How to shock a pool (with calculator)
To shock a pool, use a product labeled for shocking pools and add the amount its label gives for your pool size. Labels usually give an amount per 10,000 gallons, so a 15,000-gallon pool needs 1.5 times that amount.
Always follow your product’s label directions. Pool and spa chlorine, bromine, shock and algaecides are EPA-registered pesticides: use products labeled for pool or spa use, and don’t swim or soak until levels are within the limit on your product’s label. If this page and a label disagree, the label wins.
On this page (8 sections)
The calculator scales your label's amount to your gallons. It doesn't set a chlorine level for the shock: the label does, because it is the legal limit for the product you use.
Calculator: your label's shock dose for your pool
The calculator above needs a few numbers. Each one matters, which is why "one bag per 10,000 gallons" is only a starting point:
- Pool volume. Every dose scales with gallons. Not sure? Run the pool volume calculator once and save the number.
- Free chlorine (FC) and total chlorine (TC). From a fresh test. The difference is combined chlorine (CC), the main reason to shock a clear pool. At 0.5 ppm CC or more, shock per your label.
- Your product's label. Read the shock directions: the amount, the gallons it treats, and when the pool can be used again. Liquid chlorine (10% or 12.5%), calcium hypochlorite (cal-hypo, 65–78%), and dichlor (56–62%) contain very different amounts of chlorine, so never use the dose from one product for another.
"Shock" here means adding a larger dose of chlorine to break down combined chlorine (chloramines) and kill algae, then letting the level fall back. How high the level goes and how often you repeat it are set by your product's label, not by us. Some water-care methods, such as SLAM, aim for chlorine levels above the swim limit on many labels; we don't give those numbers. See the SLAM method, explained.
What one bag adds in different pool sizes
This table shows why one bag can't fit every pool. It is the rise in free chlorine from one 1-lb bag, from our engine's formulas. It is not a dose recommendation: follow your label.
| Product | 10,000 gal | 15,000 gal | 20,000 gal |
|---|---|---|---|
| Cal-hypo 65% | 7.8 | 5.2 | 3.9 |
| Dichlor 56% | 6.7 | 4.5 | 3.4 |
Side effects: cal-hypo adds about 0.7 ppm calcium hardness (CH) per 1 ppm of chlorine; dichlor adds about 0.9 ppm stabilizer (CYA) per 1 ppm (PHTA). Estimates, not label directions. Always follow the product label.
Look at the dichlor row. Each 1-lb bag of dichlor in 10,000 gallons also adds about 6 ppm of stabilizer, and stabilizer only leaves with water you drain. That is one reason many owners shock with liquid chlorine or cal-hypo and keep dichlor for small top-ups. The guide to chlorine types compares all five forms.
Step by step: test, balance pH, add at dusk, brush, run pump
- Test free chlorine, total chlorine, pH, and CYA. Use a fresh sample.
- Read your product's shock directions before you start, including any pH range, how to add it, and when the pool can be used again.
- Bring pH to about 7.2–7.6 first if it is high. Chlorine works much better at lower pH: the CDC's Model Aquatic Health Code puts the active share near 50% at pH 7.5 and about 20% at pH 8.0. Wait at least 30 minutes after acid, with the pump running, before you add chlorine.
- Enter your numbers in the calculator above and measure out your label's amount for your gallons. Don't go above the label amount.
- Add the shock at dusk, with the pump running, the way your label says. Liquid chlorine is usually poured slowly in front of a return jet. Some cal-hypo labels say to pre-dissolve it in a bucket of pool water (product into water, never water into product); follow yours.
- Brush the walls and floor, especially shaded corners and steps where algae hide.
- Run the pump overnight and keep the cover off if you use one.
- Retest FC and combined chlorine (CC) in the morning. If CC is still 0.5 ppm or more, or the water is still green or cloudy, check your label: some allow a repeat treatment, and some list a separate dose for algae.
Safety
Never mix chlorine products with each other or with acid, not even in an empty bucket or scoop. Add one product at a time, chemical to water, with the pump running. Wear gloves and eye protection, keep containers closed, and store chlorine and acid apart, out of reach of children. In an emergency, call 911 or Poison Control at 1-800-222-1222. Always follow the product label. It is a violation of federal law to use a pesticide product in a manner inconsistent with its labeling.
Why one bag per 10,000 gallons is only a guess
A typical 1 lb bag of 65% cal-hypo in 10,000 gallons raises FC by about 7.8 ppm. In a 20,000-gallon pool the same 65% bag gives only 3.9 ppm. So the label's "per 10,000 gallons" amount has to be scaled to your pool, which is what the calculator does.
Stabilizer matters too. The CDC's Model Aquatic Health Code (2024 Annex) explains that stabilizer (CYA) holds part of the chlorine in reserve, so the more CYA you have, the more free chlorine it takes to do the same work. A shock that works in a fresh pool can seem weak in a pool that has been on tablets all summer. The answer is not to go above the label. It is to keep CYA moderate, around 30–50 ppm, so label-level chlorine can do its job. If CYA is high, replacing some water is the fix: see what cyanuric acid does and chlorine lock.
Chlorine shock vs non-chlorine (MPS)
"Non-chlorine shock" is potassium monopersulfate (MPS). It is an oxidizer, not a sanitizer: it breaks down sweat, sunscreen, and other organic waste, but it does not kill algae or germs the way chlorine does. It is sold for a quick refresh after a pool party when chlorine is already in range. Don't use it to clear green or cloudy water.
MPS is dosed by its label, not by ppm, and it can make the common DPD test show false combined chlorine (PHTA/AQUA technical notes). How to scale its label dose and when to test are on the non-chlorine shock calculator.
When you can swim again
Follow your product label's re-entry directions, and wait until free chlorine has dropped to the swim limit on the label or lower. Many pool chlorine labels say not to swim above 4 ppm; some say 3 ppm. The CDC's model code treats 10 ppm as an absolute upper limit for public pools, but the label limit is lower and takes priority. Retest before anyone gets in, and check that pH is between 7.2 and 7.8. The test decides, not the clock: sun, bather load, and CYA all change how fast chlorine drops. The "can I swim yet" check is on how long after shocking a pool you can swim.
Cloudy after shocking?
Some haze for a day is common: dead algae and oxidized waste are now particles your filter has to catch. Run the pump continuously, clean or backwash the filter when its pressure rises, and brush. If the water stays milky, check three things:
- FC dropped fast overnight. Something is still using up chlorine, often algae. See chlorine not holding and the green pool steps.
- You used cal-hypo and your pH or calcium is high. Extra calcium can form fine calcium carbonate particles. Lower pH toward 7.4, and consider liquid chlorine for the next shock.
- The filter can't keep up. Sand filters in particular take days on a heavy load.
More causes and fixes are in the cloudy pool guide.
FAQ
How much shock is too much?
More than the amount on your product's label, scaled to your gallons. Going above the label is against federal law for a registered pesticide, and it keeps the pool closed longer. Extra dichlor also raises stabilizer, extra cal-hypo raises calcium, and neither goes away without draining.
How much shock per gallon?
Read the amount and the gallons it treats on your label, then scale it: label amount × (your gallons ÷ label gallons). For example, a label that says 1 lb per 10,000 gallons works out to 1.5 lb for 15,000 gallons. The calculator above does this for you.
Why is my pool cloudy after shocking?
Usually dead algae and oxidized waste that the filter hasn't caught yet, or calcium carbonate if you used cal-hypo in water with high pH or calcium. Keep the pump running, clean the filter, and retest FC in the morning. If FC keeps falling, something is still using up chlorine; a clarifier won't fix that.
Why is my pool still green after shocking it?
Common reasons: high stabilizer (CYA), a dirty filter, algae left in corners and on steps that weren't brushed, or a single dose that faded overnight. Check your label for an algae or repeat dose, brush daily, clean the filter, and lower CYA if it is high. The green pool steps walk through it.
Sources
- CDC, Model Aquatic Health Code 2024, Annex (FC–CYA relationship, chlorine share by pH, 10 ppm limit): cdc.gov (PDF)
- CDC, Home pool and hot tub water treatment and testing: cdc.gov
- U.S. EPA, examples of pool chlorine labels with a re-entry limit of 4 ppm and 3 ppm: EPA label (PDF), EPA label (PDF)
- PHTA fact sheet, Dichlor (0.9 ppm CYA per 1 ppm chlorine): phta.org
- AQUA / PHTA Tech Notes, Potassium monopersulfate (non-chlorine shock): PDF
- CDC, pool chemical injuries: archive.cdc.gov