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, which is exactly backwards: that smell means you need more chlorine, not less. The only way to remove them is hit breakpoint and hold there until the pool consumes the excess FC.

Three ways to pick a target, and they are not equally good:

1. SLAM level, 40% of your CYA. The one to use if you are clearing algae. Chlorine’s strength depends on how much stabilizer is holding it, so the level that actually kills things scales with CYA. At CYA 40 that is 16 ppm; at CYA 80 it is 32.

2. Breakpoint, 10 × CC. The classic formula, and the right one when the problem is chloramine smell rather than algae. It aims to oxidise the combined chlorine and nothing more, so it uses less product.

3. Flat shock, a 10 ppm raise. Simple, and fine on a low-CYA pool. In a pool at CYA 80 it is close to useless, because 10 ppm is barely above where that pool should sit on an ordinary Tuesday.

That last point is worth dwelling on. The old advice to “shock with 10 ppm” was written when nobody ran much stabilizer. Apply it to a modern trichlor-fed pool at CYA 90 and you will pour in chlorine, watch nothing happen, and conclude that shocking does not work. The calculator will warn you when the mode you picked aims below the SLAM level for the CYA you entered.

Cal-hypo vs liquid for shock. Cal-hypo 65-73% is the standard shock product. It is granular, dissolves quickly, and one pound raises 10,000 gallons by about 8 ppm at 65% strength or 8.7 ppm at 73%. Liquid chlorine works too, at about 107 fluid ounces of 12.5% per 10 ppm raise per 10,000 gallons, which is a little under a gallon.

Cal-hypo is the cheaper way to move a lot of chlorine, but it drags calcium in with it: roughly 0.7 ppm of calcium hardness for every 1 ppm of FC. Shock a pool from 3 to 20 ppm and you have added about 12 ppm of hardness in an afternoon. Do that a dozen times a season in already-hard water and the scale on the tile is your own doing. In hard-water areas, shock with liquid.

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 back in its normal range overnight before anyone swims.

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. Ammonia contamination behaves the same way. It takes enormous doses, which is why opening a green pool sometimes needs three shock cycles in a week.