Easy Pool Calc

How much liquid chlorine to SLAM a pool?

Short answer

To SLAM a 15,000 gallon pool with 40 ppm of stabilizer, raise free chlorine to 16 ppm (40% of CYA). From zero that takes about 1.92 gal (7.7 qt) of 12.5% liquid chlorine, then daily top-ups to hold it.

How to work it out

  1. In short: raise chlorine from where it is to the shock level, then work out how much of your product holds that much chlorine.
  2. Shock level: 40 ppm CYA × 40% = 16 ppm
  3. 16 − 0 = 16 ppm × 56,781 L = 908 g of chlorine
  4. 908 g ÷ 125 g per litre (12.5%) = 1.92 gal (7.7 qt) (2 jugs)

SLAM — Shock Level And Maintain — is the Trouble Free Pool method for clearing algae: raise chlorine to 40% of your CYA and keep it there, testing and topping up, until the pool clears and holds its chlorine overnight. It is a hobbyist method, not a label instruction, but it is what most forum advice on green pools means by “shock”.

Adjust for your project

gal
Don’t know it? Work it out with the pool volume calculator or the above-ground pool calculator.
The % printed on the jug.
ppm
Don’t know it? CYA (stabilizer, cyanuric acid) is what shields chlorine from sunlight — see pool words explained. Most test strips have a CYA pad, and pool stores test it free. The SLAM level is 40% of CYA, so test before a SLAM. For the CDC accident response, CYA must be 15 ppm or less.
Chlorine works better at pH 7.2–7.5. If pH is high, lower it first — the result tells you how much acid.
ppm
Add this much shock1.92 gal (7.7 qt)
1-gallon jugs
2 jugs
Shock level
16 ppm
Then
Hold it there, testing a few times a day and topping up, until the water is clear and chlorine stops dropping overnight.
  • SLAM (Shock Level And Maintain) is a Trouble Free Pool hobbyist method, not a label instruction. Because it holds chlorine high for days, liquid chlorine is the usual choice — cal-hypo adds calcium and dichlor adds stabilizer with every top-up.
  • Your test strips can’t read this high: strips and DPD tablet tests bleach out and read falsely low above about 10 ppm. Use a FAS-DPD drop test kit (a drop-count chlorine test that reads high levels) or take a sample to a pool store for a free water test.
Chemical safetyFollow the product label. Add chemicals to water, never water to chemicals. Never mix chemicals — especially chlorine and acid, or different types of chlorine. Add one at a time with the pump running, keep them away from children, and don’t swim until levels are back in range. This is an estimate: add it, let it circulate, then retest before adding more. Pool and spa water only — never use these doses for drinking water.

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FAS-DPD drop test kit (chlorine, pH, TA, CH, CYA)At 16 ppm, strips and DPD tablets read falsely low. A FAS-DPD drop kit tells you whether you’re really holding the shock level.
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First SLAM dose (12.5% liquid, 15,000 gal, from 0 ppm) by CYA

First SLAM dose (12.5% liquid, 15,000 gal, from 0 ppm) by CYA
CYAShock levelAdd this much shock1-gallon jugs
20 ppm8 ppm122.9 fl oz (3.8 qt)1 jug
30 ppm12 ppm1.44 gal (5.8 qt)2 jugs
40 ppm16 ppm1.92 gal (7.7 qt)2 jugs
50 ppm20 ppm2.4 gal (9.6 qt)3 jugs
60 ppm24 ppm2.88 gal (11.5 qt)3 jugs
70 ppm28 ppm3.36 gal (13.4 qt)4 jugs
80 ppm32 ppm3.84 gal (15.4 qt)4 jugs

Why the SLAM level depends on stabilizer

Cyanuric acid holds most of the chlorine in a bound, slow-acting form. The more CYA, the more total chlorine it takes to get enough active hypochlorous acid to kill algae quickly — so the shock level is a share of CYA rather than a fixed number. At 40 ppm CYA it is 16 ppm; at 80 ppm it is 32 ppm, which is twice the liquid chlorine every day. That is the strongest argument for keeping CYA moderate.

  • First dose. About 1.9 gallons of 12.5% chlorine takes 15,000 gallons from zero to 16 ppm.
  • Holding it. A green pool can burn through most of that in a day. Test morning, afternoon and evening, and add back what was lost each time.
  • Brush and filter. Brush walls and floor daily, run the pump around the clock and clean or backwash the filter as pressure rises.
  • When to stop. The method’s own stopping criteria are on the Trouble Free Pool site; broadly, clear water, little or no combined chlorine and an overnight chlorine loss of about 1 ppm or less.
  • Very high CYA? Above roughly 80 ppm, drain and refill first — see how much water to drain to lower CYA.

For a routine shock rather than algae, see how much liquid chlorine to shock a 10,000 gallon pool or, with bags, how many bags of shock for a 20,000 gallon pool. The target table by CYA is on the chlorine and stabilizer calculator.

Different size? Use the full Pool Shock Calculator.

What you’ll need

  • Liquid pool chlorine (sodium hypochlorite)Budget price

    Adds chlorine without stabilizer or calcium. Strength is printed on the label — commonly 10% or 12.5% — and it weakens with age and heat, so buy it fresh.

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  • FAS-DPD drop test kit (chlorine, pH, TA, CH, CYA)Mid-range price

    A drop-count kit reads free chlorine to 0.5 ppm (or 0.2 ppm), plus pH, alkalinity, calcium and stabilizer. Every dose on this site starts from a test, so this is the one thing worth buying first.

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  • Pool brush (18 in)Budget price

    Brush walls and floor after shocking or adding chemicals. Stainless bristles are for plaster only — use nylon on vinyl.

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Frequently asked questions

How much liquid chlorine to SLAM a pool with 60 ppm CYA?

The SLAM level is 24 ppm: about 2.88 gal (11.5 qt) of 12.5% for 15,000 gallons from zero.

How much liquid chlorine to SLAM a 20,000 gallon pool at 30 ppm CYA?

Raise to 12 ppm: about 1.92 gal (7.7 qt) of 12.5% liquid chlorine from zero.

Other common sizes

Sources