Pool Chlorine Calculator: How Much Chlorine to Add?
Raising the free chlorine of a 20,000 gallon pool from 1 ppm to 4 ppm takes 61 fl oz (0.48 gal) of 12.5% liquid chlorine. That is 3 ppm × 20,000 gal × 8.3454e-6 lb per gallon per ppm = 0.5 lb of pure chlorine, divided by the 12.5 g per 100 mL (1.04 lb per gallon) in 12.5% trade liquid chlorine. Pick your product, enter the volume, the free chlorine you tested and the level you want. The calculator also sizes a breakpoint shock from your combined chlorine and checks the target against your cyanuric acid. Never mix chlorine products, and add chemical to water, never water to chemical.
Figures based on the sources listed belowUpdated How we calculate
61 fl oz (0.48 gal) of 12.5% liquid chlorine.
61 fl oz (0.48 gal) of 12.5% liquid chlorine
- Product to add: 61 fl oz (0.48 gal) of 12.5% liquid chlorine
- In metric: 1.82 L
- Pure chlorine needed: 0.501 lb (8.01 oz): 3 ppm × 20,000 US gal × 8.3454e-6 lb per gal per ppm
- Per 1 ppm in this pool: 20 fl oz (0.16 gal) of 12.5% liquid chlorine raises 20,000 US gal by 1 ppm
- About this product: 12.5% trade means 12.5 g of available chlorine per 100 mL (125 g/L), about 10.8% by weight; at a typical density of 1.16 g/mL (an estimate) a gallon holds 1.043 lb of available chlorine
- Shock to breakpoint: 0.5 ppm combined chlorine × 10 = 5 ppm to add: 102 fl oz (0.8 gal) of 12.5% liquid chlorine
- CYA check: at 40 ppm CYA the minimum FC is about 3 ppm (7.5% of CYA, an estimate from Trouble Free Pool’s chart). Your 4 ppm target is at or above it
- Target check: 4 ppm is inside the typical 1 to 4 ppm residential range (an estimate)
Shopping list
- Liquid chlorine, 1 gal
- Cal-hypo shock, 73%
- FAS-DPD pool test kit (FC, CC, CYA)
- Cyanuric acid (stabiliser)
- Chemical splash goggles and gloves
Your project
- Pool volume: 20,000
- Volume unit: US gallons
- Chlorine product: Liquid chlorine, 12.5% (trade)
- Current free chlorine: 1 ppm
- Target free chlorine: 4 ppm
- Cyanuric acid (CYA): 40 ppm
- Combined chlorine: 0.5 ppm
Also pick up
- Liquid chlorine, 1 gal
- Cal-hypo shock, 73%
- FAS-DPD pool test kit (FC, CC, CYA)
- Cyanuric acid (stabiliser)
- Chemical splash goggles and gloves
Reopen this calculation: /calculators/pool-chlorine/?s=eyJyb29tIjp7Imxlbmd0aEZ0IjowLCJ3aWR0aEZ0IjowfSwiZGVkdWN0aW9ucyI6W10sIndhc3RlUGN0IjowLjEsInBhcmFtcyI6eyJwb29sVm9sdW1lIjoyMDAwMCwidm9sdW1lVW5pdCI6MCwicHJvZHVjdCI6MSwiY3VycmVudEZjIjoxLCJ0YXJnZXRGYyI6NCwiY3lhUHBtIjo0MCwiY29tYmluZWRDbCI6MC41fX0
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Chlorine product to raise 10,000 gallons by 1 ppm
| Product | Strength | Amount | Metric |
|---|---|---|---|
| Liquid chlorine, 10% (trade) | 10 g/100 mL (8.8% by weight) | 13 fl oz (0.1 gal) | 379 mL |
| Liquid chlorine, 12.5% (trade) | 12.5 g/100 mL (10.8% by weight) | 10 fl oz (0.08 gal) | 303 mL |
| Household bleach, 6% | 6.1 g/100 mL (5.7% by weight) | 21 fl oz (0.16 gal) | 619 mL |
| Household bleach, 8.25% | 8.6 g/100 mL (7.9% by weight) | 15 fl oz (0.12 gal) | 438 mL |
| Calcium hypochlorite (cal-hypo), 65% | 65% by weight | 2.1 oz by weight (0.13 lb) | 58 g |
| Calcium hypochlorite (cal-hypo), 73% | 73% by weight | 1.8 oz by weight (0.11 lb) | 52 g |
| Dichlor, 56% | 56% by weight | 2.4 oz by weight (0.15 lb) | 68 g |
| Trichlor, 90% | 90% by weight | 1.5 oz by weight (0.09 lb) | 42 g |
12.5% liquid chlorine by pool size and ppm to add
| Pool volume | +1 ppm | +2 ppm | +3 ppm | +5 ppm |
|---|---|---|---|---|
| 5,000 gal | 5 fl oz | 10 fl oz | 15 fl oz | 26 fl oz |
| 10,000 gal | 10 fl oz | 20 fl oz | 31 fl oz | 51 fl oz |
| 15,000 gal | 15 fl oz | 31 fl oz | 46 fl oz | 77 fl oz |
| 20,000 gal | 20 fl oz | 41 fl oz | 61 fl oz | 102 fl oz |
| 30,000 gal | 31 fl oz | 61 fl oz | 92 fl oz | 154 fl oz |
73% calcium hypochlorite by pool size and ppm to add
| Pool volume | +1 ppm | +2 ppm | +3 ppm | +5 ppm |
|---|---|---|---|---|
| 5,000 gal | 0.9 oz | 1.8 oz | 2.7 oz | 4.6 oz |
| 10,000 gal | 1.8 oz | 3.7 oz | 5.5 oz | 9.1 oz |
| 15,000 gal | 2.7 oz | 5.5 oz | 8.2 oz | 13.7 oz |
| 20,000 gal | 3.7 oz | 7.3 oz | 11 oz | 18.3 oz |
| 30,000 gal | 5.5 oz | 11 oz | 16.5 oz | 27.4 oz |
Breakpoint shock for a 20,000 gallon pool
| Combined chlorine | Add | 12.5% liquid | 73% cal-hypo |
|---|---|---|---|
| 0.2 ppm | 2 ppm | 41 fl oz (0.32 gal) | 7.3 oz |
| 0.4 ppm | 4 ppm | 82 fl oz (0.64 gal) | 14.6 oz |
| 0.5 ppm | 5 ppm | 102 fl oz (0.8 gal) | 18.3 oz |
| 1 ppm | 10 ppm | 205 fl oz (1.6 gal) | 36.6 oz |
| 2 ppm | 20 ppm | 410 fl oz (3.2 gal) | 73.2 oz |
| 3 ppm | 30 ppm | 614 fl oz (4.8 gal) | 109.7 oz |
Minimum free chlorine by cyanuric acid
| Cyanuric acid | Minimum free chlorine |
|---|---|
| None | No CYA-based minimum; public codes use 1 ppm |
| 20 ppm | 1.5 ppm |
| 30 ppm | 2.3 ppm |
| 40 ppm | 3 ppm |
| 50 ppm | 3.8 ppm |
| 60 ppm | 4.5 ppm |
| 80 ppm | 6 ppm |
| 100 ppm | 7.5 ppm |
Methodology & assumptions
One part per million (ppm) is one milligram per litre, which for water is one pound of chemical per million pounds of water. Water weighs 8.3454 lb per US gallon (3.785411784 kg), so 1 ppm in a gallon is 8.3454 ÷ 1,000,000 = 8.3454e-6 lb, and in 10,000 gallons it is 0.0835 lb, or 1.335 oz by weight, of available chlorine. The product needed is: pounds of product = ppm to add × gallons × 8.3454e-6 ÷ the available-chlorine fraction of the product. For the default pool that is 3 × 20,000 × 8.3454e-6 = 0.5007 lb of chlorine. Dry products are weighed: 65% cal-hypo (2.05 oz per ppm per 10,000 gal), 73% cal-hypo (1.83 oz), 56% dichlor (2.38 oz) and 90% trichlor (1.48 oz); county dosing sheets round these to 2, 1.8 to 2, 2.4 to 2.5 and 1.5 oz. Dry weights are ounces by weight, not fluid ounces or scoops. Liquids are measured by volume, and their strength has two meanings. Pool liquid chlorine is sold by trade percent, the grams of available chlorine in 100 mL, so 12.5% is 125 g per litre and a gallon holds 0.125 × 8.3454 = 1.043 lb of chlorine, whatever its density. The same product is only about 10.8% available chlorine by weight, because a litre weighs more than a kilogram: weight % = trade % ÷ density, here 12.5 ÷ 1.16 = 10.8%. Densities of 12.5% product run from about 1.16 to 1.22 g/mL between makers, so the density used here is a typical figure (an estimate); it affects only the by-weight percentage shown, not the dose, as long as the container really is trade percent. Some industrial 12.5% product is labelled as 12.5% sodium hypochlorite by weight, which is stronger (about 14% trade); check which your supplier means. Household bleach labels give percent sodium hypochlorite by weight. Available chlorine is that × 70.906 ÷ 74.442 = 0.9525 (so 8.25% is 7.86% available chlorine), and a typical density (1.07 g/mL for 6%, 1.10 for 8.25%, estimates) converts it to grams per 100 mL: 6.1 g and 8.6 g. Bleach doses depend on that density estimate to within about 2%. Breakpoint shock: chloramines (combined chlorine) are destroyed once enough free chlorine is added; the usual rule, from state health department guidance, is to add 10 times the combined chlorine in ppm. Combined chlorine is total chlorine minus free chlorine. Trouble Free Pool advises a different approach, shocking to a level set by CYA and holding it until the water passes tests (SLAM), which is not calculated here. CYA: cyanuric acid binds chlorine to protect it from sunlight but slows its sanitising, so the free chlorine needed rises with CYA. Trouble Free Pool’s chart sets the minimum FC at about 7.5% of CYA (for example 3 ppm at 40 ppm CYA, 7 to 7.5 ppm at 100 ppm), which is a pool-owner community guideline, not a code; it is shown as an estimate. Ranges: public-pool codes based on the CDC Model Aquatic Health Code set a minimum of 1 ppm free chlorine (2 ppm where cyanuric acid is used, in some local codes), and the 2024 edition treats more than 10 ppm with swimmers present as an imminent health hazard. A residential range of 1 to 4 ppm is typical and is shown as an estimate; follow your local rules and your test kit. Chlorine is used up by sunlight, swimmers and organic matter, so a dose is a starting point: retest after the water has circulated, and adjust. The calculator does not account for pH, which changes how well chlorine works, or for the CYA that dichlor and trichlor add (about 0.9 ppm of CYA per ppm of FC for dichlor, 0.6 for trichlor; typical figures), so these are less suited to routine use in a pool that already has enough CYA. Litres and imperial gallons are converted to US gallons first. Safety: never mix chlorine products with each other, with acid or with ammonia products. Add chemical to water, never water to chemical, wear splash goggles and gloves, and keep swimmers out until the level is back in range.
- CDC, Model Aquatic Health Code (MAHC), 5th edition (2024), water quality section 5.7.3: free chlorine above 10 ppm while bathers are present is an imminent health hazard; maximum free chlorine otherwise follows the product label. The 1 ppm minimum for pools (2 ppm with cyanuric acid in some codes) is read from county summaries of MAHC-based codes, not from the code text, so check the current MAHC and your local code
- San Francisco Department of Public Health, State standards for chemical levels in swimming pools and spas: public pool free chlorine 1.0 to 10.0 ppm without cyanuric acid and 2.0 to 10.0 ppm with it
- Salt Lake County Health Department, Pool chemical dosages: ounces per 10,000 gallons to raise chlorine 1 ppm (cal-hypo 2 oz, dichlor 2.5 oz, trichlor 1.5 oz), which this calculator reproduces to within rounding
- Yakima County Health District, Chemical Dosing: the same per-ppm doses, and about 13 fl oz of 10% liquid chlorine per ppm per 10,000 gallons
- Illinois Department of Public Health, Procedure for hyperchlorination: required product is the pure chlorine needed divided by the available-chlorine fraction of the product
- Indiana Department of Health, How to Shock the Pool (2022): breakpoint chlorination adds 10 times the combined chlorine (BPC = CC × 10)
- California State Water Resources Control Board, Sodium hypochlorite dosing examples: percent sodium hypochlorite as weight per volume and the role of specific gravity (about 1.2 for 12.5% product)
- Trouble Free Pool forum, “15% Sodium Hypochlorite” thread: weight % available chlorine = trade % ÷ specific gravity, with about 1.16 g/mL for 12.5% product. A community source, used for the density estimate only
- Trouble Free Pool, FC/CYA Levels chart (Pool School): minimum free chlorine about 7.5% of cyanuric acid. Read from forum posts quoting the chart (for example CYA 100: minimum 7 ppm), not from the chart itself, so treat the percentage as an estimate
- Weight of water: 8.3454 lb per US gallon (3.785411784 kg, from the NIST definition of the US gallon as 231 cubic inches and water at about 1 kg per litre); 1 ppm = 1 mg/L. Atomic weights: Cl2 70.906, NaOCl 74.442
How to work out how much chlorine to add to a pool
- Find the pool volume. Use the volume from your pool’s papers, or work it out from its size and water depth with the pool volume calculator.
- Test free chlorine, combined chlorine and CYA. Use a DPD test kit for free and total chlorine (combined is total minus free) and a CYA test.
- Choose the product and strength. Read the strength on the container: trade percent for pool liquid chlorine, percent sodium hypochlorite for bleach, percent available chlorine for dry products.
- Set the target. Pick the free chlorine you want, at least the minimum for your CYA (about 7.5% of it) and not above 10 ppm.
- Add safely and retest. With the pump running, pour the liquid slowly into the water, or dissolve dry product in a bucket of water as its label says, never the reverse and never two products together. Retest after the water has circulated.
Worked examples
| Project | Result |
|---|---|
| 20,000 gallon pool, free chlorine 1 to 4 ppm, 12.5% liquid chlorine | 61 fl oz (0.48 gal) of 12.5% liquid chlorine |
| Same pool with 6% household bleach | 126 fl oz (0.98 gal) of 6% bleach |
| Same pool with 8.25% household bleach | 89 fl oz (0.69 gal) of 8.25% bleach |
| Same pool with 73% cal-hypo | 11 oz by weight (0.69 lb) of 73% cal-hypo |
| Same pool with 56% dichlor | 14 oz by weight (0.89 lb) of 56% dichlor |
| Same pool with 90% trichlor | 8.9 oz by weight (0.56 lb) of 90% trichlor |
| 10,000 gallon pool, 0 to 2 ppm, 65% cal-hypo | 4.1 oz by weight (0.26 lb) of 65% cal-hypo |
| 75,000 litre pool, 1 to 3 ppm, 12.5% liquid chlorine | 41 fl oz (0.32 gal) of 12.5% liquid chlorine, 1.2 L |
| 15,000 gallon pool at target with 1 ppm combined chlorine | Nothing to add: free chlorine is at or above the target; breakpoint shock 154 fl oz (1.2 gal) of 12.5% liquid chlorine |
| Pool already above target (5 ppm against 4 ppm) | Nothing to add: free chlorine is at or above the target |
Frequently asked questions
- How much chlorine do I add to raise free chlorine by 1 ppm?
- In 10,000 gallons, 1 ppm takes 0.0835 lb (1.34 oz by weight) of pure chlorine. That is about 10 fl oz (0.08 gal) of 12.5% liquid chlorine, 1.8 oz by weight (0.11 lb) of 73% cal-hypo, 2.4 oz by weight (0.15 lb) of 56% dichlor or 1.5 oz by weight (0.09 lb) of 90% trichlor. Scale by the ppm and by gallons ÷ 10,000.
- What does 12.5% trade mean for liquid chlorine?
- Trade percent is weight of available chlorine per volume: 12.5% means 12.5 g in each 100 mL, or 125 g per litre. Because the liquid is denser than water (about 1.16 g/mL, a typical figure) the same product is only about 10.8% available chlorine by weight. Dose liquids by volume with the trade percent and the density does not matter. Be careful with industrial product labelled 12.5% by weight, which is stronger.
- How much household bleach is that?
- Plain household bleach has to be unscented and without added thickeners for pool use, and its label gives percent sodium hypochlorite by weight. For the default pool (1 to 4 ppm in 20,000 gallons) you need 126 fl oz (0.98 gal) of 6% bleach or 89 fl oz (0.69 gal) of 8.25% bleach. Bleach is weaker than pool liquid chlorine, and bleach strength drops with age and heat, so use a fresh container.
- Why is cal-hypo, dichlor or trichlor dosed in ounces by weight?
- They are dry, so they are weighed. For the same 3 ppm in 20,000 gallons you need 11 oz by weight (0.69 lb) of 73% cal-hypo, 14 oz by weight (0.89 lb) of 56% dichlor, or 8.9 oz by weight (0.56 lb) of 90% trichlor. Dichlor and trichlor are stabilised: they add cyanuric acid to the water each time, so they suit a pool that needs CYA, not one that already has plenty. Trichlor tablets dissolve slowly and are for a feeder, not a same-day fix.
- What is breakpoint chlorination and how much shock do I need?
- Combined chlorine (chloramines, total minus free chlorine) is what smells like chlorine and irritates eyes. Breakpoint chlorination adds enough free chlorine to destroy it; the usual rule from health departments is to add 10 times the combined chlorine in ppm. A 15,000 gallon pool with 1 ppm combined chlorine needs 10 ppm added, which is 154 fl oz (1.2 gal) of 12.5% liquid chlorine. Retest after the water has circulated; if combined chlorine is still above about 0.5 ppm, repeat. Other methods (such as SLAM) hold a level based on CYA instead.
- How much free chlorine do I need with cyanuric acid (CYA)?
- Cyanuric acid protects chlorine from sunlight but slows it, so more free chlorine is needed as CYA rises. Trouble Free Pool’s chart puts the minimum at about 7.5% of CYA: 3 ppm at 40 ppm CYA and 7.5 ppm at 100 ppm. That is a pool-owner guideline, so treat it as an estimate. Public-pool codes based on the CDC Model Aquatic Health Code set a minimum of 1 ppm, and 2 ppm where cyanuric acid is used in some local codes.
- What is a safe free chlorine range?
- The CDC Model Aquatic Health Code treats free chlorine above 10 ppm with swimmers in the pool as an imminent health hazard, and public-pool codes based on it set minimums of 1 to 2 ppm. For a home pool, 1 to 4 ppm is a typical range (an estimate), and higher with more CYA. Your local rules and your test kit’s instructions come first.
- My free chlorine is above the target. What do I do?
- Nothing to add: the calculator says so when the current level is at or above the target. Chlorine falls on its own in sunlight; wait and retest. At 5 ppm against a 4 ppm target it will come down within a day or so in sun, faster with no stabiliser. Keep swimmers out if the reading is above 10 ppm.
- Can I mix different chlorine products or add them together?
- No. Never mix chlorine products with each other, with acid or with other chemicals: it can release toxic gas or start a fire. Add one product at a time, with the pump running, and leave time between products as the labels say. Add chemical to water, never water to chemical, and wear goggles and gloves.
- What if my pool volume is in litres?
- Choose litres in the volume unit and the calculator converts to US gallons (1 US gallon is 3.785 L). A 75,000 litre pool from 1 to 3 ppm needs 41 fl oz (0.32 gal) of 12.5% liquid chlorine, which is 1.2 L. In metric, 1 ppm is 1 g of chlorine per 1,000 litres.
- Why does my test not match after I dose?
- Chlorine needs time to mix, sunlight and swimmers use it up, product strength falls as it ages, and a test kit has a margin of error. Let the pump run for a few hours, retest, and adjust in small steps. Stored liquid chlorine loses strength over weeks, especially in heat.
Handy tools
- Unit Converter: Convert fluid ounces to millilitres, or ounces to grams, if your measuring cup or scale is metric.