How to Shock a Pool the Right Way

Shocking a pool means raising free chlorine high enough to reach breakpoint, the point where chlorine burns off combined chloramines and oxidizes the organic gunk that clouds water and causes that harsh chlorine smell. The trick most people miss: the right shock level is not a fixed cup per gallon. It depends on your cyanuric acid (CYA). Test your water, shock to the target free chlorine for your CYA, do it at dusk, and hold that level until combined chlorine drops under 0.5 ppm.

Pool Shock and Testing Picks

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In The Swim 68% Cal-Hypo Granular Shock

Strong calcium hypochlorite shock for reaching breakpoint fast.

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CPDI Liquid Chlorine 12.5%, 4-Pack

No-residue liquid shock, easy to pour and pre-dissolved.

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Clorox Pool&Spa Shock XtraBlue Plus

Multi-pound shock that kills bacteria and algae quickly.

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Taylor K-1005 9-in-1 Pool Test Kit

Read free and combined chlorine so you know when breakpoint is met.

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What breakpoint chlorination really means

Chlorine in a pool exists in two forms. Free chlorine (FC) is the active sanitizer. When it combines with ammonia and nitrogen from sweat, urine, and other waste, it forms combined chlorine (CC), also called chloramines. Chloramines are weak sanitizers and the real cause of red eyes and that strong pool smell people wrongly blame on too much chlorine.

Breakpoint chlorination is the moment free chlorine climbs high enough to break those chloramine bonds apart and burn them off. Below breakpoint, you actually make more chloramines. That is why a half-hearted dose can leave the water smelling worse. You must push past breakpoint, which means reaching a specific high FC level, not just adding a random amount. Learn the two forms in detail in our guide to free vs combined chlorine.

Shock to a level set by your CYA

This is the single most important idea in shocking. Cyanuric acid binds chlorine and reduces how much is active, so the more CYA you have, the higher you must raise FC to reach breakpoint. The widely used FC/CYA ratio puts the shock target near 40 percent of your CYA reading.

These are approximate targets for clearing chloramines. An active algae bloom needs an even higher target held for longer. Rather than do the math by hand, use our shock calculator with your pool volume and CYA to get the dose, and check your CYA first with the CYA calculator. If your CYA is very high, see how to lower CYA, because extreme stabilizer makes shocking impractical.

Choosing a shock product

• Liquid chlorine (sodium hypochlorite): 10 to 12.5 percent. Pre-dissolved, leaves no residue, does not raise CYA or calcium. The cleanest choice for routine shocking. Pour it slowly around the pool with the pump running.
• Cal-hypo (calcium hypochlorite): a strong granular shock, often 65 to 73 percent available chlorine. Adds calcium, so go easy in hard-water areas. Pre-dissolve in a bucket of pool water per the label before broadcasting.
• Dichlor: stabilized granular shock that also adds CYA. Useful occasionally but it raises stabilizer with every dose, so it is not ideal for frequent shocking.

Match the product to your needs. If your CYA is already at target, liquid chlorine keeps it from climbing further. If your calcium hardness is low, a little cal-hypo can help both jobs at once.

Step by step

1. Test first. Measure FC, CC, pH, and CYA. You need CYA to set the target and pH ideally between 7.2 and 7.6 so chlorine works efficiently.
2. Calculate the dose. Use the shock calculator with your gallons and CYA to find the target FC and the amount of product to add.
3. Wait for dusk. Add shock at sundown or after dark so sunlight does not destroy it before it finishes.
4. Add it correctly. Pour liquid slowly around the perimeter, or pre-dissolve granular shock in a bucket of pool water and broadcast it. Always add chemical to water, never water to chemical.
5. Run the pump. Circulate for at least several hours, ideally overnight, to mix the shock through the whole pool.
6. Retest and maintain. In the morning, test FC and CC. Keep FC at the shock level, re-dosing as needed, until combined chlorine reads under 0.5 ppm and stays there.

Safety basics

Pool shock is concentrated and can be hazardous if mishandled. Follow these rules every time:

• Never mix shock with tablets, acid, or any other chemical. Different chlorine types and chlorine plus acid can react violently or release toxic gas.
• Always add chemical to water, never water to chemical, and broadcast or pre-dissolve exactly as the label says.
• Use a dedicated dry scoop and never reuse a scoop between products.
• Run the pump while dosing, and keep people and pets out until FC returns to a safe range.
• Store shock and acid separately, in a cool dry place, sealed and away from children and pets. Dosing figures are estimates, so test your own water before and after.

When it does not clear

If combined chlorine will not drop, you usually have not held a high enough FC long enough, often the sign of an algae bloom consuming chlorine. Recheck your CYA, raise your target accordingly, and maintain it persistently, testing every few hours and re-dosing. For salt pools, your generator's boost mode handles routine shocking, but a real bloom still calls for added liquid chlorine. See saltwater pool chemistry for that workflow, and the chlorine calculator for everyday dosing between shocks.