Pool Shock Calculator: How Much Chlorine to Shock a Pool

Pool volumegal

Free chlorine (FC) nowppm

Combined chlorine from

A combined chlorine (CC) readingTotal chlorine (TC): CC = TC − FC

Combined chlorine (CC)ppm

Cyanuric acid (CYA)ppm

Shock targetppm

Chlorine productLiquid chlorine (sodium hypochlorite)Calcium hypochlorite (cal hypo)Dichlor (sodium dichloroisocyanurate)Trichlor (trichloroisocyanuric acid)

Liquid chlorine: what your label's % isAvailable chlorine %Sodium hypochlorite % (converted × 70.90/74.44)

Liquid chlorine: % on your label%

Liquid chlorine: specific gravity (1.2 for 12.5%, 1.155 for 10%, or your SDS)

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This calculator works out how much chlorine it takes to lift free chlorine (FC) to a shock target, using the product you have and the strength printed on its label: liquid chlorine, cal hypo, dichlor or trichlor. It also reads your combined chlorine (CC), since clearing it can call for more than the gap to the target, and your cyanuric acid (CYA), which the lowest everyday free chlorine depends on. The dose scales with the pool's volume in gallons, which the pool volume calculator can work out from your measurements.

Worked example: 15,000 gallons of liquid chlorine

This example uses the figures the calculator starts with. A 15,000-gallon pool tests at 2 ppm free chlorine, 0.5 ppm combined chlorine and 50 ppm CYA, and the shock target is 10 ppm. The jug's label says 11.9% available chlorine, and its specific gravity is 1.2.

• Free chlorine has to rise by 8.0 ppm. That takes 107.6 fl oz of liquid chlorine, which is 0.84 gal.
• Free chlorine after the dose is at most 10.0 ppm, because breakpoint uses some of it up. Before swimming, wait for it to fall to 4 ppm, the re-entry limit on the liquid chlorine label the calculator cites (EPA Reg. No. 82357-1).
• At 50 ppm CYA the lowest everyday free chlorine is 2.0 ppm, the minimum with CYA set by CDC and by MAHC 5.7.3.1.1.2.2 (a public-pool code). CYA ÷ 45 gives only 1.1 ppm.

Worked example: when breakpoint sets the dose

A second example: a 10,000-gallon pool reads 3 ppm free chlorine, 1.0 ppm combined chlorine and 30 ppm CYA, the target is still 10 ppm, and the product is dichlor at 56% available chlorine.

• Breakpoint calls for 10 × 1.0 = 10.0 ppm, which beats the 7 ppm gap to the target. So the dose is 10.0 ppm of free chlorine: 23.8 oz (1.49 lb) of dichlor.
• Dichlor carries cyanuric acid with it, and this dose adds 9.1 ppm of CYA.
• Free chlorine lands at 13.0 ppm at most, since breakpoint uses some of the dose. That is above 10 ppm, so the calculator shows a warning row, Above 10 ppm after the dose, and a Before swimming row with dichlor's own 3 ppm limit.

Breakpoint and combined chlorine

The MAHC glossary (a public-pool code's definitions) describes combined chlorine as chlorine held in chloramines, and says "Combined available chlorine is less reactive than free available chlorine". Clearing it takes breakpoint chlorination: "The amount of free chlorine that must be added to the water to achieve breakpoint chlorination is approximately 10 times the … amount of combined chlorine in the water." So the calculator doses the larger of two figures: the gap between your free chlorine and the target, or 10 times your combined chlorine.

If your test kit gives total chlorine instead, pick Total chlorine (TC) under Combined chlorine from. The same MAHC glossary, part of a public-pool code, works combined chlorine out as "CC = TC – DPD-FC." (DPD-FC is free chlorine read by a DPD test), and the calculator subtracts your free chlorine reading the same way.

Shock levels on product labels

HTH's cal hypo label says to "super chlorinate the pool by adding 9-18 ounces … per 10,000 gallons of water to provide 5 to 10 ppm available chlorine." A liquid chlorine label, EPA Reg. No. 55852-3, says to "superchlorinate with 52 to 104 oz. of product for each 10,000 gallons of water to yield 5 to 10 PPM available chlorine by weight." The calculator's default target, 10 ppm, is the top of that range. Set a lower Shock target if your own label calls for one.

Those shock levels run past what the labels allow for swimming. The HTH label says "Re-entry into treated pools is prohibited above 4 ppm due to risk of bodily harm." Other labels set their own limits: EPA Reg. No. 82357-1 also prohibits swimming above 4 ppm, and the trichlor label (EPA Reg. No. 42177-18) sets 3 ppm: "Reentry into treated pools is prohibited above 3 ppm chlorine due to risk of bodily injury." The calculator shows a Before swimming row with the picked product's limit whenever free chlorine after the dose is above it.

The trichlor label describes its tablets as "[For routine use in automatic feeders, float- and plastic skimmers]", so the calculator shows that line with any trichlor dose.

The dichlor label (EPA Reg. No. 7124-31) sets 3 ppm, but this site's scan of it interleaves three columns, so no sentence of it survives word for word. The calculator paraphrases it instead, and names the label: bathers should not be allowed to enter the pool until the chlorine residual has dropped to 3.0 ppm or less. Another panel of the same label puts the line one step lower, saying the pool should not be entered until the chlorine residual is below 3.0 ppm. The calculator follows the stricter of the two, so dichlor's Before swimming row shows at 3.0 ppm as well as above it.

Some pool-care communities scale shock levels to CYA, setting free chlorine as a percentage of the CYA reading. This site found no primary source for that method, so the calculator doesn't use it. The everyday floor the calculator does use is the higher of two figures:

• CDC's minimum for home pools: at least 2 ppm with CYA, and 1 ppm without. MAHC 5.7.3.1.1.2, a public-pool code, sets the same 2.0 and 1.0 ppm.
• CYA ÷ 45, the 45:1 ratio of MAHC 6.6.3.1 (a public-pool code). It only goes above 2 ppm when CYA is above 90 ppm.

The cyanuric acid calculator explains both.

Which percentage is on your jug

Liquid chlorine labels print two kinds of percentage, and the calculator needs the right one. EPA Reg. No. 82357-1 lists sodium hypochlorite at 12.5% and "Available Chlorine 11.9%". That is a weight figure (12.5 × 0.953 = 11.9), and it's the number Liquid chlorine: % on your label expects.

EPA Reg. No. 55852-3 lists the same 12.5% sodium hypochlorite and "[Available Chlorine 15%]". That is more than the sodium hypochlorite itself, so it can't be a percentage by weight; it's a trade percentage. HASA explains trade %: "1 gallon of (X trade %) sodium hypochlorite added to 10,000 gallons of water results in (X ppm) of free chlorine." Entering 15 for an 11.9% jug would underdose by about 21% (1 − 11.9 ÷ 15). The calculator adds a note when the available chlorine comes out above 12.5%, more than any weight-% label this site saved.

If a label gives only the sodium hypochlorite %, set Liquid chlorine: what your label's % is to Sodium hypochlorite %, and the calculator multiplies by 70.90/74.44, the molar masses of chlorine and sodium hypochlorite. EPA Reg. No. 1258-1094 prints the same conversion: "Conversion of sodium hypochlorite to available chlorine is 0.953."

The calculator also needs the liquid's specific gravity to turn a weight of chlorine into fluid ounces. It defaults to 1.2 for 12.5% product, from HASA's SDS ("1.2 g/mL or 10 lb/gallon"). For 10% product HASA's SDS gives "1.155 g/mL or 9.6 lb/gallon", and EPA Reg. No. 1258-1094 prints 1.158. Use your own SDS if it differs.

The solid products' defaults

Cal hypo. The HTH Granular 73 label lists 73% calcium hypochlorite and a minimum of 70% available chlorine. The calculator defaults to the 70% minimum. The label's own chart, 16 oz in 10,000 gallons for 8.75 ppm, works out at 73%. At 70% the calculator asks for about 4% more product than it would at 73%.

Dichlor. EPA Reg. No. 7124-31 says it "Provides 56% Available Chlorine", the calculator's default.

Trichlor. EPA Reg. No. 42177-18 lists "Available Chlorine: 90%", the default for trichlor. Dichlor and trichlor both add cyanuric acid, and the calculator shows how much.

Calcium from cal hypo. PHTA's fact sheet says "0.8 ppm of calcium hardness is added to the water for each ppm of available chlorine added". The molar masses give 0.706. The fact sheet doesn't show where 0.8 comes from, so the Calcium hardness this adds row gives the 0.706 figure as a minimum, with PHTA's figure beside it.

Public-pool code: MAHC's fecal-incident hyperchlorination

This section is public-pool procedure, not a home-pool recipe. For diarrheal contamination, MAHC 6.5.3, a public-pool code, says to "raise the DPD-FC concentration to at least 20.0 ppm (mg/L) and maintain for at least 12.75 hours".

That 12.75 hours is for water without cyanuric acid. In water with CYA, MAHC 6.5.3.2.1, part of the same public-pool code, first calls for "Lowering the CYA concentration to less than or equal to 15 ppm (mg/L) by draining and then refilling with water from the approved water source per MAHC 4.11.1, if necessary", and then for "Raising the DPD-FC concentration to 20 ppm (mg/L); 30 ppm (mg/L); or 40 ppm (mg/L)", each for its own time in Table 6.5.3.2.1 of that code: at 15 ppm CYA or less, 20 ppm of chlorine has to hold for 28 hours. The calculator doesn't model any of this procedure.