How Many to Use and Where to Put Them

Every summer, we receive some version of the same question:

How many 20g chlorine tablets should I put in my pool?

It is a fair question, although the correct answer depends on two things: how much water your pool holds and what your water test shows.

The number of tablets gives you a starting point. Your chlorine and pH readings tell you whether that starting point is working.

The quick answer

For CrystaLux 20g Multifunctional Chlorine Tablets in swimming pools:

• Start with one tablet per 2,000 litres of water

• Place the tablets inside a suitable floating dispenser

• Adjust the dispenser opening to control how quickly they dissolve

• Aim to maintain approximately 2 to 3 ppm free chlorine

• Keep the pH between 7.2 and 7.6

• Retest the water every two to three days, and more frequently during hot weather or heavy use

Never drop chlorine tablets directly into the pool. They can create a highly concentrated patch of chlorine that may bleach or damage liners and pool finishes.

First, work out your pool volume

Before deciding how many tablets to use, you need to know approximately how much water your pool holds.

You can work this out quickly using our pool and hot tub chemicals dosage calculator, or calculate it manually using the formulas below.

Rectangular or square pool

Multiply:

Length × width × average depth × 1,000

Measure everything in metres. The result will be the approximate volume in litres.

For a pool with a sloping floor, calculate the average depth by adding the shallow and deep measurements together and dividing by two.

Round pool

Multiply:

Radius × radius × 3.14 × average depth × 1,000

The radius is half the pool’s diameter.

Example

A rectangular pool measuring:

• 4 metres long

• 2 metres wide

• 0.75 metres average depth

Contains approximately:

4 × 2 × 0.75 × 1,000 = 6,000 litres

Using the starting dose of one tablet per 2,000 litres, you would begin with three 20g tablets in a floating dispenser.

Write your pool volume somewhere you can find it again. Future you will be grateful, which is unusually cooperative behaviour from future you.

20g tablet starting guide

Pool volume	Starting number of tablets
2,000 litres	1 tablet
4,000 litres	2 tablets
6,000 litres	3 tablets
10,000 litres	5 tablets
20,000 litres	10 tablets

This table is only a starting guide. Chlorine demand changes according to:

• Water temperature

• Sunlight

• Rain and fresh-water top-ups

• The number of swimmers

• Leaves, dirt and other contamination

• Filtration and circulation

• The dispenser opening

Do not break or crush tablets to create half doses. Use the dispenser vents to control the dissolve rate instead.

What does “multifunctional” mean?

A 20g multifunctional tablet is based on slow-dissolving trichlor chlorine, also known as trichloroisocyanuric acid or TCCA.

The CrystaLux formulation combines four water-care functions:

• Sanitising: maintains chlorine to help control bacteria and other contaminants

• Algae control: helps reduce algae growth during normal maintenance

• Clarifying: helps the filtration system remove fine suspended particles

• UV stabilisation: helps protect chlorine against rapid loss in sunlight

That convenience is the main attraction. One tablet handles several routine jobs without requiring several separate treatments.

The trade-off is control. You cannot increase one part of the tablet without increasing the others. If you are treating a specific problem such as green water, heavy cloudiness or very low chlorine, a separate targeted treatment will usually be more suitable.

Where should 20g chlorine tablets go?

Floating dispenser

A floating dispenser is the simplest option for most above-ground, inflatable and smaller domestic pools.

Place the tablets inside the dispenser, close it securely and adjust the vents. Start with a modest opening and increase it gradually if your chlorine reading remains low.

Keep the dispenser away from ladders, steps and areas where children can easily reach it. Never allow anyone to play with or open the dispenser.

In-line chlorine feeder

You can also use a compatible in-line or off-line chlorine feeder designed for slow-dissolving tablets.

Follow the feeder manufacturer’s instructions, including its rules on tablet type, circulation and adjustment.

Skimmer basket

Do not automatically place tablets in the skimmer basket just because they physically fit.

Only use a skimmer if both the chemical label and your pool-equipment manufacturer specifically allow it. When the circulation pump stops, concentrated acidic and highly chlorinated water can remain inside the skimmer and plumbing, potentially affecting seals, fittings, pumps or heaters.

A floating dispenser is normally the more straightforward option for a small domestic pool.

Never directly in the pool

Do not place loose tablets:

• On the pool floor

• Against a liner

• On steps

• Inside an unsuitable container

• Directly beside swimmers

A slowly dissolving tablet can bleach a liner or damage the surface beneath it.

Let the water test decide

Adding more tablets is not always the correct response to a low chlorine result.

Check the following first:

1. Are the dispenser vents open?

2. Is water flowing through or around the dispenser?

3. Is the pump running for long enough?

4. Has the pool had unusually heavy use?

5. Has hot weather or strong sunlight increased chlorine demand?

6. Is the pH within the correct range?

If the chlorine is low, open the dispenser slightly or add another tablet, then retest.

If the chlorine is too high, close the vents or temporarily remove the dispenser and keep it safely inside a suitable chemical-resistant container. Do not allow swimming until the chlorine has returned to the range stated on the product label.

Watch your pH

Trichlor tablets are acidic. With continued use, they can gradually lower both pH and total alkalinity.

Test the pH regularly and aim to maintain it between 7.2 and 7.6. Low pH can make the water uncomfortable and increase the risk of corrosion. High pH reduces chlorine’s effectiveness and can contribute to cloudy water or scale.

Only add pH Increaser when testing shows that it is needed. Pool maintenance works better when the test kit makes the decisions rather than the contents of the cupboard.

Watch cyanuric acid as well

Stabilised chlorine adds cyanuric acid, usually shortened to CYA, to the water.

CYA helps protect chlorine from sunlight, which is useful in an outdoor pool. However, it can gradually build up when stabilised chlorine is used repeatedly.

When the level becomes too high, chlorine works more slowly and it becomes harder to maintain effective sanitisation, even when a test still shows free chlorine in the water.

Test CYA monthly during periods of regular tablet use.

If the level is above the range recommended for your pool:

• Pause the use of stabilised tablets

• Partially drain and replace some of the water

• Retest before resuming treatment

• Use an appropriate unstabilised chlorine product if advised by your pool manufacturer or water-care professional

Switching from stabilised tablets to stabilised chlorine granules will not stop CYA from increasing. Both products contain stabilised chlorine.

What about hot tubs?

The swimming-pool dosage table above should not be used for a hot tub.

Hot tubs contain a much smaller volume of warmer water, experience heavier bather loading and usually require more frequent testing. These conditions change both chlorine demand and the rate at which tablets dissolve.

Only use 20g tablets in a hot tub when the product’s approved label and the hot-tub manufacturer both confirm that they are suitable. Follow the specific hot-tub dosage rather than adapting the pool instructions.

Stabilised chlorine also adds CYA, which can accumulate quickly in a small volume of water. Chlorine granules formulated for spas or bromine may offer easier day-to-day control, depending on the hot tub and the manufacturer’s recommendations.

Essential safety rules

• Always read the product label before use

• Wear suitable gloves and eye protection

• Handle tablets with dry hands

• Never mix chlorine with acids or other pool chemicals

• Never add another chemical to a tablet dispenser

• Do not crush, cut or pre-dissolve tablets

• Keep the container tightly closed and completely dry

• Store in a cool, dry and well-ventilated place

• Keep away from children, pets, heat and combustible materials

• Test the water before swimming

Use biocides safely. Always read the label and product information before use.