Works out the dose for the product you actually have — and warns you what else that product is putting in the water besides chlorine.
Not sure? Work it out with the pool volume calculator first — every dose below scales directly with this number.
Routine range is 2–4 ppm for most pools. Shocking to clear algae or after heavy use typically means 10–20 ppm — set the target accordingly.
Stabiliser protects chlorine from sunlight but too much locks it up. Above roughly 80–100 ppm the only fix is diluting the water.
Add this much
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Chlorine gap to close—
Alternative measure—
Metric—
Stabiliser (CYA) added—
Calcium hardness added—
CYA after this dose—
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The calculation
Everything starts from one constant. Raising 10,000 US gallons by 1 ppm needs 0.0835 lb — about 1.335 oz — of available chlorine, because 10,000 gallons weighs 83,450 lb and one part per million of that is 0.0835 lb.
Dividing by the product's strength gives the amount to actually measure out. A pound of 73% cal-hypo therefore delivers 16 ÷ 1.335 × 0.73 = 8.75 ppm in 10,000 gallons, which is the figure printed on most labels.
Product
Strength
ppm from 1 lb in 10,000 gal
Cal-hypo
73%
8.8
Cal-hypo
65%
7.8
Dichlor
56%
6.7
Trichlor
90%
10.8
What else each product adds
This is the part labels tend to bury. Chlorine is never delivered alone:
Liquid chlorine and bleach — sodium hypochlorite. Adds nothing but a trace of salt. Raises pH briefly, which settles as the chlorine is consumed.
Cal-hypo — adds roughly 0.8 ppm calcium hardness for every 1 ppm of chlorine. Fine in soft water, a scaling problem in hard water.
Dichlor — adds about 0.9 ppm stabiliser (CYA) per 1 ppm of chlorine.
Trichlor — adds about 0.6 ppm CYA per 1 ppm of chlorine, and is strongly acidic, pulling pH down.
The stabiliser figures are why a pool run all summer on tablets slowly stops responding to chlorine. CYA does not evaporate or burn off; it only leaves with the water.
The dose itself is simple arithmetic. What makes pool chemistry frustrating is that the three common products each change something else about the water, and those changes accumulate invisibly across a season until the pool stops behaving.
Free chlorine should track stabiliser
A pool with 30 ppm CYA is well served by 2–4 ppm free chlorine. A pool that has crept to 80 ppm CYA needs considerably more chlorine to achieve the same sanitising effect, because the stabiliser holds it in reserve rather than in active form. This is why a "target" number in isolation is misleading — it is the ratio to stabiliser that matters.
Practical handling
Add chemicals to water, never water to chemicals, and never mix two chlorine products in the same container or feeder — trichlor and cal-hypo together can react violently. Broadcast granular products over the deep end with the pump running rather than dumping them in one spot, and pre-dissolve cal-hypo in a bucket if you have a vinyl liner, since undissolved granules bleach it.
Dose after sunset
Unstabilised chlorine added at midday can lose a substantial fraction to UV within a couple of hours. Adding in the evening gives it a full night to work and circulate. Retest after one complete turnover rather than immediately, or the reading will simply reflect however well the water has mixed so far.
These figures are estimates for planning. Always follow the dosing and safety instructions on the product you are actually holding, and test the water rather than trusting the calculation alone.
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Frequently asked questions
How much chlorine do I need for a 20,000 gallon pool?
To raise free chlorine by 1 ppm in 20,000 gallons you need about 2.7 oz of available chlorine — roughly 27 fl oz of 12.5% liquid chlorine, 3.7 oz of 73% cal-hypo, or 4.8 oz of 56% dichlor. Multiply by the number of ppm you need to gain.
How much liquid chlorine raises 10,000 gallons by 1 ppm?
About 10 fluid ounces of 12.5% liquid chlorine, or roughly 21 fl oz of standard 6% household bleach. A full gallon of 12.5% raises 10,000 gallons by about 12.5 ppm.
Does cal-hypo or dichlor add anything besides chlorine?
Yes. Cal-hypo adds around 0.8 ppm of calcium hardness for each 1 ppm of chlorine, and dichlor adds about 0.9 ppm of stabiliser. Trichlor adds roughly 0.6 ppm stabiliser per ppm and is also strongly acidic.
What should my free chlorine level be?
Between 2 and 4 ppm for routine sanitising in most pools, though the right figure depends on your stabiliser level — higher CYA needs proportionally more free chlorine to stay effective. Shocking usually means raising it to 10 to 20 ppm temporarily.
Why does my chlorine stop working?
Usually stabiliser build-up. Every dose of dichlor or trichlor leaves CYA behind, and it never breaks down. Once CYA passes roughly 80 to 100 ppm, chlorine becomes progressively locked up and the only remedy is partially draining and refilling with fresh water.