PooloraHow to shock a pool
Shocking means raising free chlorine well above its normal level for a while, to burn off contaminants, combined chlorine and algae. The label sets how high you can go.
- Shock when combined chlorine is above 0.4 ppm, after heavy use or storms, or when algae shows up.
- In 10,000 gallons, going from 2 to 10 ppm takes about 12.75 cups (102 fl oz · 3.05 L) of 10% liquid chlorine.
- The labels we checked cap superchlorination at 10 ppm. Swim again only when chlorine is back in the label's swimming range.
When to shock
- Combined chlorine above 0.4 ppm (the ANSI/APSP/ICC-11 limit): the "chlorine smell" and red eyes come from it.
- After a pool party, a storm or a lot of debris.
- Cloudy water with low chlorine, or the first signs of algae.
How much shock to reach 10 ppm
The sodium hypochlorite labels we checked allow superchlorination at 5 to 10 ppm. Amounts to reach 10 ppm in 10,000 gallons:
| Current FC | Liquid chlorine 10% | Liquid chlorine 12.5% | Bleach 6% | Cal-hypo 65% |
|---|---|---|---|---|
| 1 ppm | 14.5 cups 3.4 L | 11.5 cups 2.75 L | 1.5 gal 5.5 L | 1.2 lb 525 g |
| 2 ppm | 12.75 cups 3.05 L | 10.25 cups 2.4 L | 1.3 gal 4.9 L | 1 lb 465 g |
| 3 ppm | 11.25 cups 2.65 L | 9 cups 2.1 L | 1.1 gal 4.3 L | 14.5 oz 410 g |
For other sizes see the pool chemical charts by size. If you are fighting algae and your stabilizer is above 25 ppm, 10 ppm is not enough; read how to fix a green pool first.
Which shock to choose
| Product | What else it adds | Good for |
|---|---|---|
| Liquid chlorine or bleach | Nothing else | Every shock; the best choice for algae |
| Cal-hypo | Calcium: about 7.1 ppm per 10 ppm of chlorine | Occasional shocks if calcium is low |
| Dichlor | Stabilizer: about 9.1 ppm CYA per 10 ppm of chlorine | Rarely; repeated use pushes CYA up |
| Non-chlorine shock (potassium monopersulfate) | No chlorine | Burning off contaminants; it does not kill algae |
What a 1 lb bag of shock really does
Many bags say one pound treats 10,000 gallons. Here is what that pound does in 10,000 gallons, according to the engine: 65% cal-hypo raises free chlorine about 7.8 ppm (and calcium about 5.5 ppm); 56% dichlor raises it about 6.7 ppm and adds about 6.1 ppm of stabilizer. So "one bag" is not a fixed result: it depends on where your chlorine starts and on your pool's real volume.
How to do it
- Test free chlorine, pH and stabilizer. Bring pH into 7.2–7.8 first if needed.
- Add shock in the evening, when the sun will not burn it off, with the pump running. Pour liquids in front of a return; pre-dissolve cal-hypo in a bucket of water.
- Keep everyone out while you add it and until chlorine is back in the swimming range.
- Retest the next morning. If combined chlorine is still above 0.4 ppm, repeat.
When it is safe to swim
Your label decides, not a clock. The labels we checked say, for example, do not re-enter until chlorine is between 1.0 and 3.0 ppm (a 10% product) or not above 4 ppm (a 12.5% product). Test before anyone swims.
Common questions
Can I shock during the day?
You can, but sunlight burns chlorine, especially with low stabilizer. Evening doses last longer.
How often should I shock?
Only when there is a reason: combined chlorine above 0.4 ppm, heavy use, a storm or algae. Keeping free chlorine at the right level for your stabilizer every day is what prevents most shocks.
Do it with your own products
Poolora turns your test into the exact amount of the products on your shelf, in cups and ounces, in the right order, and tells you when it's safe to swim. Free to start; every chemical, water balance and history are part of Pro.
Keep going
Sources
- U.S. EPA registered labels for sodium hypochlorite pool products (Reg. No. 70271-10 and 82357-1): swimming re-entry range and 5–10 ppm superchlorination
- ANSI/APSP/ICC-11 (combined chlorine 0.4 ppm or less)
- CDC: Home pool and hot tub water treatment and testing
- Doses: Poolora engine (mass balance for chlorine; carbonate-equilibrium model for pH with constants from the USGS PHREEQC database)