Liquid chlorine per m³: calculate the exact dose for your pool

Liquid chlorine is popular with pool owners: it works quickly, leaves no stabilizer in the water, and can be dosed to the nearest milliliter. You still need to know how much to add. The container shows a concentration; the test strip gives a result in mg/L; the pool instructions specify a volume in m³. To dose liquid chlorine, simply connect these three figures: 1 gram of active chlorine in 1 m³ of water gives 1 mg/L, and the container's concentration tells you how many grams of active chlorine each liter contains. With this rule, you can calculate the exact dose for your pool, whether for routine maintenance or shock chlorination.

What is meant by “liquid chlorine”?

Liquid chlorine sold for pools is a sodium hypochlorite solution, from the same chemical family as household bleach, but generally at a higher concentration and without fragrance or additives. It comes in containers, often holding 5 to 20 liters, and is used either by hand or with a dosing pump controlled by a probe.

Its advantages are clear. It works immediately, contains no cyanuric acid (the stabilizer in chlorine tablets and pucks), so it does not accumulate in the water over the course of the season, and it can be dosed precisely. It also has limitations: it degrades over time and with heat and light, slightly raises pH, and is not protected against the sun's UV rays, which increases consumption in midsummer in an unstabilized pool.

Key point: the actual concentration of a container decreases with age. Liquid chlorine stored all summer in an overheated garden shed no longer has the strength indicated on the label. Buy quantities suited to the season, store them in a cool, dark place, and always close the cap tightly.

The calculation rule, step by step

The calculation is based on a simple equivalence: 1 mg/L is 1 gram per cubic metre. To increase the free chlorine in a pool, you therefore first calculate how many grams of active chlorine are needed, then what volume of product contains that amount.

1. Determine the water volume in m³. For a rectangular pool: length × width × average depth. For a round pool, our article on calculating the volume of a round pool explains the formula.
2. Measure the current free chlorine level and set the target value.
3. Calculate the grams of active chlorine required: volume (m³) × desired increase (mg/L).
4. Convert to product volume: grams required ÷ product concentration (g/L), then × 1000 to obtain milliliters.

Example: a 40 m³ pool with free chlorine measured at 0.5 mg/L and a target of 1.5 mg/L. The difference is 1 mg/L, so 40 grams of active chlorine are needed. With a 150 g/L product: 40 ÷ 150 = 0.27 liters, or approximately 270 mL.

If the label states a percentage rather than a concentration in g/L, refer to the manufacturer's technical data sheet: depending on whether the percentage is expressed by mass or by volume, the conversion differs slightly. When in doubt, dose slightly below the calculated amount, then measure again after a few hours of filtration.

For a product with a different concentration, simply adjust proportionally: liquid chlorine that is half as concentrated requires twice the volume.

What chlorine level should you aim for?

For a private pool treated with unstabilized chlorine, the commonly recommended maintenance range for free chlorine is around 1 to 2 mg/L, with the pH maintained at approximately 7.0 to 7.4. These two factors are inseparable: chlorine is truly effective only in the form of hypochlorous acid, whose proportion decreases as the pH rises. At pH 8, a large portion of the measured chlorine works much less effectively.

That is why the golden rule is always the same: correct the pH before chlorinating. Adding chlorine to water whose pH is too high means paying for a product that works at reduced efficiency.

For shock chlorination, after a storm, heavy use, cloudy water, or the beginning of algae growth, free chlorine is temporarily raised to several mg/L, often around 5 to 10 mg/L depending on the situation and the manufacturer's instructions. Swimming is then suspended until the level returns to the normal range.

How to pour liquid chlorine, step by step

The right action matters just as much as the right dose.

• In the evening, preferably: without direct sunlight, unstabilized chlorine is not immediately destroyed by UV rays and works throughout the night.
• Filtration running, to distribute the product throughout the entire volume.
• In front of a return jet, pouring slowly over the surface, or diluted in a bucket of pool water.
• Never in the skimmer if another product has just been put in it, and never at the same time as an acidic pH adjuster.
• Check a few hours later, or the next morning.

For safety: gloves, goggles, clothing you do not mind staining, and a measuring cup reserved for this use. Never mix liquid chlorine with an acid, including liquid pH reducer: the reaction releases toxic chlorine gas. If you need to adjust the pH and chlorinate on the same day, space the two operations several hours apart, with the filtration running.

Why chlorine “disappears” so quickly in summer

Many pool owners are surprised to have to redose every day. Several mechanisms combine.

The sun comes first: UV rays quickly destroy unstabilized chlorine. A pool in full sun without a cover can lose a large part of its free chlorine in a single summer day. Then there is organic matter: sweat, sunscreens, pollen, leaves, and insects continuously consume chlorine. Finally, temperature: the warmer the water, the faster the reactions and the more microorganisms develop.

In practice, in midsummer, daily loss may be around 1 to a few mg/L. In a 40 m³ pool, with a product at 150 g/L, this corresponds to a few hundred milliliters per day. A cover or roller shutter, properly adjusted filtration, and regular skimmer cleaning significantly reduce this consumption.

Liquid chlorine and pH: the pair to monitor

Sodium hypochlorite is a highly alkaline solution. Each addition therefore slightly raises the pool's pH, especially when alkalinity (TAC) is high. In practice, a pool treated with liquid chlorine requires regular additions of pH reducer.

The simplest routine over a typical week is to measure pH and chlorine two or three times, correct the pH first if necessary, wait a few hours, then adjust the chlorine. With a dosing pump and probes, regulation is automatic, but a weekly manual check remains essential to verify calibration.

Liquid chlorine or stabilized chlorine: when to prefer liquid chlorine

Liquid chlorine is not necessarily the best choice for every pool, but it is the clear choice in several situations. In a covered pool, under a shelter, or fitted with a cover that remains closed for much of the day, UV degradation is low and the absence of stabilizer becomes a clear advantage. It is also suitable for pools whose water already contains a lot of cyanuric acid after several seasons of tablets: switching to liquid chlorine avoids adding more while water replacements lower the level. Finally, it is the natural product for automatic dosing pumps.

However, in a pool in full sunlight without a cover, liquid chlorine alone requires daily additions. Many owners therefore combine a low stabilizer level with liquid additions, keeping an eye on cyanuric acid with each complete analysis.

The most common dosing mistakes

Relying on total chlorine rather than free chlorine. Total chlorine includes chloramines, which are already “spent” and cause the smell of chlorine and red eyes. Free chlorine is what disinfects.

Adjusting the dose without taking pH into account. Above approximately 7.6, chlorine loses a significant part of its effectiveness; you overdose to compensate, and the pH rises further.

Using a container that is too old. The actual concentration has fallen, the calculation underestimates the necessary dose, and the water remains under-chlorinated despite repeated additions.

Confusing product volume with grams of active chlorine. One litre of liquid chlorine does not provide one kilogram of chlorine: it provides the amount indicated by its concentration.

Mixing products. Liquid chlorine and acid in the same bucket release toxic gas.

If the water has already turned green, shock chlorination is not always enough: see our complete method for treating green pool water, and for yellowish deposits on shaded walls, see our article on mustard algae.

Also read

The exact volume, the basis of every dosage: How to calculate the volume of a round pool?

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Frequently asked questions

— How much liquid chlorine is needed for 1 m³ of water?

With a product containing around 150 g of active chlorine per litre, approximately 6 to 7 mL per m³ increases free chlorine by 1 mg/L. Multiply by the desired increase and by the pool volume.

— Can you swim right after adding liquid chlorine?

After a maintenance dose, wait for the product to be distributed by the filtration and check that the free chlorine has returned to the normal range. After shock chlorination, wait until the level returns to normal before swimming.

— Does liquid chlorine raise the pH?

Yes. Sodium hypochlorite is highly alkaline: regular additions raise the pH, which must be corrected with pH minus, never at the same time as the chlorine.

— How long can a container of liquid chlorine be stored?

Its concentration gradually decreases, faster in heat and light. Store it in a cool, dark place and favor quantities appropriate for one season.

In summary

• 1 g of active chlorine in 1 m³ gives 1 mg/L: this is the basis of every calculation.
• With a 150 g/L product, allow approximately 6 to 7 mL per m³ for +1 mg/L.
• Aim for around 1 to 2 mg/L of free chlorine for maintenance, with a pH of around 7.0 to 7.4.
• Correct the pH before chlorinating, pour in the evening, with the filtration running.
• Never mix liquid chlorine and acid, and store the containers in a cool place.

Last updated: October 3, 2026.