In a 15,000 gallon pool with 40 ppm cyanuric acid (CYA), free chlorine should sit at about 4.5 ppm and never below 3 ppm. If your test reads 1 ppm, you need 3.5 ppm more: 54 fl oz of 12.5% liquid chlorine (0.42 gallons, 1.6 L), or 112 fl oz of 6% household bleach (0.88 gallons, 3.3 L). The calculator sets the target from your CYA, then works out the amount for the product in your hand.
How the chlorine amount is calculated
Product = (target FC − current FC) × gallons ÷ 10,000 × product per ppm per 10,000 gal
Product per ppm comes from strength. Liquid chlorine is sold by trade percent, which is grams of chlorine per 100 mL, so 12.5% gives 12.5 ppm per gallon in 10,000 gallons and needs 10.24 fl oz per ppm. Solids are worked from available chlorine: 73% cal-hypo needs 1.83 oz per ppm per 10,000 gallons, 56% dichlor 2.38 oz, 90% trichlor 1.48 oz.
For the example, 3.5 ppm × 1.5 × 10.24 fl oz = 53.8 fl oz. One gallon jug covers it, and the rest keeps for tomorrow.
The free chlorine target: which authority
Free chlorine does less work as CYA rises, so the authorities do not agree on one number.
| CYA | Minimum (7.5% of CYA) | Target (11.5% of CYA) |
|---|---|---|
| 20 ppm | 1.5 ppm | 2.5 ppm |
| 30 ppm | 2.25 ppm | 3.5 ppm |
| 40 ppm | 3 ppm | 4.5 ppm |
| 60 ppm | 4.5 ppm | 7 ppm |
| 80 ppm | 6 ppm | 9 ppm |
We follow the Trouble Free Pool ratio in the table, with a floor of 1 ppm, because it scales with CYA and Orenda calls the chlorine to CYA ratio paramount. PHTA and the Pentair manuals quote a fixed range of 2 to 4 ppm ideal with 1 ppm minimum, regardless of CYA. The CDC Model Aquatic Health Code sets 1 ppm without CYA and 2 ppm when CYA is used. At CYA 30 to 40 all three are close. At CYA 60 or more the fixed range is too little, which is why the tool offers "Fixed 2 to 4" and "My number" beside the default.
Household bleach and pool bleach
Household bleach runs from 1.5% to 6% sodium hypochlorite by weight according to PHTA, and stores now sell 8.25% too. Pool liquid chlorine is 10% or 12.5%. All of them add no CYA and no calcium, so the dose is a volume: 112 fl oz of 6% or 81.5 fl oz of 8.25% for the example above. Use plain bleach with no scent, thickener or detergent. Percent by weight is not quite grams per 100 mL, so the tool's bleach numbers run a few percent high: add most of it and retest. Bleach and pool chlorine leave about 0.8 ppm of salt per ppm of chlorine, and they raise pH, so check the pool pH calculator afterwards.
Cal-hypo, dichlor and trichlor
Cal-hypo 73% is 9.6 oz for the example and adds 2.8 ppm of calcium hardness. Dichlor 56% is 12.5 oz and adds 3.15 ppm of CYA. Trichlor 90% tablets are 7.8 oz of chlorine and add 2.1 ppm of CYA. That CYA never leaves, and it lifts the target you need next time, so a pool on tablets needs the pool stabilizer calculator checked every few weeks. Tablets belong in a feeder or floater, not in a bucket.
How much chlorine per day
Sun, heat and swimmers burn chlorine. The daily box asks for your own loss in ppm, and multiplies. At 1 ppm per day in 15,000 gallons the pool needs 15.4 fl oz of 12.5% liquid chlorine. On a hot week it can be double, so measure: test after sunset and again the next evening. If the loss is 1 ppm or more overnight after a SLAM, the pool is not clean yet.
Litres and cubic metres
Switch the header to metric and the answer shows litres: 1.59 L of 12.5% liquid chlorine for a 56.8 m³ pool. Doses scale with volume, so a 30,000 litre pool needs about half of that for the same ppm.
What to do next
If the water is green or smells strongly of chlorine, use the pool shock calculator instead. Salt pool owners can compare targets in the pool salt calculator. Enter all your test results at once in the pool chemical calculator, and find your gallons with the pool volume calculator. Everything else lives in the pool chemistry hub.