Quick answer: 10 fl oz of 12.5% liquid chlorine raises 10,000 gallons by 1 ppm. But the target matters more than the dose: free chlorine should be about 11–12% of your cyanuric acid level, not a fixed 1–3 ppm. At 50 ppm CYA you need roughly 6 ppm free chlorine — three times what most charts tell you.
Safety. Never mix pool chemicals together, and never mix any of them with acid or ammonia — chlorine gas is produced within seconds and is dangerous in an enclosed space. Add chemicals to water, never water to chemical. Store products separately in a cool dry place; trichlor and cal-hypo react violently with each other. Keep everything locked away from children and pets.
The part most chlorine calculators get wrong
Almost every pool chart says "keep free chlorine at 1–3 ppm". For an outdoor stabilised pool that advice is actively wrong, and it is the single most common reason people fight algae all summer while their test kit reads "fine".
Here is why. Cyanuric acid — stabiliser or conditioner — is added to outdoor pools to stop ultraviolet light destroying chlorine. Without it, an outdoor pool can lose most of its free chlorine in a single sunny day. It works by binding reversibly to chlorine and holding most of it in reserve.
That reserve is the catch. At 50 ppm CYA, the overwhelming majority of your "free chlorine" is bound to cyanurate and is not doing any sanitising at that instant. The tiny unbound fraction — hypochlorous acid — is what actually kills things. So the total free chlorine reading has to rise as CYA rises, just to keep the same active concentration.
Algae-clearing shock level ≈ 40% of CYA
| Cyanuric acid | Minimum FC | Target FC | Shock level | Comment |
|---|---|---|---|---|
| 0 ppm (indoor) | 1.0 | 1–3 | 10 | The classic advice — only valid here |
| 20 ppm | 1.5 | 2.3 | 8 | Low stabiliser |
| 30 ppm | 2.3 | 3.5 | 12 | Common outdoor minimum |
| 40 ppm | 3.0 | 4.6 | 16 | |
| 50 ppm | 3.8 | 5.8 | 20 | Typical well-run outdoor pool |
| 60 ppm | 4.5 | 6.9 | 24 | |
| 80 ppm | 6.0 | 9.2 | 32 | Getting high — often from trichlor use |
| 100 ppm | 7.5 | 11.5 | 40 | Consider partial drain and refill |
The trap this explains. Someone uses trichlor tablets all season. CYA creeps to 100 ppm. Their test kit reads 3 ppm free chlorine — squarely inside the "1–3 ppm" range every chart recommends — and yet the pool goes green. At 100 ppm CYA, 3 ppm is below the 7.5 ppm minimum. The water was under-sanitised the whole time while every reading said otherwise.
These percentages come from the chlorine/CYA relationship widely used in the pool-care community and supported by hypochlorous acid equilibrium chemistry. Some official bodies and local codes still specify fixed ranges — where a code applies to you, follow the code.
The dose, derived
Everything below follows from that one line plus the strength on the label. Per 10,000 gallons, to raise free chlorine by 1 ppm:
| Product | Available chlorine | Dose per 10,000 gal / 1 ppm | Also adds |
|---|---|---|---|
| Liquid chlorine (pool grade) | 12.5% | 10.2 fl oz | Salt only |
| Liquid chlorine | 10% | 12.8 fl oz | Salt only |
| Household bleach | 6% | 21.3 fl oz | Salt only |
| Calcium hypochlorite | 65% | 2.05 oz wt | ~0.8 ppm calcium hardness |
| Calcium hypochlorite | 73% | 1.83 oz wt | ~0.7 ppm calcium hardness |
| Dichlor granular | 56% | 2.38 oz wt | ~0.9 ppm cyanuric acid |
| Trichlor tablets | 90% | 1.48 oz wt | ~0.6 ppm cyanuric acid, strongly acidic |
Worked example — a 20,000 gallon pool reading 1.5 ppm, CYA at 50 ppm so the target is 50 × 0.115 = 5.75 ppm:
- Rise needed: 5.75 − 1.5 = 4.25 ppm
- Scale for volume: 20,000 ÷ 10,000 = 2×
- Dose: 10.24 fl oz × 4.25 × 2 = 87 fl oz ≈ 0.68 gallons of 12.5% liquid chlorine
Carry the unrounded target through the multiplication. Rounding 5.75 to 5.8 first gives 88 fl oz — a small difference here, but the same habit on a shock dose in a large pool costs real money.
You need your pool's real volume for this. The pool volume calculator handles rectangular, oval, round and irregular pools including sloping floors — and guessing it is the largest single source of dosing error, because every figure above scales directly with it.
Choosing a product — and what each one does to your water
The chlorine is the same in all of them. What differs is everything else the product carries in, and that accumulates over a season.
| Product | Effect on pH | Accumulates | Best for |
|---|---|---|---|
| Liquid chlorine / bleach | Slightly raises, largely self-cancelling | Salt (harmless) | Routine dosing and shocking. The default choice |
| Calcium hypochlorite | Raises | Calcium hardness | Soft-water areas; poor choice in hard water |
| Dichlor | Near neutral | Cyanuric acid, fast | Starting a fresh pool that needs CYA anyway |
| Trichlor tablets | Lowers strongly | Cyanuric acid | Slow feed while away; not for daily use all season |
| Salt chlorine generator | Raises | Nothing (regenerates) | Hands-off maintenance; higher upfront cost |
The trichlor problem, quantified. Trichlor adds about 0.6 ppm CYA for every 1 ppm of chlorine it delivers. A pool consuming 2 ppm of chlorine a day, dosed entirely with trichlor, gains roughly 1.2 ppm CYA per day — around 36 ppm a month. Start the season at 30 ppm and by August you are near 100 ppm, needing 11.5 ppm free chlorine just to stay sanitised.
And there is no chemical fix. Cyanuric acid is not consumed and cannot be neutralised in any practical way; the only route down is draining and replacing water. This is why experienced pool owners use trichlor tablets sparingly and dose routinely with liquid.
Total alkalinity, briefly
Alkalinity buffers pH — get it right and pH stops drifting. Target is 80–120 ppm for most pools.
| Pool volume | +10 ppm TA | +20 ppm TA | +40 ppm TA |
|---|---|---|---|
| 10,000 gal | 1.4 lb | 2.8 lb | 5.6 lb |
| 15,000 gal | 2.1 lb | 4.2 lb | 8.4 lb |
| 20,000 gal | 2.8 lb | 5.6 lb | 11.2 lb |
| 30,000 gal | 4.2 lb | 8.4 lb | 16.8 lb |
On pH, an honest answer: there is no reliable formula. The acid needed to move pH depends on the current alkalinity, the borate level and the cyanuric acid, all of which buffer the water. Anyone giving you a clean pH-adjustment formula is guessing. The practical method is to add a modest measured dose of acid, circulate for a few hours, and retest — approaching the target from above rather than overshooting. Fix alkalinity first and pH usually settles on its own.
Common mistakes & pro tips
- Targeting 1–3 ppm regardless of CYA. The headline error. Correct only for an unstabilised indoor pool.
- Not testing cyanuric acid. Test it at least monthly. Without it, the chlorine reading is uninterpretable.
- Guessing the pool volume. Every dose scales with it directly — a 25% volume error is a 25% dosing error, every time.
- Using trichlor as the only sanitiser all season. Adds roughly 36 ppm CYA per month at typical consumption.
- Adding cal-hypo in hard water. Calcium accumulates and eventually scales the heater and surfaces.
- Throwing granular product straight in. Undissolved granules sitting on a vinyl liner bleach and weaken it. Pre-dissolve in a bucket.
- Shocking in daylight. Add shock at dusk. Sunlight destroys unstabilised chlorine fastest exactly when you have most of it.
- Pro tip — dose liquid chlorine at night. Same reason. The chlorine gets all night to work before UV starts consuming it.
- Pro tip — if chlorine vanishes overnight, suspect algae. No sunlight means no UV loss, so anything more than about 1 ppm of overnight loss is something in the water consuming it.
How to use this calculator
- Pick your product from the row of buttons — the strength is on the label.
- Enter your pool volume in gallons or litres.
- Enter current free chlorine and cyanuric acid. The CYA reading sets the target automatically.
- Or set your own target if you have a code or a service company figure to follow.
- Check the side-effect lines — how much calcium or CYA this dose adds, which matters cumulatively.
Frequently asked questions
How much liquid chlorine do I add to my pool?
About 10 fl oz of 12.5% liquid chlorine raises free chlorine by 1 ppm in 10,000 gallons of water. For a 20,000 gallon pool needing a rise of 3 ppm, that is 10 × 2 × 3 = 60 fl oz, or just under half a gallon. Weaker 10% product needs about 12.8 fl oz per 10,000 gallons per ppm.
What should my free chlorine level be?
It depends entirely on your cyanuric acid level, which is the part most guidance leaves out. Free chlorine should be about 11–12% of the cyanuric acid concentration, with an absolute minimum near 7.5%. At 30 ppm CYA the target is about 3.5 ppm; at 50 ppm it is about 6 ppm; at 80 ppm it is about 9 ppm. The commonly quoted 1–3 ppm is only adequate in a pool with little or no stabiliser.
How much chlorine does it take to shock a pool?
A conventional shock raises free chlorine to about 10 ppm, which in a 20,000 gallon pool takes roughly 1.6 gallons of 12.5% liquid chlorine. If you are clearing algae, the target is far higher and is set by cyanuric acid: shock level is about 40% of the CYA reading, so a pool at 50 ppm CYA needs around 20 ppm free chlorine held until the water clears.
Which type of chlorine should I use?
Liquid chlorine (sodium hypochlorite) adds nothing but chlorine and salt and is the usual choice for routine dosing. Calcium hypochlorite is cheaper to store but adds calcium hardness. Dichlor and trichlor tablets add cyanuric acid with every dose, which is convenient at first and becomes a problem once the level climbs, because the only way to reduce cyanuric acid is to drain and replace water. Trichlor is also strongly acidic and pushes pH down.
Why does my pool use chlorine so fast?
The most common reason is too little cyanuric acid, which leaves chlorine unprotected from sunlight — an unstabilised outdoor pool can lose most of its free chlorine in a single sunny day. Other causes are a high bather load, organic debris, algae consuming chlorine faster than you add it, and warm water accelerating everything. If chlorine disappears overnight with no sunlight involved, suspect algae.
Can I swim right after adding chlorine?
Wait until free chlorine has fallen back to your normal target range and the water has circulated for at least 15–30 minutes after a routine dose. After a shock, wait until free chlorine drops below about 10 ppm. Never swim while granular product is still visible undissolved on the pool floor, and never enter a pool immediately after adding concentrated chemical near the point where it went in.
References & further reading
- CDC Model Aquatic Health Code — disinfection and water quality parameters for public pools.
- WHO Guidelines for Safe Recreational Water Environments, Volume 2: Swimming Pools and Similar Environments.
- The chlorine/cyanuric acid relationship and hypochlorous acid equilibrium, as characterised in pool water chemistry literature and applied in the widely used FC/CYA charts.
- Product labels for sodium hypochlorite, calcium hypochlorite, dichlor and trichlor — always the authority on strength.
Doses are derived arithmetically from product strength and water volume. Actual results vary with chlorine demand, temperature, sunlight and bather load. Where a local health code applies to your pool, follow the code. See our accuracy policy.