Pool Shock Formula

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.
• The dosing math is identical in kind to liquid chlorine (formula-02) — the same mass of 100%-available chlorine is needed per ppm per gallon regardless of whether the product is liquid or dry; only the available-chlorine percentage changes. At 65% cal-hypo this works out to about 2.05 oz per 10,000 gallons per 1 ppm, consistent with the Pool & Hot Tub Alliance's own "Water Chemistry Adjustment Guide" (2 oz per 10,000 gal per 1 ppm for 67% cal-hypo) and this site's own dosage-matrices dataset (2.0 oz for 65%, 1.8 oz for 73%).

**As of Phase 7W, this equation is directly implemented** by the Pool Shock Calculator, Hot Tub Shock Calculator, and the granular-chlorine options on the All-in-One Chemical Calculator: the user selects a specific product (liquid chlorine, calcium hypochlorite, sodium dichlor, or trichlor tablets), and the calculator applies this formula using that product's dataset-verified available-chlorine percentage. This replaces a prior generic, unsupported divisor that corresponded to no real product (see Phase 7T's SHOCK-DIVISOR-AUDIT.md).

**What the live calculators do NOT compute:** a breakpoint-chlorination dose (10× combined chlorine) -- the preset ppm values (5/10/15/20) are flat target-FC-increase amounts a user chooses directly, not a value derived from a measured combined-chlorine reading, and the calculators do not collect combined chlorine as an input. A "generic/unspecified product" option is also not offered as a numeric calculation -- Phase 7T/7U established that no single divisor is defensible across products, so users who do not know their product receive qualitative guidance only, with no ounce or pound figure.

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. Sodium dichlor and trichlor add cyanuric acid with repeated use; trichlor is also acidic. Calcium hypochlorite must not be mixed with trichlor or other chlorinating agents (mixing hazard). The 0.013344 constant assumes 8.34 lb/gal water density and treats the product's labeled available-chlorine percentage as active-ingredient-by-weight; PHTA's own Calcium Hypochlorite fact sheet notes exact dosage depends on the specific product and directs users to the product label for precise amounts — treat this as a starting dose, not an exact figure. A generic/unspecified-product dose remains REQUIRES_EXPERT_REVIEW: no single divisor is defensible across products (Phase 7T `SHOCK-DIVISOR-AUDIT.md`), which is why the live calculators require a specific product selection for a numeric result. Breakpoint-chlorination dosing (10× combined chlorine) is a distinct, unimplemented use case -- see Phase 7T `SHOCK-ARCHITECTURE-AUDIT.md` and Phase 7U `SHOCK-ARCHITECTURE-DECISION.md`.