How much shock for a 25,000 gallon pool?
To shock a 25,000 gallon pool from 1 ppm up to the 16 ppm algae-killing level (CYA 40), add 3 gal of 12.5% liquid chlorine or 4 lb 13 oz of 65% cal-hypo. For a routine weekly shock that raises free chlorine 5 ppm, use a third of that.
By the label: most cal-hypo shock says 1 lb per 10,000 gallons, which is 2 lb 8 oz for 25,000 gallons. Liquid chlorine labels usually say 1 gallon per 10,000, which is 2.5 gallons. Those raise free chlorine about 8 ppm and 12 ppm, enough for a weekly shock but not always for a green pool.
Shock doses for 25,000 gallons
| Product | Routine shock (+5 ppm) | Cloudy water (+10 ppm) | Green pool, CYA 40 (+15 ppm) |
|---|---|---|---|
| Liquid chlorine 12.5% | 1 gal | 2 gal | 3 gal |
| Liquid chlorine 10% | 1 gal 32 fl oz | 2 gal 64 fl oz | 3 gal 96 fl oz |
| Household bleach 6% | 2 gal 11 fl oz | 4 gal 21 fl oz | 6 gal 32 fl oz |
| Cal-hypo 65% | 1 lb 10 oz | 3 lb 3 oz | 4 lb 13 oz |
| Cal-hypo 73% | 1 lb 7 oz | 2 lb 13 oz | 4 lb 4 oz |
| Dichlor 56% | 2 lb 4 oz | 4 lb 7 oz | 6 lb 11 oz |
Why CYA decides the dose
Stabilizer binds chlorine, so the level that kills algae is about 40% of your CYA reading. At CYA 40 that's 16 ppm; at CYA 80 it's 32 ppm and the doses above double. If you don't know your CYA, test it before shocking a green pool, or you'll underdose and do it all again tomorrow.
Which product
Liquid chlorine adds nothing but chlorine. Cal-hypo raises calcium hardness about 7 ppm per pound per 10,000 gallons, and dichlor raises CYA about 9 ppm per pound, which pushes your future shock level up. For a green pool buy liquid.
How to add it
At dusk, pump running, poured slowly in front of a return. Leave the pump on overnight and retest in the morning. A green pool needs the dose repeated each evening until free chlorine holds overnight.
Other pool sizes
Shock for a 5,000, 8,000, 10,000, 12,000, 15,000, 18,000, 20,000, 30,000 gallon pool. Not a round number? Use the shock calculator with your exact volume, or find it with the volume calculator.
Other chemicals for 25,000 gallons
How much chlorine for a 25,000 gallon pool, How much salt for a 25,000 gallon pool, How much baking soda for a 25,000 gallon pool.