Pool Shock Calculator
How much shock to add for your pool size and product, for a routine boost or to clear algae at the right shock level for your stabilizer (CYA), with what each product adds to your water.
To raise free chlorine by 10 ppm in 10,000 gallons you need about 1.3 lb of 65% cal-hypo, 1.5 lb of dichlor, or 0.8 gal of 12.5% liquid chlorine. For algae, hold free chlorine at about 40% of your CYA (16 ppm at 40 ppm CYA) until the water clears.
Shock dose per 10,000 gallons (+10 ppm free chlorine)
| Product | Amount | Side effect |
|---|---|---|
| Cal-hypo shock 65% | 1.28 lb | +7 ppm calcium |
| Cal-hypo shock 73% | 1.14 lb | +7 ppm calcium |
| Dichlor shock 56% | 1.49 lb | +9 ppm CYA |
| Liquid chlorine 12.5% | 0.80 gal | Adds a little salt only |
| Liquid chlorine 10% | 1.00 gal | Adds a little salt only |
| Plain bleach 8.25% | 1.21 gal | Adds a little salt only |
Choosing a shock
- Liquid chlorine adds no stabilizer or calcium, so it's the cleanest choice for regular shocking.
- Cal-hypo stores well but raises calcium hardness; avoid it in hard-water areas.
- Dichlor dissolves fast but raises CYA by about 0.9 ppm for every 1 ppm of chlorine, which makes later shocks weaker.
- Shock at dusk, with the pump running, and keep swimmers out until free chlorine falls below about 10 ppm.
Frequently asked questions
How much shock do I need for my pool?
For a routine shock, raise free chlorine by about 10 ppm. In 10,000 gallons that takes about 1.3 lb of 65% cal-hypo or 0.8 gallon of 12.5% liquid chlorine. Scale it to your volume: twice the water, twice the shock.
How much liquid chlorine to shock a pool?
About 0.8 gallon of 12.5% liquid chlorine per 10,000 gallons raises free chlorine by 10 ppm. For a 20,000 gallon pool that is about 1.6 gallons.
What is the SLAM method?
SLAM (Shock Level And Maintain) means raising free chlorine to a shock level tied to your stabilizer, about 40% of CYA, and holding it there with repeated doses until the water is clear, chlorine holds overnight and the water tests clear for combined chlorine. It is popularized by the Trouble Free Pool community.
How many bags of shock per gallon of water?
One 1 lb bag of 65% cal-hypo raises 10,000 gallons by about 7.8 ppm. Product labels often say "1 bag per 10,000 gallons", which is a light shock; algae needs more.
How long after shocking can you swim?
When free chlorine is back below about 10 ppm, typically the next day for a routine shock. During SLAM, stay out while chlorine is held at shock level.
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Doses use available chlorine by weight for dry products and trade percentage (weight per volume) for liquids: 12.5% liquid supplies 125,000 mg/L. CYA and calcium side effects and the FC/CYA shock ratio follow Trouble Free Pool guidance. Always read the product label and add chemicals separately, never mixed. Volume formulas use 7.48 gallons per cubic foot; round and oval pools use π/4 × 7.48 ≈ 5.9; kidney pools use the industry approximation (A + B) × length × depth × 3.38. Real pools with steps, benches and sloped floors vary by a few percent.