Pool shock calculator

Add chemicals safely

Pool chemicals are hazardous. Read this before you open a bag.

• Follow the label. Every product’s label is the final word on dose, handling and storage. This calculator estimates; the label and your health department’s rules win.
• Add one chemical at a time. Never put two chemicals in the pool or a bucket together, and let the pump run a while before the next one.
• Never mix them. Chlorine mixed with acid, or with another chlorine product, can release toxic gas or start a fire. Use a separate, clean scoop and bucket for each chemical, and never store different chemicals together. The Tulsa and Indiana guides both say never to mix chemicals of any kind.
• Add chemicals to water, never water to chemicals, and add acid to water. Dissolve dry products in a bucket of water first where the label says so.
• Keep swimmers out while you add chlorine, and protect your eyes and skin.
• Add large amounts in thirds over a couple of hours, as the Tulsa and Indiana guides advise, rather than all at once.

How it works

Shocking is arithmetic. A part per million of free chlorine is one milligram of chlorine in a litre of water, so the chlorine you need is the rise in ppm times the litres. The product is that chlorine divided by its strength.

rise     = target free chlorine − free chlorine now          (ppm)
chlorine = rise × pool volume (L) ÷ 1,000            (grams of available chlorine)
product  = chlorine ÷ strength       (dry: grams;  liquid: mL, strength as grams per 100 mL)

• The target is one of three things: the SLAM level for your stabilizer, ten times your combined chlorine (breakpoint), or any level you choose.
• Side effects are worked out from the chemistry. Dichlor adds about 0.9 ppm of cyanuric acid for every 1 ppm of chlorine, and calcium hypochlorite adds about 0.7 ppm of calcium hardness. Liquid chlorine adds neither.
• The chart plots the minimum, the target range and the SLAM level against CYA, using Trouble Free Pool’s chart rows joined by straight lines, with your pool marked on it.

Worked example

A 15,000 gallon pool with 40 ppm of stabilizer and 3 ppm of free chlorine, shocked to its SLAM level:

• SLAM level at 40 ppm CYA: 16 ppm, so the rise is 13 ppm
• Chlorine: 13 × 56,781 L ÷ 1,000 = 738 g
• 12.5% liquid chlorine: 738 g ÷ 0.125 g/mL = 5,905 mL = 1.56 gal (200 fl oz)
• Or 65% calcium hypochlorite: 738 g ÷ 0.65 = 1,136 g = 2.5 lb, which also adds 9 ppm of hardness

Keep the free chlorine at 16 ppm until the water is clear and passes the overnight chlorine loss test, re-dosing as it falls.

Tips from the pool pad

• Test with the right kit. High chlorine levels overwhelm basic strips and DPD drops. A FAS-DPD test is the one that reads a SLAM level.
• Balance pH first. High pH makes chlorine far less effective, and a pH test is unreliable during a SLAM, so set it before you begin.
• A bigger bottle is not a bigger dose. The calculator’s “have on hand” chip is about twice the first dose for a SLAM, because you add again as the chlorine is used.
• Shock at dusk. Sunlight breaks down chlorine quickly; adding it in the evening lets it work overnight.

FAQ

Sources

1. Pool Operators Permit Instruction Manual (section 3 and tables A and B, reprinted from Taylor Technologies), Tulsa Health Department. Amount of chlorine compound for 1 ppm by strength (65% calcium hypochlorite 2.05 oz per 10,000 gal; 10% liquid 12.8 fl oz); a 30 ppm algae shock table; breakpoint chlorination = combined chlorine × 10; never mix chemicals and follow the label.
2. Adjusting Chemical Levels in a Swimming Pool (Water Chemistry Adjustment Guide), Indiana Department of Health (adapted from the NSPF Pool & Spa Operator Handbook). Per ppm in 10,000 gal, 67% calcium hypochlorite 2 oz, 12% sodium hypochlorite 10.7 fl oz, dichlor 56% 2.4 oz and 62% 2.1 oz; worked examples of shocking a 200,000 gal and a 40,000 gal pool.
3. Swimming Pool Course Operator's Manual (Appendix B, breakpoint superchlorination worksheet), Idaho Public Health District 5. Combined chlorine × 10 is the amount to reach breakpoint; a 55,000 gal worked example using 2.0 oz of calcium hypochlorite per ppm per 10,000 gal.
4. The Free Chlorine and Cyanuric Acid Relationship (FC/CYA chart), Trouble Free Pool. Read from a Wayback Machine copy, because the site itself blocks automated access. Minimum, target and SLAM free chlorine for each CYA from 20 to 100 ppm; the SLAM level is about 40% of CYA. Also the SLAM article, which says to buy double the first dose.
5. New Thinking, Chlorine/Cyanuric Acid in Balance (Aqua Magazine, October 2015), Horizon Commercial Pools (reprint). Robert Lowry's recommendation that free chlorine be kept at 7.5% of the CYA level, e.g. about 2 ppm at 30 ppm CYA and 4 ppm at 50.

Formula and sources last checked September 30, 2026. How we test formulas.

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Safety note: Estimates only. Equipment ratings vary by manufacturer, so always follow the manufacturer’s instructions and ratings, and ask a qualified professional when safety is at stake.