Pool Shock Calculator
Shocking your pool kills bacteria, breaks down combined chlorine, and clears cloudy or green water. Enter your pool volume and select your water condition — this calculator gives you the exact amount of calcium hypochlorite shock to add.
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Enter your pool volume and readings, then click calculate.
Need to dial in chlorine, pH, or alkalinity too? See the full pool chemical calculator →
Dosing Table by Pool Size and Product
Routine maintenance doses, which raise free chlorine roughly 5–10 ppm. Double every figure for cloudy water and triple it for a green pool. Product strengths vary between brands, so treat this as a starting point and follow the directions on the bag or bottle in front of you.
| Pool volume | Cal-hypo 65–73% | Dichlor 56–62% | Liquid chlorine 10–12.5% |
|---|---|---|---|
| 5,000 gal | 0.5 lb | 0.6 lb | 0.5 gal |
| 8,000 gal | 0.8 lb | 1.0 lb | 0.8 gal |
| 10,000 gal | 1.0 lb | 1.2 lb | 1.0 gal |
| 15,000 gal | 1.5 lb | 1.8 lb | 1.5 gal |
| 20,000 gal | 2.0 lb | 2.4 lb | 2.0 gal |
| 25,000 gal | 2.5 lb | 3.0 lb | 2.5 gal |
| 30,000 gal | 3.0 lb | 3.6 lb | 3.0 gal |
Dichlor figures are higher because it carries less available chlorine per pound, and it also adds cyanuric acid with every dose — useful once, a problem if you use it all season. Not sure of your volume? Work it out with the pool volume calculator first, since every figure on this page depends on getting that number right.
The Safety Order
Pool chemicals sent an estimated 13,508 people to US emergency departments between 2015 and 2017, and the CDC found that more than half of those injuries happened at a home rather than a public pool. The order you do things in is what prevents most of them.
- Read the label on the product you actually bought. Available chlorine differs by brand, and the label instructions override any table, including the one above.
- Never mix pool chemicals. The CDC's guidance is to store each chemical separately and keep incompatible products apart. Chlorine mixed with acid produces chlorine gas; cal-hypo mixed with dichlor or trichlor can ignite.
- Add chemical to water, never water to chemical. The CDC warns that doing it the other way round can cause a violent, potentially explosive reaction. If you pre-dissolve granular shock, fill the bucket with water first.
- Wear eye protection and gloves, and open bags away from your face. Dust clouds and fumes released when opening a container are among the most common causes of injury.
- Add at dusk, with the pump running. Sunlight destroys unstabilised chlorine before it can do its job, and circulation stops the dose settling on the floor and bleaching the liner.
- Test before anyone gets back in. Free chlorine at 4 ppm or below is the re-entry threshold on the labels we reviewed — check the one on your own product, and treat it as one condition among the others that make a pool safe.
Sources are listed at the foot of this page.
Liquid Chlorine vs. Cal-Hypo Shock
Cal-hypo (calcium hypochlorite) is a granular shock, typically 65–73% available chlorine. It is the default for most home pools because it dissolves well, keeps for a long time if stored dry, and the dosing above is built around it. The trade-off is calcium: repeated use raises calcium hardness, which eventually shows up as scale.
Liquid chlorine (sodium hypochlorite) is much weaker per unit, typically 10–12.5%, and sold by the gallon. It works faster because it is already in solution and adds no calcium, which suits pools that already run hard. It degrades in storage within weeks, though, and you carry a lot more of it for the same dose.
Dichlor dissolves quickly and is stabilised, which sounds ideal until you notice it adds cyanuric acid every single time. Occasional use is fine; a whole season of it drives CYA up until your chlorine stops working properly and the only fix is replacing water.
For a pool already prone to scale, liquid is usually the better choice. For a pool where shock sits in a shed for months between uses, cal-hypo is.
Quick Answer
For routine maintenance use 1 lb of 65–73% cal-hypo shock per 10,000 gallons, double it for cloudy water and triple it for green or algae-filled water. Add it at dusk with the pump running, and do not swim again until a test shows free chlorine has fallen to 4 ppm or below — the re-entry threshold on the EPA-registered pool chlorine labels we reviewed. Your own product label governs.
How It Works: Formula & Variables
lbs of cal-hypo shock = (Gallons / 10,000) × Shock Factor
Shock factor: 1 for normal maintenance, 2 for cloudy water, 3 for green/algae water.
Worked Examples
Example 1: A green 15,000-gallon pool
Green water is the triple dose: (15,000 ÷ 10,000) × 3 = 4.5 lb of cal-hypo. Brush the walls and floor first, add it at dusk with the pump running, and leave the pump on overnight. Re-test the next morning: green pools consume chlorine fast, and if the water is still tinted the dose has been used up rather than wasted, so a second treatment is normal. Nobody swims until free chlorine reads 4 ppm or less, which after a dose this size can take well over a day.
Example 2: Weekly maintenance on a 20,000-gallon pool
Clear water, routine weekly shock: (20,000 ÷ 10,000) × 1 = 2 lb of cal-hypo, which lifts free chlorine by roughly 5–10 ppm. Using liquid instead, that is about 2 gallons of 10–12.5% sodium hypochlorite. Added at dusk on a Friday, you test on Saturday afternoon rather than assuming it has cleared, because bather load, sunlight and your stabiliser level all change how fast it falls.
Example 3: Cloudy water in a small 8,000-gallon pool
Cloudy but not green is the double dose: (8,000 ÷ 10,000) × 2 = 1.6 lb. Cloudiness after heavy use is usually chloramines rather than algae — chlorine already bound up with sweat and sunscreen — which is exactly what a shock dose breaks apart. If the water is still cloudy a day later with chlorine holding steady, the problem is more likely filtration or high pH than a shortage of chlorine.
Key Concepts
Shock vs. daily chlorine: Shock is a large one-time dose to break down contaminants and combined chlorine — it's not a substitute for ongoing sanitizer.
Always shock at night: UV rays burn off unstabilised chlorine before it can fully work.
CYA and target chlorine levels: Cyanuric acid (CYA) acts as a stabilizer that shields chlorine from UV breakdown, but higher CYA also means you need more free chlorine to get the same sanitizing power. As a rough guide, keep free chlorine at roughly 7.5% of your CYA level (so around 3-4 ppm free chlorine at a CYA of 40-50 ppm). If CYA climbs too high, chlorine becomes less effective even at normal shock doses, and partial water replacement is usually the only way to bring it back down.
Common Mistakes
Swimming too soon: the EPA-registered pool chlorine labels we reviewed restrict re-entry above 4 ppm of chlorine because of the risk of injury; your own label may word it differently. Test before anyone gets in — do not judge it by how long it has been or by how the water looks.
Shocking without brushing first: For green pools, brush the walls and floor before shocking so the chlorine can reach algae clinging to surfaces rather than only treating open water.
Shocking in daylight: Unstabilised chlorine is destroyed by UV within hours. A dose added at midday can be largely gone before it has finished working, which is why a pool sometimes stays green after what looked like a full treatment.
Chasing a smell with more chlorine: A strong chlorine smell means chloramines, which is a sign of too little effective chlorine rather than too much. Shocking is the right response; adding a little more daily sanitiser is not.
Guessing the pool volume: Every dose here scales directly off gallons. A 20% error in volume is a 20% error in the dose, in both directions.
Frequently Asked Questions
For routine maintenance, 1 lb of 65-73% cal-hypo shock per 10,000 gallons is the standard label dose, which raises free chlorine by roughly 5-10 ppm. Double it for cloudy water and triple it for a green, algae-filled pool. So a 15,000-gallon pool needs about 1.5 lb for maintenance, 3 lb if cloudy, and 4.5 lb if green. Always check your own product label first, because available chlorine varies by brand and the label overrides any general rule.
Not by the clock — by the test kit. Each EPA-registered pool chlorine label reviewed for our guide restricts re-entry above 4 ppm; one reads "Reentry into treated swimming pools is prohibited above levels of 4 ppm of chlorine due to risk of body injury." So test the water and wait until free chlorine has fallen to 4 ppm or below, then follow the directions on the product you used. How long that takes varies too much between pools to put a number on, and a number would not be a re-entry criterion anyway.
Roughly triple the maintenance dose — about 3 lb of cal-hypo per 10,000 gallons. Brush the walls and floor first so the chlorine can reach algae clinging to surfaces, run the pump continuously, and re-test after 24 hours. A pool that is still green the next day usually needs a second treatment. Green water also burns through chlorine quickly, so expect the level to fall faster than normal.
Yes. About 1 gallon of 10-12.5% liquid chlorine (sodium hypochlorite) per 10,000 gallons delivers a comparable maintenance dose, and you triple it for a green pool the same way. Liquid acts faster because it is already dissolved and it adds no calcium, which matters if your calcium hardness is already high. The trade-offs are that it degrades in storage within weeks and you have to carry far more volume for the same effect.
Do not shock in direct sunlight, because unstabilised chlorine burns off before it can work — shock at dusk or after dark with the pump running. Do not shock when free chlorine is already high, because you cannot swim until it drops back to 4 ppm anyway. And if your cyanuric acid is very high, adding more chlorine will not fix the problem; the chlorine is being locked up by the stabiliser, and partial water replacement is usually the only real remedy.
About once a week during swim season is a common routine, plus after anything that spikes chlorine demand: heavy rain, a hot spell, a pool party, or a fouling incident. If you are testing regularly, a better trigger than the calendar is combined chlorine — the difference between total and free chlorine. When that gap climbs, chlorine is bound up with contaminants and a shock is what breaks it apart.
No. The CDC is explicit that you should never mix different pool chemicals, and that when pre-dissolving you add the chemical to water and never water to the chemical, because a violent and potentially explosive reaction can occur. Mixing cal-hypo with dichlor or trichlor, or chlorine with acid, is a leading cause of pool chemical injury. Add one product at a time, let the pump circulate it, and keep products in their original labelled containers.
Shock is a large, deliberate overdose that breaks apart chloramines — chlorine already bound to sweat, sunscreen and other contaminants, which no longer sanitises and is what produces the strong "chlorine smell" people associate with a heavily treated pool. That smell is a sign of too little effective chlorine, not too much. Your daily sanitiser maintains a level; shock resets it.
Related Calculators
Pool Volume Calculator
Calculate your pool volume in gallons, liters, and cubic feet.
Pool Chemical Calculator
Get exact doses for chlorine, shock, pH, and alkalinity.
Pool Chlorine Calculator
Calculate liquid, granular, or bleach chlorine doses.
Muriatic Acid Pool Calculator
Calculate exact muriatic acid doses to lower pH.
Guides
The dose is one part of it. These cover the rest — what order to add things in, which product to reach for, and how to know the water is safe again.
- Pool Shock Dosage ChartCal-hypo, dichlor and liquid chlorine dosed separately by volume — and what each leaves behind in your water.
- When Is It Safe to Swim After Shocking a Pool?Re-entry is a chlorine reading, not a waiting time. What the EPA label limit means and how to test for it.
- How Much Muriatic Acid to Lower Pool pH?High pH stops chlorine working properly, which is often why a shock appears not to have taken.
Shock fixes the symptom — check what caused it
A pool that needs shocking often has something else out of range. If the water went green or cloudy, check pH and alkalinity with the pool chemical calculator, which doses chlorine, pH, alkalinity and stabiliser together. If pH is high, the muriatic acid calculator works out how much it takes to bring it down — worth doing, because chlorine sanitises poorly above pH 7.8.
Sources
- Re-entry threshold of 4 ppm. One of three EPA-registered pool chlorine labels reviewed, EPA Reg. No. 11411-3, accepted 2 February 2015: “Reentry into treated swimming pools is prohibited above levels of 4 ppm of chlorine due to risk of body injury.” EPA Pesticide Product Label System. Verified 6 September 2026.
- Never mix chemicals; add chemical to water. CDC, Pool Chemical Safety.
- Injury figures. An estimated 13,508 US emergency department visits from pool chemical injuries during 2015–2017, at least 56.3% of them at a residence: CDC, MMWR 68(19), Pool Chemical Injuries in Public and Residential Settings.
- Target water chemistry. CDC recommends pH 7.0–7.8 and free chlorine of at least 1 ppm in pools, or at least 2 ppm where cyanuric acid is used, tested at least twice a day: CDC, Home Pool and Hot Tub Water Treatment and Testing.
- Product strengths and the 1 lb per 10,000 gallon dose are taken from manufacturer labels for 65% and 73% calcium hypochlorite shock, which state that 1 lb per 10,000 gallons provides 5–10 ppm available chlorine. Your own product label always takes precedence over this page.
This page is general guidance for domestic pools. It is not a substitute for the directions on your product, and public or commercial pools are subject to separate local health regulations.