Pool Chlorine Calculator
Enter your pool volume and your current and target free chlorine to get the exact dose, in the unit of whatever chlorine you have. Liquid chlorine, cal-hypo, or dichlor, the math is done for you.
Add liquid chlorine to raise FC by 0 ppm
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Heads up: dichlor is stabilized chlorine, so this dose also adds about 0 ppm of CYA. Watch your stabilizer over the season with the CYA calculator.
Every product, side by side
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Safety first. Never mix chlorine types or chlorine with acid. Always add the chemical to the water, never water to the chemical. Run the pump while dosing so it circulates, then wait and retest before you add any more.
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How the Chlorine Dose Is Calculated
Raising free chlorine is simple arithmetic once you know three things: how many gallons your pool holds, how far you need to raise free chlorine, and which chlorine product you are using. First the tool finds the gap, your target free chlorine minus your current free chlorine. If you are already at or above target, you need nothing. Then it multiplies that gap by the dose it takes to raise free chlorine by 1 ppm per 10,000 gallons for your chosen product, scaled to your actual volume. Every dose on this page comes straight from standard pool-care chemistry, not a guess.
The Numbers Behind Each Product
To raise free chlorine by 1 ppm per 10,000 gallons, you need about 10.7 fluid ounces of 12.5 percent liquid chlorine, about 13.3 fluid ounces of 10 percent liquid chlorine, about 2.0 ounces by weight of 73 percent cal-hypo, or about 2.7 ounces by weight of 56 percent dichlor. The differences come from the available chlorine in each product. Cal-hypo and dichlor are far more concentrated than liquid, which is why a little granular goes a long way. The calculator converts the result into friendly units too, so liquid is shown in cups and gallons and granular in pounds when the amounts get large.
Pick the Right Chlorine for the Job
Liquid chlorine (sodium hypochlorite) is the workhorse for day-to-day free-chlorine maintenance because it adds nothing but a trace of salt. Cal-hypo is a strong granular shock, but it raises calcium hardness, so it is a poor choice if your calcium is already high. Dichlor is convenient granular chlorine, but it is stabilized, meaning it carries cyanuric acid with it. Trichlor tablets are even more stabilized and also push pH down. For a pool that already has enough CYA, lean on liquid chlorine or cal-hypo so your stabilizer does not creep up over the summer.
Watch Your CYA With Stabilized Chlorine
Cyanuric acid (CYA), also called stabilizer or conditioner, shields chlorine from sunlight so it lasts longer in an outdoor pool. That is helpful up to a point. Every 1 ppm of free chlorine you add with dichlor also adds roughly 0.9 ppm of CYA, and trichlor adds even more per dose. Over a long season of stabilized chlorine, CYA quietly climbs, and once it is too high your chlorine becomes sluggish and algae gets a foothold. The catch is that CYA only leaves the water by dilution, so the only way to lower it is to drain part of the pool and refill. If you dose with dichlor or trichlor, keep an eye on your stabilizer and switch to liquid chlorine when CYA reaches the top of your target range.
Target Free Chlorine Depends on CYA
There is no single perfect free-chlorine number. The right target rises with your CYA, because more stabilizer means you need more chlorine to keep the same sanitizing power. A good working rule is to keep free chlorine near 7.5 percent of your CYA reading. A pool with CYA of 40 wants free chlorine around 3 ppm, while CYA of 60 wants closer to 4 to 5 ppm. Most backyard pools sit comfortably in the 2 to 5 ppm range. The important thing is never to let free chlorine fall below the minimum for your CYA, because that low point is exactly when algae starts and a small problem becomes a green pool.
Dose Safely, Every Time
Chlorine is a strong oxidizer, so handle it with respect. Never mix two pool chemicals together, and that includes two different chlorine products or chlorine and any acid, since the reaction can release toxic gas or cause a fire. Always add the chemical to the water, never pour water into the chemical. Broadcast or pre-dissolve granular chlorine per the label, and run the pump so the dose circulates fully. Dose in the evening when you can, give it time to mix, then retest before you add anything else or get in the water. These dosing figures are estimates based on standard formulas, so test your own water and adjust. Store chlorine in a cool, dry place, separate from acid, and well away from kids and pets.
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Frequently Asked Questions
How much chlorine do I need to raise my free chlorine?
It depends on your pool volume, how far you need to raise free chlorine, and which product you use. As a rule of thumb, about 10.7 fluid ounces of 12.5 percent liquid chlorine raises free chlorine by 1 ppm per 10,000 gallons. So a 20,000-gallon pool that needs to go up 3 ppm needs roughly 64 fluid ounces, or half a gallon. This calculator does the math for liquid chlorine, cal-hypo, and dichlor so you can dose with whatever you have on hand.
What is the difference between liquid chlorine, cal-hypo, and dichlor?
Liquid chlorine is sodium hypochlorite, usually 10 to 12.5 percent, and it adds nothing but salt as a byproduct, so it is the cleanest way to raise free chlorine. Cal-hypo is calcium hypochlorite granular shock, around 73 percent available chlorine, and it adds calcium hardness over time. Dichlor is a stabilized granular, about 56 percent, and it adds cyanuric acid (CYA) every time you use it. Trichlor tablets also add CYA and lower pH. Match the product to what your water needs.
Why does dichlor or trichlor raise my CYA?
Dichlor and trichlor are stabilized chlorine, meaning cyanuric acid is built into the chemical. Every 1 ppm of free chlorine you add with dichlor also adds about 0.9 ppm of CYA, and trichlor adds even more. CYA protects chlorine from the sun, but too much CYA weakens chlorine and forces you to run higher free-chlorine levels. If you rely on stabilized chlorine all season, your CYA climbs and the only way to lower it is to drain and refill. Use our CYA calculator to track it.
How long after adding chlorine can I swim?
For a normal maintenance dose that keeps free chlorine in the 1 to 5 ppm range, you can usually swim once the chlorine has circulated, about 15 to 30 minutes with the pump running. After a heavy shock dose, wait until free chlorine drops back to about 5 ppm or lower before swimming, which can take several hours or overnight. Always retest before getting in, and never swim in water you have not tested.
Should I add chlorine in the morning or at night?
Add chlorine in the evening or at night whenever you can. The sun burns off unstabilized chlorine quickly, so dosing after sunset gives the chlorine all night to work without UV loss. Run the pump while and after you dose so the chlorine mixes fully. Morning testing then tells you how much chlorine held overnight, which is the single best indicator of whether your water is sanitary.
What free chlorine level should I target?
Target free chlorine is tied to your CYA, not a single fixed number. For a stabilized chlorine pool, aim for free chlorine around 7.5 percent of your CYA reading, so CYA of 40 wants roughly 3 ppm and CYA of 60 wants closer to 4 to 5 ppm. Salt pools run similar ratios. Most owners keep free chlorine between 2 and 5 ppm. Never let it fall below the minimum for your CYA, since that is when algae starts.
Can I mix two types of chlorine to dose faster?
No. Never mix pool chemicals, and that includes different chlorine products. Combining cal-hypo and trichlor, or any chlorine with acid, can react violently and release toxic gas or even catch fire. Add one product at a time, pour it into the water (not water into the product), run the pump to circulate, and wait and retest before adding anything else. When in doubt, slow down and dose in stages.