To shock a 20,000 gallon pool with CYA 40, raise free chlorine from 2 to 16 ppm with 3.6 lb (1.6 kg) of 65% cal-hypo shock (about 3.6 one-pound bags). Each 1 lb bag raises free chlorine by 3.9 ppm in 20,000 gallons. The shock level is 40% of your CYA, so test CYA first.
| From FC 2 to | 65% cal-hypo | 73% cal-hypo | Calcium added |
|---|---|---|---|
| CYA 30 → 12 ppm | 2.6 lb | 2.3 lb | +7 ppm |
| CYA 40 → 16 ppm | 3.6 lb | 3.2 lb | +10 ppm |
| CYA 50 → 20 ppm | 4.6 lb | 4.1 lb | +13 ppm |
| CYA 60 → 24 ppm | 5.6 lb | 5 lb | +16 ppm |
| CYA 80 → 32 ppm | 7.7 lb | 6.9 lb | +21 ppm |
Add
3.6lb
of Cal-hypo shock 65%
58 oz1.6 kg
Raises free chlorine from 2 ppm to the shock level of 16 ppm (40% of CYA 40) in 20,000 gal.
Check the product label. Strengths vary by brand, so enter the % printed on yours. Add one chemical at a time, into the water, and never mix chemicals.
Save this test and get reminded when it is time to retest.
Pool Calc for iPhoneComing soon to the App StoreCal-hypo adds calcium: about 0.7 ppm of calcium hardness for every 1 ppm of chlorine. One shock at CYA 40 adds about 10 ppm, which matters in vinyl pools and where the fill water is already hard. If you shock often, 2.8 gal of 10% liquid chlorine does the same job with no calcium.
Dissolve cal-hypo in a bucket of pool water first and pour it in front of a return. Never mix it with any other chlorine product, especially trichlor tablets: the combination can ignite.
A shock (SLAM) is not one dose. Hold free chlorine at the shock level, testing and topping up several times a day, until combined chlorine is 0.5 ppm or less, the overnight loss is 1 ppm or less, and the water is clear.
That is about the size of a 30 ft round × 52 in above-ground pool (about 20,270 gal), or a 16 × 32 ft in-ground pool averaging 5 ft deep (about 19,150 gal). 20,000 gallons is a typical backyard pool. Measure the volume once rather than trusting the builder’s brochure: an error of 20% in gallons makes every dose 20% off. The pool volume calculator takes a minute.
At CYA 40, about 3.6 lb of 65% cal-hypo, from 2 ppm to the 16 ppm shock level. Each 1 lb bag adds 3.9 ppm.
Swim when free chlorine is back within your normal range and at or below the re-entry level on your chlorine label (many labels say 4 ppm). Never swim during a SLAM or with free chlorine above 10 ppm. Your product label always comes first.
Both are chlorine. Liquid chlorine adds no calcium or stabilizer, so it suits repeated shocking. Cal-hypo is easier to store but raises calcium hardness.