“Shocking” a pool isn't a single dose — it's a math problem. If you don't calculate the breakpoint correctly, you're not shocking. You're just adding chlorine that will bond with more contaminants and make combined chlorine worse.
The 10× rule
Breakpoint = 10 × Combined Chlorine (CC)
To eliminate chloramines, you must raise free chlorine to at least 10 times the combined chlorine reading, all at once. Less than 10× and you form more mono/di/trichloramine instead of destroying it. This is the chemistry that distinguishes a real shock from a wasted bag.
Worked example
Pool: 55,000 gallons.
Test: FC 1.2 ppm, CC 0.3 ppm.
Breakpoint target: 10 × 0.3 = 3.0 ppm FC.
FC needed: 3.0 − 1.2 = 1.8 ppm increase.
Cal-hypo (65%): dose factor 0.21 oz per 10,000 gal per ppm.
Dose: 1.8 × (55,000 ÷ 10,000) × 0.21 = 1.8 × 5.5 × 0.21 = ~2.1 oz cal-hypo
That's a small real-world dose because CC was low. Run the same math with CC of 1.0 and you need roughly 20× more product.
Algae-removal SLAM math
For algae removal, you don't use the 10×CC rule — you use the FC/CYA ratio. Target FC is roughly 40% of your CYA level and maintained there until the pool passes the three SLAM completion tests:
- Combined chlorine < 0.5 ppm
- Overnight chlorine loss < 1 ppm
- Water is crystal clear
When non-chlorine shock is the right call
Monopersulfate (MPS, also called non-chlorine shock) oxidizes organic contaminants without adding chlorine. Use it when:
- You want to clear a pool for same-day swim (MPS has a short waiting period)
- FC is already high and you don't want to push it higher
- You're treating a PHMB (biguanide) pool
MPS does NOT replace chlorine for sanitation — it just handles oxidation. You still need a chlorine or bromine residual.
Why partial shocking makes it worse
If CC is 1 ppm and you add enough chlorine to get FC to 5 ppm (not 10 ppm), the chlorine bonds with more ammonia and organics, creating additional chloramines. Combined chlorine climbsinstead of dropping. This is why “I shocked it and it got worse” is a real operator experience — they didn't actually shock, they dosed.