How many chlorine tablets per m³? The dosage of chlorine tablets according to your pool
Chlorine tablets and pucks are the preferred solution for many above-ground and in-ground pool owners: they are placed in a dispenser or the skimmer, dissolve slowly, and disinfect the water for several days. Yet the question comes up every spring: how many should I use for my pool volume? As a rough guide, use one 200 g tablet for 20 to 30 m³ of water per week, or one 20 g tablet for 2 to 3 m³ per week, adjusting according to the temperature, sunlight, pool usage, and above all the free chlorine readings. Here’s how to calculate the right quantity, where to place the tablets, and which limits to observe to keep your water healthy all season.
— The short answer
For weekly maintenance with slow-release chlorine, the usual guideline is around 7 to 10 g of product per m³ per week: one 200 g tablet for 20 to 30 m³, or one 20 g tablet for 2 to 3 m³. A 15 m³ above-ground pool therefore requires roughly half a tablet to one 200 g tablet per week, or five to seven 20 g tablets. These values are starting points: follow the manufacturer’s instructions, measure free chlorine twice a week, and adjust the number of tablets. Stabilized tablets cause cyanuric acid to build up in the water; monitor it and replace some of the water if necessary.
Tablets, pucks, multifunction products: what are we talking about?
Most solid products sold for the slow disinfection of pools are based on trichloroisocyanuric acid. This is known as “stabilized” chlorine: as it dissolves, it releases both active chlorine and cyanuric acid, which protects the chlorine from being destroyed by the sun’s UV rays. This allows it to last for several days in an exposed pool, whereas unstabilized chlorine disappears much more quickly.
They are available in several formats:
- 200 g tablets (sometimes 250 g), for medium to large pools;
- 20 g tablets, for small above-ground pools, paddling pools, and precise dosing;
- Multifunction tablets, which combine chlorine with an algaecide, a flocculant, and sometimes a pH stabilizer; practical, but less flexible to dose;
- Fast-dissolving “shock” chlorine, in granules or tablets, for quickly raising the level; do not confuse it with slow-release chlorine.
The dosage described here concerns slow-release chlorine for maintenance. Always read the label: the available chlorine content, unit weight, and recommended doses vary from one manufacturer to another.
How many tablets according to the volume
The first figure to determine is the pool's actual volume. For a round pool, multiply the radius squared by 3.14 and then by the water depth; for a rectangular pool, length × width × average depth. Our article on calculating the volume of a round pool explains the formula in detail with examples.
Then, the starting guideline: around 7 to 10 g of slow-release chlorine per m³ per week during the season, with water between 20 and 26 °C and typical family use.
| Pool volume | 20 g tablets per week (approximate amount) | 200 g tablets per week (approximate amount) |
|---|---|---|
| 5 m³ (small above-ground pool) | 2 to 3 | More suitable tablets |
| 15 m³ | 5 to 7 | ½ to 1 |
| 30 m³ | 10 to 15 | 1 to 1.5 |
| 50 m³ | More suitable tablets | 2 to 2.5 |
| 80 m³ | More suitable tablets | 3 to 4 |
These figures are averages. During a heatwave, with water at 28 °C and daily swimming, consumption can double. At the beginning and end of the season, with cool water and infrequent pool use, it drops significantly. The only way to find the right dose for your pool is to measure the free chlorine twice a week and add or remove one tablet according to the result.
The target level and how to check it
For a private pool treated with stabilized chlorine, the commonly targeted free chlorine maintenance range is around 1 to 3 mg/L, slightly higher than with unstabilized chlorine, because cyanuric acid “binds” some of the chlorine. The pH should remain around 7.0 to 7.4: above this, chlorine loses a significant part of its effectiveness.
The recommended testing schedule:
- twice a week during the season: free chlorine and pH;
- once a week: alkalinity (TAC);
- about once a month: cyanuric acid, using a specific test or at a store.
If free chlorine is too low despite the planned number of tablets, first look for the cause: pH too high, filtration time too short, a lot of organic matter, or cyanuric acid too high. Adding tablets without correcting the cause often makes the problem worse.
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Where to place the tablets: dispenser, skimmer, or chlorinator
The floating dispenser is the most common solution for above-ground pools. It is adjusted with a ring that opens the dissolution slots more or less. Advantage: simple, visible, and easy to refill. Disadvantage: it drifts toward calm areas and can locally create more highly chlorinated water; remove it while swimming and do not leave it stuck against the liner, which could become discolored.
The skimmer basket is practical for inground pools: the water drawn in dissolves the tablet, and the chlorine passes directly into the filtration circuit. It is effective, but the highly concentrated water passes through the pump, filter, and downstream equipment. Check with the manufacturer of your heat pump or filter that this method is approved, and never put another product, such as a pH adjuster, in the skimmer at the same time as the tablets.
The inline chlorinator, installed after the filter and sensitive equipment, is the cleanest solution for an inground pool installation: adjustable flow, no tablet in the skimmer, and consistent dosing.
Whatever the method, never place a tablet directly on the bottom of the pool: it discolors and weakens the liner or coating where it rests.
The stabilizer trap: when tablets are no longer enough
Each stabilized chlorine tablet adds cyanuric acid. Unlike chlorine, it hardly breaks down: it accumulates week after week and only decreases through dilution, when some of the water is replaced.
At moderate levels, it protects chlorine from the sun. At high levels, it « blocks » it: the measured free chlorine appears correct, but the water still turns green or cloudy because the chlorine that is actually active is too low. This is known as overstabilization. Common recommendations advise not exceeding approximately 50 to 75 mg/L of cyanuric acid in a private pool.
Warning signs include water that remains dull despite free chlorine being « within range », algae that reappear for no apparent reason, and tablet consumption that continues to increase. The solution is not to add more tablets, but to measure cyanuric acid, replace some of the water and, if necessary, temporarily switch to unstabilized chlorine, either liquid or in unstabilized shock granules.
Tablets and pH: an effect opposite to liquid chlorine
Trichloroisocyanuric acid-based tablets are acidic. Their dissolution therefore slowly lowers the pH and alkalinity of the water, unlike liquid chlorine, which raises them. In a region where tap water is hard and the pH is naturally high, this effect is often welcome. In soft water, or when filling with rainwater, the pH may drop too low and make the water aggressive toward metal parts and the liner.
This is why it is important to measure pH at every check, not just chlorine. A pH that drops week after week is corrected with pH plus; a pH that remains high despite the tablets is corrected with pH minus, always separately from the chlorine.
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Start of season, shock treatment and winterization: the doses change
When restarting, the water has often lost all its chlorine during the winter. Start by cleaning, adjusting the pH, running the filtration continuously, then carrying out shock chlorination with a fast-dissolving product. Only once the free chlorine has returned to the maintenance range should you add the slow-release tablets. Our advice on cleaning an above-ground pool after winter describes this restart step by step.
After a storm, intense heat, or heavy use, slow-release tablets do not react quickly enough: a one-off shock treatment is more effective than adding another tablet.
At the end of the season, gradually reduce the number of tablets as the temperature drops, before switching to a winterizing product.
Filtration, the other half of treatment
No chlorine dosage can compensate for insufficient filtration. The filter retains particles and some of the organic matter that would otherwise consume chlorine. The most widely used rule of thumb sets the daily filtration time, in hours, at approximately half the water temperature in degrees: around 12 hours per day for water at 24 °C. A properly sized sand filter, backwashed when the pressure rises, keeps the water clear with less chlorine. To choose or replace yours, see our article on choosing a sand filter for a swimming pool.
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Chlorine tablet safety and storage
Chlorine tablets are powerful oxidizing products. Store them in their original packaging, tightly closed, in a dry, cool place out of the reach of children. Never store them next to a pH adjuster, an acid, or a product from another chlorine family: some mixtures can react violently or release chlorine gas. Handle them with gloves, do not break them with your bare hands, and never put water in the tablet bucket: always put the product into the water, never the other way around.
Read also
Before the tablets, restart your pool: How to clean an above-ground pool after winter?
READ THE ARTICLEDoes your tablet consumption seem abnormal, or does the water remain cloudy despite «correct» chlorine levels? Send us your pool volume, the type of tablets used and your latest free chlorine, pH and, if possible, cyanuric acid measurements: the Aquiflor team in Nodebais and the Foudebassin service will help you restore the right balance.
Frequently asked questions
— How many chlorine tablets for a 20 m³ pool?
As a rule of thumb, one 200 g tablet per week, or seven to ten 20 g tablets, adjusted according to temperature, usage and free chlorine measurements.
— Can chlorine tablets be placed directly in the pool?
No. Placed on the bottom, a tablet discolors and damages the liner or coating. Use a floating dispenser, the skimmer basket if the equipment allows it, or a chlorinator.
— Why does the water turn green even though I add tablets every week?
Often due to pH that is too high, insufficient filtration or excess cyanuric acid accumulated from the tablets. Measure these parameters before adding chlorine.
— 20 g tablets or 200 g tablets: which should you choose?
20 g tablets are suitable for small pools and allow finer dosing; 200 g tablets are more convenient above about twenty cubic metres.
In summary
- Starting point: approximately 7 to 10 g of slow-release chlorine per m³ per week.
- Either one 200 g tablet for 20 to 30 m³, or one 20 g tablet for 2 to 3 m³, each week.
- Adjust according to the free chlorine measured twice a week, with a pH around 7.0 to 7.4.
- Use a dispenser, the skimmer or a chlorinator, never the bottom of the pool.
- Monitor cyanuric acid: above a few dozen mg/L, chlorine loses its effectiveness.
Sources and references
- Manufacturers' technical data sheets: PoolCheck, AquaForte (chlorine and pH Test-Kit, Aqualoon), instructions and safety data sheets for trichloroisocyanuric acid tablets
- Wetzel R. G., Limnology, 3rd ed., Academic Press, 2001 (chlorine chemistry and pH in aquatic environments)
- Aquiflor / Foudebassin consulting experience, Nodebais (Belgium)
Last updated: October 3, 2026.
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