Pool Shock Calculator
Find how much chlorine shock to add to your pool: cal-hypo or dichlor by weight, or liquid chlorine by volume, and how many packs or containers to buy. Enter your pool's volume or its dimensions, your free chlorine and the level you want, and the product.
- Choose US or Metric. This sets the units of the results and the pack and container sizes.
- Under Pool volume, enter your pool's volume in gallons, liters, or cubic meters. If you don't know it, choose Calculate from dimensions and enter the shape, size, and water depth of the pool. For a pool with a sloped bottom, enter the shallow end and deep end depths.
- Enter your Current free chlorine from your test kit. If you leave it blank, it counts as 0, which gives a little more than you need.
- Enter the Target free chlorine. It starts at 10 ppm, the top of the 5 to 10 ppm most shock labels call for.
- Pick the Product. The Chlorine strength fills in from the product. If your label shows a different strength, enter it. The reset link under the field brings back the value for the product.
- If your pack or container is not listed, enter its size in Custom pack size or Custom container size to add it to the results.
- Optionally, enter a price per pack, container, pound, kilogram, gallon, or liter to estimate the cost.
The volume, chlorine, and strength fields accept whole numbers and
decimals, such as 15,000 or 1.5. The dimension
and depth fields also accept fractions, such as 3 1/2.
How much shock you need
Free chlorine is measured in parts per million (ppm), which is milligrams per liter. The chlorine to add is:
Chlorine (g) = (target − current free chlorine) × pool volume (L) ÷ 1,000
For granular shock, the strength is the available chlorine by weight:
Granular shock = chlorine ÷ (available chlorine % ÷ 100)
For liquid chlorine, the strength is the grams of chlorine in 100 mL, so 12.5% liquid chlorine holds 125 g of chlorine per liter:
Liquid chlorine (L) = chlorine (g) ÷ (strength % × 10)
In US units, 1 gallon of liquid chlorine raises 10,000 gallons of water by as many ppm as its strength: 1 gallon of 12.5% liquid chlorine adds about 12.5 ppm, the figure the Pool & Hot Tub Alliance (PHTA) gives in its liquid chlorine fact sheet. One pound of 65% cal-hypo raises 10,000 gallons by about 7.8 ppm.
The calculator assumes all the chlorine you add becomes free chlorine and mixes evenly. It doesn't account for chlorine used up by dirt and sunlight, or for liquid chlorine that has weakened in storage, so test the water after adding and add more if you're short of the target.
Packs and containers are rounded up to whole ones. The calculator shows these sizes:
| Product | US | Metric |
|---|---|---|
| Granular | 1 lb | 10 kg, 4 kg, 2 kg, 1 kg |
| Liquid | 1 gal | 15 L, 12.5 L, 5 L |
Product strength
The calculator uses these strengths for each product:
| Product | Strength used |
|---|---|
| Cal-hypo 65% | 65% available chlorine |
| Cal-hypo 73% | 70% available chlorine |
| Dichlor 56% | 56% available chlorine |
| Liquid chlorine 10% | 10% (100 g per liter) |
| Liquid chlorine 12.5% | 12.5% (125 g per liter) |
The strengths come from product labels: the minimum 65% available chlorine on PPG's ZAPP Shock Treatment, the minimum 70% on Assault 73 and HTH Granular 73, the 56% on Puri Tech's dichlor, the 10% on HTH Pool Care Liquid Chlorine, and the 12.5% on Pool Clear and Hy-Clor liquid chlorine (Hy-Clor gives it as 125 g per liter).
Cal-hypo labels show two numbers: the calcium hypochlorite in the product and the available chlorine it gives. The number in the name can be either one. A product called 73% has 73% calcium hypochlorite and at least 70% available chlorine, so the calculator uses 70%. A product labeled 68% calcium hypochlorite and 65% available chlorine matches Cal-hypo 65%.
Many shocks sold at big-box stores, such as some HTH products, contain 52% to 56% calcium hypochlorite and are weaker than the presets. Choose a cal-hypo product and enter the available chlorine from the label. If the label doesn't give it, entering the calcium hypochlorite percentage gives a close amount.
For dichlor, use the available chlorine (56%), not the active ingredient (99% sodium dichloro-s-triazinetrione). Entering 99% would show only about 57% of the dichlor you need.
Liquid chlorine is sold at different strengths, usually 10% or 12.5%. US labels give the sodium hypochlorite as a percentage by weight. The calculator reads the percentage as grams per 100 mL, which gives slightly more liquid than a weight percentage would. That extra helps cover the strength liquid chlorine loses in storage.
For a product not listed, such as lithium hypochlorite or granular trichlor, choose a product of the same form (granular or liquid) and enter its strength. Switching between a granular product and a liquid product resets the strength, because the two percentages mean different things.
Target free chlorine
Shock labels usually call for 5 to 10 ppm. The HTH Granular 73 label says to add 9 to 18 ounces per 10,000 gallons for 5 to 10 ppm, and the Pool Clear liquid chlorine label says 52 to 104 fl oz per 10,000 gallons for the same range.
To reach breakpoint, the Indiana Department of Health says to add 10 times the combined chlorine. Find the combined chlorine by subtracting free chlorine from total chlorine, multiply it by 10 for the target, and enter your free chlorine as the current level.
After a fecal incident, the CDC gives free chlorine levels and times for hyperchlorination, such as 20 ppm for a set number of hours after a diarrheal incident. The time depends on the stabilizer (cyanuric acid, or CYA) in the water and on the pH. Follow the CDC guidance and your local rules for those levels and times.
Chlorine works less well in water with a lot of CYA. If your pool uses a stabilizer, check the product label for a target that fits your CYA level.
Pool volume
If you choose Calculate from dimensions, the calculator finds the volume from the surface area of each section and its average depth:
Volume = surface area × (shallow end depth + deep end depth) ÷ 2
| Shape | Area |
|---|---|
| Rectangle | length × width |
| Circle | π × diameter² ÷ 4 (or π × radius²) |
| Oval | π × length × width ÷ 4 |
| Kidney | (width A + width B) × length × 0.45 |
| Known area | the area you enter |
Width A and Width B of a kidney pool are the widest points across its two rounded ends. The kidney formula is an industry approximation, so treat its volume as an estimate. Enter water depths, measured from the bottom to the water line, not to the top of the wall. For a pool with a deep end that drops off suddenly, enter the shallow part and the deep part as separate sections.
The calculator converts 1 ft³ to 7.48 gallons. Many pool guides round it to 7.5, which gives about 0.26% more water.
Examples
Cal-hypo for a 15,000 gallon pool
To raise free chlorine from 0 to 10 ppm in 15,000 gallons (56,781 L), you need 567.8 g of chlorine. With 65% cal-hypo, that's 1.9 lb (30.8 oz, or 874 g): 2 packs of 1 lb. At $8.99 per pack, the cost is $17.98. With Cal-hypo 73% (70% available chlorine), it's 1.8 lb, and with dichlor, 2.2 lb.
Liquid chlorine for the same pool
With 12.5% liquid chlorine, the same 10 ppm takes 1.20 gal (153.6 fl oz): 2 containers of 1 gal. With 10% liquid chlorine, it takes 1.50 gal.
Breakpoint chlorination
A 60,000 gallon pool tests at 2.3 ppm total chlorine and 1.5 ppm free chlorine, so the combined chlorine is 0.8 ppm and the breakpoint target is 8 ppm. Raising free chlorine by 6.5 ppm with 67% cal-hypo takes 4.9 lb (77.7 oz): 5 packs of 1 lb.
A pool entered by its dimensions
A rectangular pool 32 ft by 16 ft, 3 1/2 ft deep at the shallow end and 8 ft at the deep end, has an average depth of 5.75 ft and holds 2,944 ft³, or 22,023 gallons. Raising it by 10 ppm with 12.5% liquid chlorine takes 1.76 gal (225.5 fl oz): 2 containers of 1 gal.
Metric
To raise free chlorine from 1 to 10 ppm in 50,000 L, you need 450 g of chlorine. That's 3.60 L of 12.5% liquid chlorine, or 0.7 kg (692 g) of 65% cal-hypo.
FAQ
About 1.3 lb (20.5 oz) of 65% cal-hypo to raise free chlorine by 10 ppm. With a 73% cal-hypo product (70% available chlorine), it's about 1.2 lb (19.1 oz).
About 1 gallon of 12.5% liquid chlorine per 10,000 gallons of water for each 12.5 ppm. To raise free chlorine by 10 ppm in a 10,000 gallon pool, that's 0.8 gal (102.4 fl oz) of 12.5% or 1 gal of 10%.
You don't need to add chlorine. The calculator shows 0 and says your free chlorine is already above the target. Free chlorine drops on its own with sunlight and use.
The available chlorine. Labels for granular shock list the active ingredient and the available chlorine separately. For dichlor, they're about 99% and 56%, and using 99% would show only about 57% of the dichlor you need.
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