A 15,000 gallon pool with 2 ppm free chlorine and 0.8 ppm combined chlorine needs 8 ppm of extra chlorine to break the chloramines down: 0.96 gallons (123 fl oz, 3.6 L) of 12.5% liquid chlorine, so one gallon jug. A SLAM in the same pool with 40 ppm cyanuric acid (CYA) aims at 16 ppm free chlorine, which is 14 ppm more than the pool holds: 1.68 gallons (6.4 L), so two jugs. Pick the mode above, enter your own gallons and readings, and the amount follows.
How the shock dose is calculated
Every shock dose is a number of ppm multiplied by your water volume and by how much of the product it takes to make one ppm. The number of ppm depends on the mode.
Product = ppm to add × gallons ÷ 10,000 × product per ppm per 10,000 gal
Routine shock follows the breakpoint rule in the CDC Model Aquatic Health Code: add about ten times the combined chlorine, which is total chlorine minus free chlorine. With 0.8 ppm combined chlorine that is 8 ppm, which takes free chlorine from 2 to 10 ppm.
SLAM (Shock Level And Maintain) holds free chlorine at 40% of your CYA until three things are true: combined chlorine is 0.5 ppm or lower, the pool loses no more than 1 ppm of free chlorine overnight, and the water is clear. The 40% figure comes from Trouble Free Pool and matches the Pool Research table of 12, 16 and 20 ppm at CYA 30, 40 and 50. At CYA 40 and 2 ppm free chlorine that is 16 − 2 = 14 ppm, and 14 × 1.5 × 10.24 fl oz = 215 fl oz, or 1.68 gallons.
Algae uses the SLAM level for every colour. The colour changes what you do around the chemistry, not the target.
How much of each shock product
Strength decides the amount. These are the amounts for +1 ppm of free chlorine in 10,000 gallons, with what else each product leaves behind.
| Product | Per 1 ppm, 10,000 gal | Also adds |
|---|---|---|
| Liquid chlorine 12.5% | 10.24 fl oz | about 0.8 ppm salt |
| Liquid chlorine 10% | 12.8 fl oz | about 0.8 ppm salt |
| Cal-hypo 73% | 1.83 oz | 0.8 ppm calcium hardness |
| Cal-hypo 65% | 2.05 oz | 0.8 ppm calcium hardness |
| Dichlor 62% | 2.15 oz | 0.9 ppm CYA |
| Dichlor 56% | 2.38 oz | 0.9 ppm CYA |
The PHTA fact sheet on sodium hypochlorite says 10.6 fl oz of 12.5% per ppm, but the same sheet says one gallon gives 12.5 ppm in 10,000 gallons, which is 10.24 fl oz. We use the arithmetic. Liquid chlorine loses strength as it ages, so a jug that has sat in a hot garage gives less than its label.
For a SLAM at CYA 40, the same 14 ppm from 73% cal-hypo is 2.40 lb and adds 11.2 ppm of calcium hardness. Dichlor would be 3.13 lb and would add 12.6 ppm of CYA, which raises the SLAM level you then have to hold. That is why liquid chlorine or cal-hypo are the SLAM products.
Green, yellow and black algae
Green water is the standard case: reach the SLAM level, brush, and run the filter without breaks. Yellow (mustard) algae clings to walls and survives ordinary chlorine, so brush every surface, including steps, the skimmer and under ladders, and keep the SLAM level for a day after the water looks clear. Swim University notes that high CYA makes it harder still. Black algae roots into plaster and grout: scrub the spots with a stainless steel brush before you shock, then hold the SLAM level.
The overnight loss test
Test free chlorine after sunset, then again next morning before the sun is on the water. If you lose more than 1 ppm the pool still has something to burn off. Enter both readings on the calculator and it tells you whether you can stop, or how much to add to get back to the SLAM level. Early in a SLAM chlorine drops fast, so Pool Magazine suggests testing every two to three hours.
When CYA is high
The SLAM level is 40% of CYA, so at CYA 80 you would hold 32 ppm. Above about 80 ppm that becomes impractical. Replace part of the water first: the pool stabilizer calculator gives the percentage. Liquid chlorine and cal-hypo also raise pH, so check it afterwards with the pool pH calculator.
Non-chlorine shock
Potassium monopersulfate oxidises contaminants but is not a sanitizer or algaecide, so it never replaces chlorine for algae. Left in the water it reads as combined chlorine on a DPD test, which is why the PHTA suggests testing before adding it or the next morning. Use the amount on its own label.
What to do next
If free chlorine is simply low and there is no algae, the pool chlorine calculator sets the target from your CYA. Enter all your readings at once in the pool chemical calculator, and measure your gallons first with the pool volume calculator. More tools are in the pool chemistry hub.