How to shock a pool

When and why to shock

Everyday chlorine is a slow, steady guard. A shock is a short, deliberate overdose that burns through whatever has piled up faster than the daily level can handle. The usual reasons:

• After heavy use or a storm, when sweat, sunscreen, leaves and rain have used up chlorine and the water smells “chlorinated” (that smell is mostly combined chlorine, not too much free chlorine).
• When opening the pool after winter, or after the chlorine level has sat at zero for a while.
• Algae, from a faint green tinge on the walls to water you can’t see the floor through.
• A fecal accident, where the CDC sets specific levels and times.
• Combined chlorine creeping up. The Model Aquatic Health Code tells public pools to act when combined chlorine (total minus free) passes 0.4 ppm; it’s a reasonable trigger at home too.

“Shock” is also a legal word. The US EPA treats it as a pesticide claim, so any product sold as shock is a registered pesticide with directions you are required to follow. The label decides the dose range for that product.

How high to shock

The SLAM level scales with stabilizer because cyanuric acid holds most of the chlorine in a slow, bound form. The more CYA in the water, the more total chlorine it takes to leave enough active chlorine to kill algae quickly. That’s also why a pool with very high CYA is hard to clear: at 80 ppm CYA you are chasing 32 ppm of chlorine, every day, until it clears. The chlorine and stabilizer calculator gives the level for any CYA.

How much shock that takes

Shock is sold by strength, so the same 9 ppm rise (from 1 ppm to 10 ppm) in 10,000 gallons looks very different by product. One 1 lb bag of 73% cal-hypo holds about 331 g of chlorine, which is about 8.7 ppm in 10,000 gallons; a 65% bag gives about 7.8 ppm.

The by-products matter when you shock often. Cal-hypo slowly builds calcium, dichlor builds stabilizer (which then raises the next shock level), and liquid chlorine leaves a little salt. For repeated shocks, liquid chlorine is usually the cleanest choice. Worked examples: how much shock for a 10,000 gallon pool in bags, how much liquid chlorine to shock a 10,000 gallon pool from a jug, and how many bags of shock for a 20,000 gallon pool. Using household bleach instead? See how much bleach for a 10,000 gallon pool, and use only plain, unscented bleach.

Shocking, step by step

1. Test first: free chlorine, pH and stabilizer (CYA). The CYA sets the SLAM level, and the CDC notes chlorine loses much of its punch above pH 8.
2. Fix pH before the shock, not with it. If pH is high, lower it — how much muriatic acid to lower pH in a 10,000 gallon pool works one through — then let the pump circulate it before any chlorine goes in. Acid and chlorine never go in together.
3. Shock at dusk. Sunlight destroys unstabilized chlorine quickly, so an evening dose gets the whole night to work.
4. Run the pump and brush. Brush walls, steps and floor to knock loose anything clinging there.
5. Add the shock in front of a return, with the pump on. Pre-dissolve granular shock in a bucket of pool water if the label says so — shock into water, never water onto shock — so undissolved granules don’t bleach a vinyl liner.
6. Retest after about an hour of circulation, and again in the morning.

Clearing a green pool

A green pool is the one case where a single dose rarely works. Algae eat chlorine as fast as you add it, so the job is to reach the SLAM level and hold it there. The Trouble Free Pool method (SLAM: Shock Level And Maintain) is a hobbyist approach, not a label instruction, but it is what most forum advice on green pools means.

The order below is the same one the balancing guide uses for a green pool:

1. Keep everyone out and test free chlorine, pH and stabilizer (CYA). Most strips have a CYA pad; a pool store will test it free.
2. Very high CYA? Drain first. Above about 80 ppm the shock level gets impractical, so partly drain and refill before buying chlorine — see how much water to drain to lower CYA.
3. Clear the debris — skim leaves, empty the baskets — and start the pump. Leave it running until the pool is clear.
4. If pH is above about 7.6, lower it first to about 7.4 with acid, run the pump a few hours and retest: chlorine works better at lower pH. Leave alkalinity, calcium and stabilizer for later. Never add acid and chlorine together.
5. At dusk, raise chlorine to the shock (SLAM) level — 40% of your CYA — usually with liquid chlorine. How much liquid chlorine to SLAM a pool works the first dose.
6. Hold it there. Test two or three times a day and top up what was used. Strips can’t read this high — use a FAS-DPD drop test kit (a drop-count chlorine test) or a pool store’s free water test. Brush daily and clean or backwash the filter as its pressure rises.
7. Stop when it is clear and chlorine drops no more than about 1 ppm overnight (the Trouble Free Pool method’s rough guide). Let chlorine fall back to your label’s swimming level, then balance alkalinity, calcium and stabilizer in the usual order.

Salt pools follow the same rules; a salt cell can’t raise chlorine fast enough to SLAM, so liquid chlorine does the work there too. The usual stabilizer and chlorine levels for a salt pool are on what CYA and chlorine a salt water pool needs.

How long before you can swim

Swimming waits until free chlorine is back within what your product’s label allows. One EPA-registered pool chlorine label (reg. 82289-2) prohibits re-entering a pool above 4 ppm and a spa above 5 ppm; check yours, because labels differ. For public pools, the Model Aquatic Health Code treats free chlorine above 10 ppm as an imminent health hazard that closes the pool.

How long that takes depends on sun, temperature, stabilizer and what the chlorine is fighting, so there is no fixed number of hours. After an evening routine shock it is often the next day; after a SLAM, it is when the water has cleared and chlorine has come down on its own. pH should be back in the CDC’s 7.0–7.8 range as well. Test before anyone gets in, rather than going by the clock.

Safety while shocking

• Keep children and pets away from the pool and the chemicals while you work, and store shock locked up.
• Wear eye protection and gloves if the label calls for them; liquid chlorine splashes.
• One chemical at a time, each fully circulated before the next.
• Never use pool shock in drinking water or in a kiddie pool; the CDC says to empty kiddie pools daily rather than chlorinate them.

For storage, spills and what to do after an exposure, read pool chemical safety. For how the chlorine products differ, see chlorine types compared.