What shocking does
Shocking means raising free chlorine well above your normal level for a while. You do it for one of two reasons:
- Algae or cloudy water. You need enough chlorine to kill algae faster than it grows, held until the water is clear. Trouble Free Pool calls this SLAM: shock, level and maintain.
- Combined chlorine (chloramines). When total chlorine reads noticeably higher than free chlorine, chlorine has combined with ammonia and sweat. Those combined forms cause the “chlorine smell” and stinging eyes. Raising free chlorine to about 10 times the combined level breaks them apart.
How high to go
For algae, the target depends on stabilizer. CYA holds most chlorine in reserve, so a pool with 60 ppm CYA needs far more free chlorine than one with 20 ppm to hit algae just as hard.
| Stabilizer (CYA) | 20 ppm | 30 ppm | 40 ppm | 50 ppm | 60 ppm | 70 ppm | 80 ppm |
|---|---|---|---|---|---|---|---|
| Shock free chlorine | 10 ppm | 12 ppm | 16 ppm | 20 ppm | 24 ppm | 28 ppm | 31 ppm |
These are the shock levels from the Trouble Free Pool FC/CYA chart, roughly 40% of the CYA level. For combined chlorine, the breakpoint target is:
Breakpoint target = free chlorine + 10 × combined chlorine
Combined chlorine = total chlorine − free chlorine
Worked examples
Green pool: 15,000 gallons, FC 2, CYA 40, 12.5% liquid chlorine
- Shock level for 40 ppm CYA = 16 ppm, so raise by 14 ppm
- Available chlorine = 14 × 56,781 L ÷ 1,000 = 795 g
- Liquid chlorine = 795 ÷ 0.125 = 1.68 gal
Chlorine smell: 15,000 gallons, FC 3, total chlorine 3.8
- Combined chlorine = 3.8 − 3 = 0.8 ppm
- Target = 3 + 10 × 0.8 = 11 ppm, a rise of 8 ppm
- Liquid chlorine = 123 fl oz
What a one-pound bag of shock really does
Bagged cal-hypo shock is usually 65% or 73% available chlorine. One pound of 65% cal-hypo in 10,000 gallons raises free chlorine by about 7.8 ppm. That’s plenty for a routine boost in an unstabilized pool, but well short of a shock level once you have 40 ppm or more of CYA. The label dose of “one bag per 10,000 gallons” often isn’t enough to clear algae.
Liquid chlorine to reach shock level in 15,000 gallons
How to shock
- Test pH first and bring it to about 7.2 to 7.4. Chlorine works noticeably better at lower pH.
- Shock in the evening so sunlight doesn’t burn off the chlorine before it works.
- Pour liquid chlorine slowly in front of a return jet with the pump running. Pre-dissolve cal-hypo in a bucket of pool water.
- For algae, test every few hours and top up to stay at the shock level. Brush the walls and floor daily.
- You’re done when the water is clear, combined chlorine is 0.5 ppm or less, and free chlorine drops no more than 1 ppm overnight.
Test kits using DPD drops or tablets can bleach out above about 10 ppm and read falsely low, even zero, as the CDC notes. Use an FAS-DPD test for shock levels.
If your stabilizer is very high, shock levels become hard to reach. Lowering CYA with the drain and refill calculator first makes the shock far more effective.
Frequently asked questions
How much shock do I need for a green pool?
Enough to reach the shock level for your CYA and hold it there. In 15,000 gallons with 40 ppm CYA that means about 1.68 gal of 12.5% liquid chlorine to start, topped up as the algae uses it.
How many bags of shock for a 10,000 gallon pool?
One 1 lb bag of 65% cal-hypo raises 10,000 gallons by about 7.8 ppm. To reach a shock level of 16 ppm with 40 ppm CYA from 2 ppm, you’d need about 1.8 bags.
Can I use bleach to shock my pool?
Yes. Unscented bleach is the same chemical as pool liquid chlorine, just weaker. Choose bleach strength in the calculator and it adjusts the amount.
How long after shocking can I swim?
Follow the product label. Many people wait until free chlorine has dropped back toward their normal target. For algae treatment, stay out until the water is clear.
Why is my pool still cloudy after shocking?
Usually because the chlorine level wasn’t held long enough. Algae can take several days of steady shock-level chlorine, brushing and filtering. Also check that the filter is clean and running long enough.
Should I use dichlor shock?
It works, but it adds a lot of stabilizer. In the green-pool example, dichlor would add about 13 ppm of CYA, which raises the next shock target. Liquid chlorine is the better choice for a big shock.
Sources
- Understanding the Chlorine/CYA Chart. Trouble Free Pool.
- Home Pool and Hot Tub Water Treatment and Testing. U.S. Centers for Disease Control and Prevention (CDC), reviewed May 10, 2024.
- Model Aquatic Health Code, 5th edition (2024). CDC.
- What increases cyanuric acid (CYA) in a swimming pool?. Orenda Technologies.
Reviewed November 7, 2026. We check the formulas against the sources above and re-run the worked examples whenever we update this page. Found something wrong? Tell us.