How the calculation works

Chlorine tests report parts per million (ppm), and 1 ppm in water is 1 milligram per litre. That makes the maths short:

Chlorine needed (grams of available chlorine) = (target ppm − current ppm) × pool volume in litres ÷ 1,000. A US gallon is 3.785 litres, so 10,000 US gallons need 37.85 g (1.34 oz) of chlorine for each 1 ppm.
Granules and tablets: product weight = chlorine needed ÷ (available chlorine % ÷ 100). Use the "available chlorine" figure on the label, not the active ingredient percentage. For dichlor and trichlor the two are different.
Liquid chlorine and bleach: the calculator turns the label strength into grams of available chlorine per litre (explained below), then divides the chlorine needed by it.
What else goes in: dichlor and trichlor add stabilizer (cyanuric acid, CYA) and cal-hypo adds calcium, in fixed amounts for each ppm of chlorine (see the table further down).

Worked example

A 15,000 US gallon pool tests at 1 ppm free chlorine and you want 3 ppm. 15,000 gallons is 56,781 litres, so a 2 ppm rise needs 2 × 56,781 ÷ 1,000 = 113.6 g (4.0 oz) of available chlorine.

Pool liquid chlorine labelled 12.5% sodium hypochlorite holds about 142.9 g of available chlorine per litre, so you need 113.6 ÷ 142.9 = 0.79 litres, or 26.9 fl oz. If the label means 12.5 trade percent (125 g/L), it is 30.7 fl oz.
6% household bleach: 61.1 fl oz.
Cal-hypo at 65%: 113.6 ÷ 0.65 = 175 g (6.2 oz). It also adds 1.41 ppm of calcium hardness.
Dichlor at 56%: 7.2 oz, adding 1.82 ppm of CYA. Trichlor at 90%: 4.5 oz, adding 1.21 ppm of CYA.

In metric, a 50,000 litre pool raised from 1 to 3 ppm needs 100 g of available chlorine: 700 mL of 12.5% liquid chlorine (by weight), 800 mL of liquid labelled 125 g/L, or 154 g of 65% cal-hypo.

Liquid chlorine strength: by weight or trade percent?

Pool liquid chlorine and household bleach are both sodium hypochlorite in water. PHTA describes household bleach as 1.5% to 6% sodium hypochlorite by weight (Clorox's safety data sheet lists 5% to 10% for its regular bleach) and says pool liquid chlorine is usually made at 10% or 12.5%. Labels state that percentage in two different ways, and for a 12.5% product the two readings differ by about 14%:

Sodium hypochlorite by weight. US pesticide labels must give the active ingredient as a percentage by weight (EPA Label Review Manual), so "Sodium hypochlorite 12.5%" means 12.5 g in every 100 g of liquid. This is the calculator's default.
Trade percent, or available chlorine by volume: grams of available chlorine in 100 mL. A label reading "125 g/L available chlorine" is 12.5 trade percent. Rowell Chemical's specification lists both figures: its 12.5% by weight grade is 14.27 trade percent. PHTA's rule of thumb that one gallon of 12.5% raises 10,000 gallons by 12.5 ppm works on the trade percent basis, which is 10.2 fl oz per 1 ppm; the same PHTA fact sheet also gives 10.6 fl oz.

To turn a by-weight figure into grams per litre, the calculator multiplies it by 0.952 (the available chlorine in a gram of sodium hypochlorite, from the molar masses on PubChem) and by the liquid's density. It assumes a specific gravity of 1.10 up to 8.25% (Clorox's safety data sheet gives about 1.1 for 5–10% bleach), 1.20 at 12.5% (Formosa Plastics gives 1.15 to 1.25, Rowell Chemical 1.199) and 1.242 at 15% (Rowell Chemical's maximum), with a straight line between. That line gives 1.14 at 10%, inside Formosa's 1.13 to 1.20 for its 10% product. Real products vary by a few percent. As a check, Rowell lists 142.7 g/L of available chlorine for its 12.5% grade, and the calculator works out 142.9 g/L.

Liquid chlorine also weakens in storage. EPA asks sodium hypochlorite labels to warn that it degrades with age, sunlight and heat, and PHTA says 12.5% liquid loses half its strength in about 180 days at 77°F (25°C), or 48 days at 95°F (35°C). An old jug raises chlorine less than the calculator says, which is one more reason to add it in stages and retest.

What each product adds besides chlorine

Product	Available chlorine	Also adds, per 1 ppm of chlorine
Liquid chlorine, bleach	See above	No stabilizer or calcium (PHTA). Very alkaline: pH about 13 (PHTA).
Calcium hypochlorite (cal-hypo)	65% to 75% (PHTA)	0.71 ppm calcium hardness, as CaCO₃
Dichlor	55% to 57% (dihydrate), at least 62% (anhydrous) (FAO/WHO)	0.91 ppm CYA
Trichlor	About 90% (Wojtowicz)	0.61 ppm CYA

These amounts come from the chemical formulas. In water, each dichlor molecule splits into cyanuric acid and two units of chlorine (WHO), and trichlor carries three units of chlorine for each cyanuric acid (Wojtowicz). Calcium hypochlorite has one calcium for every two hypochlorites. Dividing the molar masses from PubChem gives the figures in the table; the calcium figure counts only the calcium in the calcium hypochlorite itself.

CYA builds up, because the only way to lower it is to replace some of the water with fresh water (MAHC annex, Oregon Health Authority). Dichlor and trichlor are also acidic: the MAHC annex warns that pH can drop quickly when they are used in water with low total alkalinity.

What free chlorine should you aim for?

Stabilizer protects chlorine from sunlight, but it also holds on to chlorine and slows down how fast it kills germs. The MAHC annex says that free chlorine should therefore rise in step with CYA. Here is who says what:

CDC, for home pools: at least 1 ppm free chlorine, or at least 2 ppm if you use CYA or a stabilized chlorine such as dichlor or trichlor, with pH between 7.0 and 7.8.
Model Aquatic Health Code (MAHC), for public pools: the same 1 and 2 ppm minimums, and CYA no higher than 90 ppm. A public pool must close if its CYA is more than 45 times its free chlorine, or if free chlorine is above 10 ppm while people are swimming. The annex recommends running pools at 2 to 4 ppm to keep a margin above the minimum. These are legal limits where a state or city adopts the code; they are not written for home pools.
Falk et al. (2019), the illness-risk study the MAHC annex draws on, recommends keeping CYA no more than 20 times the free chlorine, so free chlorine of at least CYA ÷ 20.
Orenda Technologies, a pool chemical maker, suggests free chlorine at 7.5% of the CYA level and 30 to 50 ppm of CYA in home pools. The Oregon Health Authority also calls 30 to 50 ppm the ideal CYA range.

CYA (ppm)	CDC and MAHC minimum	CYA ÷ 20 (Falk et al.)	7.5% of CYA (Orenda)
0	1	–	–
20	2	1	1.5
40	2	2	3
60	2	3	4.5
80	2	4	6

Free chlorine in ppm. The results above work these out for your own CYA reading.

Whatever level you choose, PHTA advises keeping swimmers out until free chlorine tests at 4 ppm or less. This calculator does not give shock or algae treatment doses; follow your product's label for those.

Handle pool chemicals safely

CDC says thousands of preventable pool chemical injuries happen every year. The basics:

Never mix chemicals. Chlorine mixed with acid can release toxic gas, and different pool chemicals, including different types of chlorine, must not be mixed with each other or with anything else (CDC). Trichlor and cal-hypo can react explosively (PubChem, citing NFPA).
Add chemical to water, never water to chemical. Only pre-dissolve a product if its label says so, and then add the product to the water (CDC).
Protect yourself. Read the whole label first, wear safety goggles and gloves, work where there is good airflow, and open one container at a time (CDC).
Keep people out of the water. No one should be swimming while you add chemicals (New Jersey Department of Health). Run the pump, and pour slowly, close to the water surface, in the deep end away from metal ladders and lights (PHTA).
Add big doses in stages. Pour in part of the dose, let the pump mix it, retest and then add more if needed. Hypochlorite labels tell you to use a test kit and increase the dose as necessary (EPA).
Store products safely. Keep them closed in their original labelled containers, cool, dry and out of direct sun, locked away from children and pets, with acid and chlorine kept apart and liquids stored low (CDC), never above solids (MAHC annex).

Common questions

How much bleach raises chlorine by 1 ppm in 10,000 gallons?: About 20.4 fl oz of 6% bleach or 14.8 fl oz of 8.25% bleach. Pool liquid chlorine labelled 12.5% sodium hypochlorite takes about 9 fl oz, or 10.2 fl oz if the 12.5% is trade percent (125 g/L). In metric, 1 ppm in 10,000 litres needs 10 g of available chlorine, which is about 159 mL of 6% bleach.
How much will dichlor or trichlor raise my stabilizer (CYA)?: Every 1 ppm of chlorine from dichlor adds about 0.91 ppm of CYA, and every 1 ppm from trichlor about 0.61 ppm. Raising free chlorine by 10 ppm with dichlor adds about 9.1 ppm of CYA, or 6.1 ppm with trichlor. CYA only comes down when you replace some of the water (MAHC annex), so if your CYA is already high, an unstabilized chlorine such as liquid chlorine avoids adding more.
What chlorine level should I keep with 40 ppm of stabilizer?: CDC's minimum for a pool that uses stabilizer is 2 ppm. Falk et al.'s 20:1 ratio also gives 2 ppm at 40 ppm CYA, and Orenda's 7.5% rule gives 3 ppm. The MAHC annex suggests running pools at 2 to 4 ppm. Whatever you choose, PHTA advises waiting until free chlorine is 4 ppm or less before swimming.
Can I swim straight after adding chlorine?: No one should be in the water while you add chemicals (New Jersey Department of Health). Let the pump mix the water, retest, and wait until free chlorine is 4 ppm or less, as PHTA advises, or the level your product's label gives.
Is household bleach the same as pool chlorine?: Chemically, yes: both are sodium hypochlorite, and neither adds stabilizer or calcium. Bleach is weaker, so you need more. In the worked example it takes 61.1 fl oz of 6% bleach against 26.9 fl oz of 12.5% liquid chlorine. Where the MAHC annex mentions household bleach for pools, it suggests unscented bleach whose label shows it is EPA-registered as a drinking water disinfectant.

Sources

Last checked and updated: October 31, 2026.