How Much Shock to Add to a Pool: Doses, SLAM and CYA

For a routine shock, add one label dose for your pool’s gallons. On the EPA-registered labels we read, that is 64–128 fluid ounces of 10% liquid chlorine or 9–18 ounces of 73% calcium hypochlorite (cal-hypo) per 10,000 gallons, which raises free chlorine by about 5 to 10 ppm. Green water needs more. The SLAM method holds free chlorine at about 40% of your stabilizer (cyanuric acid, or CYA) reading until the algae is gone, and nobody swims until chlorine is back to 4 ppm or less.

Many guides repeat “one pound per 10,000 gallons.” But a pound of shock is not a fixed dose. We converted each label’s directions into parts per million (ppm): the same pound adds anywhere from 4.8 ppm (dichlor) to 8.8 ppm (73% cal-hypo), and dichlor adds almost as much stabilizer as chlorine. Below are the label doses, a table by pool size, SLAM targets by stabilizer level and, from NOAA climate data, how much stabilizer a season of weekly dichlor shock leaves behind in 19 US cities.

What one shock dose adds, product by product

EPA treats the word “shock” as a pesticide claim, so chlorine shocks are registered pesticides; a product that does not kill germs must be sold as a “shock oxidizer” or “oxidizing shock.”¹¹ That makes every chlorine shock label public in EPA’s label database. We read six labels, one per common product type (five EPA-registered chlorine labels and one non-chlorine shock label), and turned each shock dose into the free chlorine it adds.

Sources for each row: the two liquid chlorine labels,^3,4 the two HTH labels,^5,6 the dichlor label,⁷ the non-chlorine shock label⁸ and PHTA’s fact sheet on monopersulfate.⁹ Every “our math” figure uses one formula: ppm added = pounds of available chlorine × 1,000,000 ÷ (gallons × 8.34). The labels confirm it. HTH Granular 73’s chart lists 8.75 ppm for one pound in 10,000 gallons, which is exactly 0.73 × 1,000,000 ÷ 83,400.⁵ The 10% liquid label says 13 fluid ounces per 10,000 gallons adds 1 ppm.³

So “one pound per 10,000 gallons” can mean a 4.8 ppm dose or an 8.8 ppm dose, and Google’s AI answer treats one pound of granular shock and one gallon of liquid as the same dose when a gallon of 12.5% liquid adds about 12 ppm. The labels also agree on when swimmers can return:

Re-entry into treated pools is prohibited above 4 ppm due to risk of bodily harm.

CDC’s pool code annex adds that EPA has not approved any pool product for use above 4 ppm free chlorine while swimmers are present.²

How much shock for your pool size

Doses scale with gallons, so a 5,000-gallon pool takes half the 10,000-gallon dose and a 20,000-gallon pool takes double. If you don’t know your gallons, measure them with our pool volume calculator first: a 20% error in volume becomes a 20% error in every dose.

The liquid and cal-hypo columns use each label’s top shock rate; half of each amount gives the bottom of the range, about 5 ppm.^3,4,5 The dichlor column is that label’s single dose.⁷ For HTH Pool Care Shock Advanced, the label’s own chart gives 12 ounces for 10,000 gallons, 24 ounces for 20,000 and 36 ounces for 30,000.⁶

Small and above-ground pools

Pre-measured bags are sized for big pools, so check the label’s small-pool line instead. HTH Pool Care Shock Advanced gives 1.2 ounces per 1,000 gallons for pools up to 10,000 gallons.⁶ The dichlor label gives 3.5 teaspoons per 500 gallons, or 1.3 ounces per 1,000 gallons, for small vinyl pools.⁷ By the formula above, a whole 1-pound bag of 73% cal-hypo in a 1,000-gallon pool would add about 88 ppm.

Routine shock or SLAM: which one you need

“Shock” covers two different jobs. A routine shock burns off the chloramines that swimmers and debris leave behind. A SLAM kills an algae bloom by holding chlorine high for as long as it takes. Your test results tell you which one you need.

Routine shock clears combined chlorine

The labels call for a shock every week and after heavy swimming, rain, wind, cloudy water, odors or eye irritation.⁶ The number to watch is combined chlorine: total chlorine minus free chlorine. CDC’s Model Aquatic Health Code (MAHC) Annex sets 0.4 ppm as the level where public pools should act.² To get there, the shock has to pass “breakpoint,” which the Annex says takes:

approximately 10 times the amount of combined chlorine in the water

SLAM: hold shock level until the algae is gone

SLAM stands for Shock Level And Maintain, a method from Trouble Free Pool, a large pool-owner forum. Its chart sets the shock level at about 40% of your stabilizer reading: 12 ppm at 30 ppm CYA, 20 ppm at 50 and 31 ppm at 80.¹² You hold that level, testing and topping up through the day, until the water is clear, combined chlorine is 0.5 ppm or less and the pool loses no more than 1 ppm overnight.¹⁶ Trouble Free Pool calls SLAM levels safe for swimmers;¹² the labels do not, so keep everyone out until chlorine is 4 ppm or less.⁵

Above about 25 ppm stabilizer, the SLAM level is higher than one label dose adds, so a SLAM means repeated doses. Table 3 shows the first top-up for a 10,000-gallon pool starting at 3 ppm.

Two cautions before you start. CDC warns that DPD test kits can bleach out above 10 ppm and read falsely low, so a SLAM needs a test that reads high levels.¹ And every pound of cal-hypo adds calcium too: about 0.7 ppm of calcium hardness per ppm of chlorine, by our calculation from its formula, which builds up over a long SLAM.¹⁴

Why stabilizer sets the shock dose

Cyanuric acid shields chlorine from sunlight, but it also holds chlorine in a weaker form. That is why CDC asks for at least 2 ppm free chlorine in stabilized pools instead of 1 ppm.¹ The MAHC Annex goes further: chlorine should rise in proportion to stabilizer.² Its superchlorination table shows how much: at the same 5 ppm free chlorine, the fast-acting form, hypochlorous acid, falls from 2.42 ppm with no stabilizer to 0.043 ppm at 50 ppm CYA, about 56 times less.² That is the logic behind a CYA-based shock level.

Dichlor and trichlor add stabilizer with every dose

PHTA, the pool industry’s trade association, says trichlor adds 0.6 ppm of cyanuric acid for each ppm of chlorine.¹⁰ Dichlor adds more. From the molecular weights in PubChem, the dichlor dihydrate in our label adds 0.91 ppm of stabilizer per ppm of chlorine; the same math gives 0.61 for trichlor, matching PHTA’s figure.¹⁴ Liquid chlorine and non-chlorine shock add none.^9,14 So the more you shock with dichlor, the higher your SLAM target climbs.

How much stabilizer a season of dichlor shock adds where you live

The dichlor label says to apply it weekly, and each dose adds 3.6 ppm of stabilizer.⁷ How many weeks that adds up to depends on your climate. We counted the days when the typical daily mean air temperature is at or above 65°F, the same threshold our guide to when to close your pool uses, from NOAA’s 1991–2020 normals for the station nearest each ZIP code.¹⁵

Across 40,906 ZIP codes, the median season is 17 weeks, which adds 62 ppm. Weekly dichlor alone would push stabilizer past 50 ppm in 73% of ZIP codes and past 90 ppm, the MAHC limit for public pools, in 19%.² At 90 ppm the SLAM level is 35 ppm, about 3½ gallons of 10% liquid chlorine in a 10,000-gallon pool starting from zero.

Non-chlorine shock: an oxidizer, not a sanitizer

Non-chlorine shock is potassium monopersulfate (MPS). It burns off organic waste without raising chlorine, which is why the label lets you swim 15 minutes later.⁸ It doesn’t replace chlorine:

Potassium monopersulfate is not a sanitizer or algaecide

The label we read calls for 1 pound per 10,000 gallons weekly and 2 pounds for the first treatment, and twice a week when the water is above 86°F or the pool sees heavy use.⁸ Two catches: leftover MPS reads as combined chlorine on DPD tests, so test before you add it,⁹ and the MAHC Annex notes it may not reduce the inorganic chloramines that a chlorine shock destroys.²

How to add shock safely

1. Test first. Free and total chlorine, pH and stabilizer decide the dose; see our ideal pool chemical levels for the targets.
2. Adjust pH. The dichlor and liquid labels both ask for pH 7.2–7.6.^7,3
3. Measure the dose for your gallons from your own label, using Table 2 as a cross-check.
4. Add it in the evening with the pump running. The 10% liquid label says to add it at night while running the pump and filter.³ Broadcast granules over the deep end and brush any that settle.⁶
5. Use the skimmer only if your label allows it. The dichlor label rules out the skimmer and chemical feeders because other chemicals there could cause a violent reaction.⁷ HTH’s label allows slow skimmer addition only with the skimmer emptied and no feeder in the system.⁶
6. Circulate and retest. Run the filter 4–8 hours after a routine dose and 24 hours for algae, then test before anyone swims.⁷

Shock at opening, at closing and in salt pools

At opening, the 10% liquid label uses its shock rate, 64–128 fluid ounces per 10,000 gallons, for start-up.³ HTH’s label says to balance the water, add a shock dose and repeat as needed until free chlorine reads 1–4 ppm.⁶

Closing doses vary more than any other. For water that is already clear, the 10% liquid label adds 40 fluid ounces per 10,000 gallons for 3 ppm,³ HTH Granular 73 uses 27 ounces for 15 ppm,⁵ and the dichlor label uses 1 pound per 6,000 gallons.⁷ Our guide to closing your pool covers timing.

Salt pools can be shocked with any of these. The MAHC Annex notes that a salt cell may not make chlorine fast enough to reach shock levels, so a separate chlorine product may be needed.² Remember that cal-hypo adds calcium and dichlor adds stabilizer, and check salt with our pool salt calculator.

Before you buy more shock

If chlorine keeps vanishing or the water stays green after a full SLAM, test stabilizer before buying more shock: a high CYA reading raises the target every time. For the full set of target ranges, see ideal pool chemical levels, and find more guides in our water chemistry section.