The quick answer: shock isn't a number of bags. It's a chlorine level. For a stabilized outdoor pool, the shock level is free chlorine (FC) equal to 40% of your cyanuric acid (CYA).
Example: with CYA at 40 and FC at 2, you need to raise FC to 16 ppm. For 15,000 gallons, that is about 269 fl oz (2.1 gal) of 10% liquid chlorine. You then hold that level until the pool passes three tests.
Enter your numbers below for the dose with your product.
Why "one bag per 10,000 gallons" fails
A standard 1 lb bag of 65% cal-hypo adds about 7.8 ppm of chlorine to 10,000 gallons. That's close to the 10 ppm shock level of an unstabilized pool. In an outdoor pool with 50 ppm CYA, though, the shock level is 20 ppm. One bag gets you under halfway, the chlorine is used up overnight, and the algae is back in two days.
The fix is to dose to the level your CYA requires, and keep it there.
The shock level for your CYA
| CYA (ppm) | Shock (SLAM) FC |
|---|---|
| 0 | 10 |
| 30 | 12 |
| 40 | 16 |
| 50 | 20 |
| 60 | 24 |
| 70 | 28 |
| 80 | 32 |
Above about 80–90 CYA, reaching and holding the shock level gets expensive and slow. It's usually better to lower CYA by partial drain-and-refill first. See cyanuric acid too high.
Don't know your CYA? Test it first. Everything depends on it.
How much product per ppm
To raise FC by 1 ppm in 10,000 gallons:
| Product | Amount | Side effect per 1 ppm FC |
|---|---|---|
| Liquid chlorine 12.5% | 10.2 fl oz | none significant |
| Liquid chlorine 10% | 12.8 fl oz | none significant |
| Household bleach 6% (unscented) | 20.6 fl oz | none significant |
| Cal-hypo 65% | 2.05 oz (58 g) | +0.7 ppm calcium hardness |
| Cal-hypo 73% | 1.83 oz (52 g) | +0.7 ppm calcium hardness |
| Dichlor 56% | 2.38 oz (68 g) | +0.9 ppm CYA |
| Trichlor 90% | 1.48 oz (42 g) | +0.6 ppm CYA, lowers pH |
To find your dose, multiply by the ppm you need and by (your gallons ÷ 10,000). In metric, 1 ppm in 10,000 litres = 10 g of available chlorine, which is 100 mL of 10% liquid chlorine.
Which to use for shocking:
- Liquid chlorine is the best choice for a multi-day SLAM, because it adds nothing you'll regret.
- Cal-hypo is fine if your calcium hardness is low. A SLAM can use 50–100 ppm or more of FC in total, which would add 35–70 ppm or more of calcium.
- Avoid dichlor and trichlor for anything beyond a one-off top-up. A SLAM with dichlor can add dozens of ppm of CYA, which raises your shock level even further: the problem feeds itself.
- Non-chlorine shock (MPS) oxidizes, but it does not kill algae. It's fine for a quick post-party boost in a clean pool. It can't fix green water.
How to SLAM (Shock Level And Maintain)
The idea behind SLAM is not to hit the shock level once, but to hold it until the problem is gone.
- Test CYA, FC and pH. Bring pH to about 7.2–7.4 first if it's high; that's easier to manage during a SLAM.
- Raise FC to the shock level for your CYA, using the calculator.
- Brush the whole pool, run the filter 24/7, and clean or backwash the filter whenever pressure climbs.
- Retest FC several times a day, including late evening, and top it back up to the shock level each time. A green pool can use a lot of chlorine in the first few days. That's expected.
- Keep going until all three conditions are met:
- Combined chlorine (CC) is 0.5 ppm or less.
- The pool passes an overnight chlorine loss test (OCLT): it loses 1 ppm or less of FC from dusk to dawn.
- The water is clear, with the main drain visible.
- Then stop adding and let FC fall back to your normal target. 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.
Depending on how bad it was, a SLAM can take a weekend or two weeks. Stopping early is the most common reason it fails.
When should you shock?
- The water is green, cloudy or slimy (algae). See green pool fix.
- Overnight FC loss is more than 1 ppm. This is invisible early algae.
- Combined chlorine stays above 0.5 ppm despite good FC levels.
- Opening the pool after winter.
- After something unpleasant (a fecal accident, a dead animal), follow public-health guidance for raised chlorine levels.
You don't need to shock weekly "just because". If you keep FC above the minimum for your CYA every day, routine shocking is mostly unnecessary.
Common mistakes
- Shocking at noon. Add chlorine at dusk so the sun doesn't burn it off before it works.
- Shocking without knowing CYA. You can't know the target.
- Using stabilized shock repeatedly. CYA climbs, so your shock level climbs too.
- Adding algaecide instead of chlorine. Algaecide can help prevent algae; it doesn't replace reaching the shock level.
- Testing pH at shock-level chlorine. High FC distorts phenol red pH readings.
SLAM with Pool Calc
Start a SLAM in Pool Calc and the app:
- sets the target to 40% of your latest CYA;
- calculates each top-up dose for the product on your shelf;
- reminds you to retest;
- tracks the three conditions (CC, overnight loss and clarity), and tells you when you're done.
One pool? Every feature, free. No account, no ads, and iCloud backup is included.
Safety
- Keep chemicals out of reach of children and pets, and keep them out of the water until it is back in range.
- Never mix chemicals. Never add acid and chlorine together: mixing them releases toxic chlorine gas.
- Add chemicals to water, never water to chemicals.
- Store chemicals sealed, cool and dry, apart from each other.
- Read and follow the product label. It always comes first.