How to Shock a Pool: Step by Step
“Shocking” a pool simply means raising free chlorine well above its normal level for long enough to destroy whatever is using it up: algae, bacteria, or the chloramines that cause the classic “chlorine smell”. Done right, it clears a green pool in days. Done halfway, the algae comes straight back. This guide covers when to shock, how much chlorine you really need, and how to know you are finished.
When to shock a pool
- Combined chlorine is 0.5 ppm or more. Combined chlorine is total chlorine minus free chlorine. It is the chlorine already tied up with contaminants.
- You can see algae or the water is cloudy. Even a faint green tint or slippery walls means algae is growing.
- After a heavy swim party, a big storm, or anything that dumps organic matter into the pool.
- When opening the pool after winter.
- When chlorine keeps disappearing overnight even though the sun is down. That is a sign something in the water is consuming it.
How much shock: it depends on your CYA
The most common mistake is adding “one bag of shock” regardless of the pool. Stabilizer (CYA) binds part of the chlorine, so the more CYA you have, the higher the chlorine level must be to work. The SLAM method (Shock Level And Maintain), developed by the Trouble Free Pool community, sets the target at about 40% of your CYA level:
| CYA | Shock FC | 12.5% liquid | 65% cal-hypo |
|---|---|---|---|
| 0 ppm | 10 ppm | 2.24 qt | 14.4 oz |
| 30 ppm | 12 ppm | 2.88 qt | 1.16 lb |
| 40 ppm | 16 ppm | 1.04 gal | 1.67 lb |
| 50 ppm | 20 ppm | 1.36 gal | 2.18 lb |
| 60 ppm | 24 ppm | 1.68 gal | 2.7 lb |
| 80 ppm | 32 ppm | 2.32 gal | 3.72 lb |
Per 10,000 gallons, starting from 3 ppm free chlorine. Computed with the same formulas as the shock calculator.
The pool shock calculator does this for your exact volume, current chlorine and product. If your CYA is above about 80 ppm, the shock level gets very high; it is often cheaper to lower CYA first with a partial drain using the CYA calculator.
Which product to use
- Liquid chlorine (sodium hypochlorite, 10–12.5%): the best choice for most pools. It adds nothing but chlorine and a little salt, mixes instantly and is easy to dose in large amounts.
- Plain household bleach (6–8.25%): the same chemical, weaker. Fine if it is unscented and has no thickeners.
- Cal-hypo (65–73%): strong and convenient, but each dose adds calcium. Pre-dissolve it in a bucket of pool water.
- Dichlor: adds almost as much CYA as chlorine, so it is a poor choice for repeated shocking.
- Trichlor tablets: dissolve too slowly to reach shock levels and add stabilizer. Do not use them to shock.
Do not mix different chlorine products, dry or dissolved. Some combinations, such as cal-hypo and trichlor, can react violently.
Step by step
- Test. Measure free chlorine, combined chlorine, pH and CYA. A drop-based test kit is far more reliable than strips at high chlorine levels.
- Lower pH to about 7.2–7.4 if it is higher. Chlorine is noticeably more effective at lower pH. Use the pH calculator.
- Brush and clean. Brush walls, steps and floor, empty the skimmer and pump baskets, and clean or backwash the filter. Algae hides in corners and under ladders.
- Shock in the evening. Sunlight destroys chlorine, so adding it at dusk keeps more of it working overnight.
- Pour slowly in front of a return jet with the pump running, walking around the pool if it is large.
- Maintain the level. This is the “maintain” in SLAM. Retest every few hours while you are awake and top up whenever free chlorine drops below the target. Letting it fall back gives the algae time to recover.
- Keep the pump running and clean the filter whenever pressure rises; dead algae clogs it quickly.
- Check if you are done with the three tests below.
How to know shocking is finished
You can stop holding the shock level when all three are true:
- The water is clear and you can see the main drain.
- Combined chlorine is 0.5 ppm or less.
- The overnight chlorine loss test passes: measure free chlorine after sunset and again before sunrise. A loss of 1 ppm or less means nothing in the water is consuming chlorine anymore.
After that, let chlorine drop back into its normal range for your CYA level and resume normal dosing with the chlorine calculator.
Common shocking mistakes
- Stopping too early. The water turns blue before the algae is fully gone. If you stop then, it comes back within days.
- Ignoring CYA. A bag of shock that would clear a pool at CYA 30 does very little at CYA 90.
- Shocking at midday. Much of the chlorine is lost to sunlight before it does any work.
- Using the wrong product. Repeated dichlor shocks push CYA up and make the next shock harder.
- Skipping the filter. Dead algae has to go somewhere. Clean the filter often during a shock.
Is it safe to swim after shocking?
Wait until free chlorine has dropped back to your normal range and always retest first. With liquid chlorine and normal stabilizer that is often the next day, but large SLAM processes can take several days. Follow the product label.