How much shock for a 25,000 gallon pool

To shock a 25,000 gallon pool with CYA 40, raise free chlorine from 2 to 16 ppm with 4.5 lb (2 kg) of 65% cal-hypo shock (about 4.5 one-pound bags). Each 1 lb bag raises free chlorine by 3.1 ppm in 25,000 gallons. The shock level is 40% of your CYA, so test CYA first.

Shock doses for 25,000 gallons by CYA level

25,000 US gallons (94,635 L).
From FC 2 to	65% cal-hypo	73% cal-hypo	Calcium added
CYA 30 → 12 ppm	3.2 lb	2.9 lb	+7 ppm
CYA 40 → 16 ppm	4.5 lb	4 lb	+10 ppm
CYA 50 → 20 ppm	5.8 lb	5.1 lb	+13 ppm
CYA 60 → 24 ppm	7.1 lb	6.3 lb	+16 ppm
CYA 80 → 32 ppm	9.6 lb	8.6 lb	+21 ppm

Run it with your numbers

Add

4.5lb

of Cal-hypo shock 65%

72 oz2 kg

Raises free chlorine from 2 ppm to the shock level of 16 ppm (40% of CYA 40) in 25,000 gal.

Side effects: CH +9.9 ppm.
Pour it slowly in front of a return jet with the pump running.
Dissolve it in a bucket of pool water first, then pour it in front of a return jet.
Hold free chlorine at 16 ppm: retest and top up at least morning, midday and evening.
You are done when combined chlorine is 0.5 ppm or less, the overnight chlorine loss is 1 ppm or less, and the water is clear.
No swimming during the SLAM. 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.
Repeated shocks with this product build up its side effect. Liquid chlorine adds nothing but chlorine and a little salt.

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.

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About shock

Cal-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, 3.5 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.

Dosing a 25,000 gallon pool

That is about the size of an 18 × 36 ft in-ground pool averaging 5 ft deep (about 24,240 gal). In a 25,000 gallon pool doses run to gallons and bags, so pour or broadcast them while walking along the deep end or in front of several returns rather than in one spot. Larger pools take longer to mix: give the pump at least an hour before you retest.

Questions

How many bags of shock for a 25,000 gallon pool?

At CYA 40, about 4.5 lb of 65% cal-hypo, from 2 ppm to the 16 ppm shock level. Each 1 lb bag adds 3.1 ppm.

Can I swim after shocking?

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.

Is liquid chlorine better than shock?

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.