Pool Shock Treatment Calculator

What “shocking” actually means. Shock = raising free chlorine high enough to oxidize chloramines (combined chlorine, CC) and break them apart. The standard target is 10 × CC. If your CC is 1.0 ppm, your FC needs to hit 10 ppm to break through.

Why FC × 10 and not just “add a pound of cal-hypo”. Below the breakpoint threshold, chloramines are stable. They cause the chlorine smell people blame on chlorine itself — it is actually CC, not chlorine. The only way to remove them is hit breakpoint and hold there until the pool consumes the excess FC.

The two common shock approaches:

1. Targeted breakpoint shock — measure CC, multiply by 10, that is your target FC. Calibrated and uses less product.

2. Flat shock — add enough cal-hypo for a 10 ppm FC raise regardless of CC. Simpler, but uses more product than necessary on a clean pool.

This calculator does both. Pick a mode at the top.

Cal-hypo vs liquid for shock. Cal-hypo 65-73% is the standard shock product. It is granular, dissolves quickly, and one pound treats roughly 8-10 ppm in 10,000 gallons. Liquid chlorine works too — about a half gallon of 12.5% per 10 ppm raise per 10,000 gallons.

Worked example. 20,000 gallons, FC 3, CC 0.8.

• Breakpoint target = 0.8 × 10 = 8 ppm
• Raise needed = 8 - 3 = 5 ppm
• Cal-hypo 65%: 2.05 oz × 5 × (20,000 / 10,000) = 20.5 oz = 1.28 lb

Add it gradually over the surface (or pre-dissolve in a 5-gal bucket if your CH is borderline high). Pump runs the whole time. Recheck at 30 minutes and again at 4 hours — you want FC to drop back to normal range overnight before swimming.

When shock will not work. If your CYA is over 100, chlorine is so neutered that you cannot reach effective shock levels with normal doses. Partial drain and refill is the only fix at that point. Same is true for ammonia contamination — that takes massive doses and is why opening a green pool sometimes needs three shock cycles in a week.