swimming. Persistent CC above 1 ppm may need a SLAM process, not a single shock.
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The breakpoint chlorination formula
combined chlorine (CC) = total chlorine − free chlorine FC rise needed = 10 × CC fl oz = 10.6 × (12.5 ÷ strength %) × (gallons ÷ 10,000) × rise
"Shock" isn't a product — it's a dose big enough to oxidize the chloramines (combined chlorine) that make water smell and eyes sting. The 10× multiplier is the industry-standard breakpoint ratio.
A worked example
Your kit reads FC 2.0, TC 2.8 in a 15,000-gallon pool: CC is 0.8 ppm, so you need an 8 ppm rise. With 12.5% liquid chlorine: 10.6 × 1 × 1.5 × 8 ≈ 127 fl oz — one full gallon jug. Add it at dusk, run the pump overnight, and CC should read near zero by morning.
When a single shock isn't enough
If CC bounces right back, or the water is green, you're not fighting chloramines — you're fighting algae, and that takes sustained elevated chlorine (a SLAM: Shock Level And Maintain) rather than one dose. The shock level depends on your CYA; test stabilizer before starting.
Frequently asked questions
How much shock do I need for my pool?
For breakpoint chlorination, raise free chlorine by 10× your combined chlorine reading. With CC at 0.6 ppm in a 15,000-gallon pool, that's a 6 ppm rise — about 95 fl oz of 12.5% liquid chlorine.
What is combined chlorine (CC)?
Chlorine that has already reacted with sweat, oils, and other contaminants — it's used up, smells like 'pool chlorine,' and irritates eyes. CC = total chlorine minus free chlorine. Above 0.5 ppm is the usual trigger to shock.
Why 10 times the combined chlorine?
Breakpoint chlorination needs roughly a 10:1 molar ratio of free chlorine to chloramines to fully oxidize them. Underdosing makes the problem worse — you create more chloramines without breaking them down.
Should I shock at night?
Yes — after sunset with the pump running. UV burns off chlorine before it can finish the job, and you want the elevated level held for several hours. Don't swim until FC returns to your normal range.