Pool Shock Formula
The Formula
| Symbol | Description | Unit |
|---|---|---|
Target FC | Target free chlorine level. For breakpoint chlorination: 10× combined chlorine. For maintenance: 10 ppm. For algae: 30 ppm. | ppm |
Current FC | Current free chlorine in ppm | ppm |
Volume | Pool volume in gallons | gallons |
Available Cl% | Available chlorine % of the shock product (e.g. 65 for cal-hypo, 56 for dichlor) | % |
800 | Conversion constant: 1 lb of 100% chlorine raises 10,000 gal by 120 ppm | constant |
Worked Example
Pool: 20,000 gallons. Current FC: 1 ppm. Combined Chlorine: 0.5 ppm. Breakpoint target = 10 × 0.5 = 5 ppm. Using 65% cal-hypo.
- Target FC = 5 ppm
- Increase needed = 5 − 1 = 4 ppm
- Numerator = 4 × 20,000 = 80,000
- Denominator = 65 × 800 = 52,000
- Dose = 80,000 ÷ 52,000 = 1.54 lbs
You need approximately 1.5 lbs of 65% calcium hypochlorite.
How This Formula Works
Shock treatment raises free chlorine quickly to a high level to eliminate combined chlorine (CC), algae, or pathogens. The required dose depends on the shock product's concentration, the pool volume, and how high the FC must be raised.
- Breakpoint chlorination requires raising FC to at least 10× the combined chlorine level.
- Maintenance shock raises FC to 10 ppm as a routine preventive treatment.
- Algae recovery typically requires 20–30 ppm FC.
- Calcium hypochlorite (cal-hypo) comes in 65% and 73% available chlorine strengths.
Limitations & Notes
Calcium hypochlorite increases calcium hardness — significant doses can raise hardness noticeably, especially in smaller pools or spas. Using cal-hypo repeatedly in a pool with already-high calcium can contribute to scaling. Consider sodium hypochlorite (liquid chlorine) for routine shocking in pools with high calcium hardness.
Last reviewed: 2026-06-01