How to shock a pool, including a green one
Shocking means raising free chlorine to a much higher level than usual and holding it there until algae and contaminants are gone. The key to doing it well is choosing the right level for your stabilizer, and keeping it there until the water is genuinely clean, not just one big dose.
When to shock
- The water is green, cloudy or slimy, or algae is visible on walls or in corners.
- Combined chlorine (total chlorine minus free chlorine) is above about 0.5 ppm. A strong "chlorine" smell usually means chloramines, not too much chlorine.
- Free chlorine drops by more than 1 ppm overnight, when there's no sun. That suggests something in the water, often early algae, is using it up.
- When opening the pool after winter, and after a heavy rainstorm, a pool party or a contamination incident. For fecal or vomit contamination in pools, follow the CDC's specific contamination response steps.
Choose the shock level
Because stabilizer (CYA) holds most chlorine in reserve, the shock level should scale with CYA. A commonly used guideline sets it at about 40% of the CYA: 16 ppm at 40 ppm CYA, 20 ppm at 50 ppm, 32 ppm at 80 ppm. The shock calculator does the math and gives the dose of your product. Free chlorine and stabilizer explained covers the reasoning.
If CYA is very high (above about 80–100 ppm), the shock level becomes impractical. Lower CYA by partly draining and refilling first, then shock.
Which product to use
| Product | Good for shocking? | Notes |
|---|---|---|
| Liquid chlorine (10–12.5%) or plain bleach | Yes, best for repeated doses | Adds nothing that builds up except a little salt. Buy it fresh: it weakens in storage. |
| Cal-hypo (65–73%) | Yes | Adds about 0.7 ppm calcium per ppm of chlorine. Fine for soft water, less so for hard water. |
| Dichlor | Once, at low CYA | Adds about 0.9 ppm CYA per ppm of chlorine, so repeated shocks keep raising the shock level needed. |
| Trichlor | No | Dissolves too slowly and adds CYA. |
| Non-chlorine shock (MPS) | Not for algae | Oxidizes waste but doesn't sanitize or kill algae. |
Step by step
- Test. Measure free chlorine, combined chlorine, pH and CYA. If you can, use a drop test kit that can read high chlorine levels (see testing guide). Many test strips and standard kits can't read shock-level chlorine.
- Lower pH to about 7.2–7.4. Chlorine is more effective at lower pH. The pH calculator gives the acid dose.
- Clean what you can. Scoop out leaves and debris, brush walls, floor and steps, and clean or backwash the filter.
- Add chlorine to reach the shock level, preferably at dusk so sunlight doesn't burn it off. Pour liquid chlorine slowly in front of a return jet with the pump running. Pre-dissolve granular products in a bucket of pool water unless the label says otherwise.
- Hold the level. Retest every few hours while you're awake, and add chlorine to bring it back to the shock level. A green pool can use up a lot of chlorine in the first day or two.
- Brush daily, run the pump continuously and clean the filter whenever its pressure rises noticeably.
When are you done?
Stop shocking when all three are true:
- The water is clear: you can see the main drain clearly.
- Combined chlorine is 0.5 ppm or less.
- The pool loses 1 ppm of free chlorine or less overnight. Test at dusk after the sun is off the water, and again before sunrise. This "overnight chlorine loss test" is a good sign that no algae is left using up chlorine.
Stopping early, as soon as the water turns from green to blue-cloudy, is the most common reason algae comes back within days.
Clearing a green pool: what to expect
A light green pool may clear in a day or two. A dark green or black "swamp" can take a week or more of holding shock level, plus a lot of filter cleaning. The water usually goes from green to gray or cloudy blue as algae dies, then clears gradually as the filter removes the debris. A flocculant or clarifier can help a cartridge or sand filter that's struggling, but follow its label closely.
If the cost of the chlorine is getting high, check CYA. A pool at 100 ppm CYA needs 40 ppm free chlorine to shock, so draining part of the water first is often cheaper.
Frequently asked questions
How long after shocking can I swim?
Once free chlorine has dropped back to a normal level for your CYA and below your chlorine product's label limit for swimming. That's often the next day for a routine shock, and longer while clearing algae.
Should I shock every week?
Not if chlorine stays at the right level for your CYA. Pools kept properly chlorinated rarely need a full shock. A weekly "boost" is mainly a fix for chlorine levels that were too low.
Why is my pool still cloudy after shocking?
Dead algae and particles take time to filter out. Keep the pump running, clean the filter and hold chlorine until the overnight loss test passes. If it stays cloudy for days, see why is my pool cloudy.