Helpful Guide How to Balance The Calcium Hardness in Your Pool

The calcium hardness in the pool is one of the most important parameters to keep the water in the pool balanced. But which concentration is the right one, and how is the calcium hardness measured and adjusted?

The calcium hardness concentration in pool water should be between 200 and 400 ppm – 300 ppm is considered optimal. If the calcium measurement is above 600 ppm, you can expect lime in the pool. The calcium hardness is optimized with pool chemistry, or you exchange the water.

I will explain how you can determine the calcium hardness level in your pool, how often you have to measure it, and how the levels can be optimized so that the pool water remains crystal clear.

What does calcium hardness mean in the pool?

We all learned that calcium is essential to humans, but what does calcium mean in water?

The calcium hardness determines how hard and how soft the water in the pool is.

It would be best to remember that the pool’s calcium hardness interacts with the pH level and the total alkalinity (acid capacity).

This means that if the calcium hardness level in your pool is not ideal, you will have long-term problems with a strongly fluctuating pH level and alkalinity.

The result is cloudy water and even limescale deposits in the pool and equipment.

If the concentration of calcium ions in the pool is not tested and adjusted regularly, expensive repair costs can arise over the long term.

It is therefore important to test the calcium with a suitable water tester and maintain and optimize the water hardness in your pool.

How high should the calcium hardness in the pool be?

Disinfecting the pool with pool chemicals is not the only criterion for clean water. An optimal pH level, total alkalinity, and calcium hardness are equally important.

And how is calcium hardness measured, and how high should the level be?

The proportion of calcium ions is measured using the titration or photometric method.

The concentration of calcium in the pool must not exceed 600 ppm, as this could lead to various problems with the pool water.

The calcium hardness in the pool should be between 200 and 400 ppm – a concentration of 300 ppm is a good average value.

In a vinyl pool or a fiberglass pool, the calcium hardness level should be between 200 and 250 ppm.

And how does the water hardness relate to the pH level in your pool?

After all, the calcium hardness affects the pH level, and then it must logically be the other way around, right?

It’s good that you ask. I have prepared a table for you:

Water hardness pH level °dH mmol/l
Very soft 7,8 – 8,2 Until 7,3 Until 1,3
Soft 7,4- 8,0 7,3 – 14,0 1,3 – 2,5
Medium hard 7,2 – 7,8 14 – 21,3 2,5 – 3,8
Hard 6,8 – 7,4 Above 21,3 Above 3,8

List of abbreviations:

  • Water hardness: The parameter says how hard or soft the water is. Source.
  • pH level: Strength of the hydrogen atom in water.
  • °dH: Degree of German hardness.
  • Mmol: Alkaline earth ions in millimoles – the mole is the SI unit of the substance.

The table shows the interaction between water hardness and the pH level in swimming pools.

Furthermore, the old hardness level can be read off with the current status (alkaline earth ions in millimoles) – 1 mmol corresponds to 100 ppm.

With the table, you can determine exactly how hard or how soft the pool water should be.

Balance the pool water hardness level

Correcting the calcium hardness in the pool upwards and downwards is essential so that the water stays clean.

In the meantime, suitable products can optimize water hardness for private pool owners. These products are available from pool retailers.

However, the powder for calcium hardness is not always necessary. Sometimes a change of water is enough to optimize the calcium hardness in the pool.

In the following chapters, you will find out exactly when and how to use which method.

What if the pool water is too hard?

If there is too much calcium in the pool water, it is called hard water (see table above).

The optimal level is exceeded from a concentration above 400 ppm: The pool walls and the filter sand fog up, and the pool water becomes milky.

Limescale formation in the pool is expected from a calcium hardness level above 600 ppm.

A limestone rim can often be seen in the pool. Here you can be sure that the calcium hardness in your pool is too high.

Believe me: Removing limescale deposits in your pool is a lot of work.

For this reason, you should test the calcium hardness regularly and, if necessary, correct the concentration of calcium in your pool downwards as soon as possible.

Next, I will explain how you can lower the calcium hardness in your pool or hot tub.

How can the calcium hardness in the pool be reduced?

If the calcium level measured in your pool was too high, this must be corrected downwards.

  • Water exchange: First, measure the calcium hardness in the pool and then empty the pool to a certain extent until the concentration is again in the green range. Before filling with freshwater, you should also measure the water source so that too much calcium does not end up in the pool again.
  • Use flocculants: If exchanging water is not an option for you, you can try to lower the calcium level with a pool flocculant. The resulting flakes must later be vacuumed with a pool vacuum from the floor.
  • Calcium lowering agent: In the meantime, chemicals are available in pool shops that can lower the calcium hardness in your pool. Here it would help if you kept in mind that the pH level will also be pulled down and will have to be optimized again later.

What if the pool water is too soft?

Don’t make the logic of assuming that too much calcium in the pool will cause problems.

Too soft water can also damage the pool.

The pH level drops, the water becomes acidic, and has a corrosive effect on the pool.

Metal parts in the pool will rust, and joints will be damaged. This problem leads to algae in the pool and other issues.

In the worst case, your pool has to be renovated because the problem with black algae cannot be brought under control in any other way.

Another disadvantage if the calcium hardness negatively affects the pH level is that the disinfecting effect of chlorine, bromine and other disinfectants becomes weak.

For these reasons, you should quickly optimize the water hardness in the swimming pool upwards again.

How can the calcium hardness in the pool be raised?

Raising calcium hardness is much easier than decreasing the calcium concentration in your swimming pool.

All you have to do is pour a calcium hardness increaser into the pool.

Follow the manufacturer’s instructions when using the hardness stabilizer.

Info: Calcium gets into the pool through the pool shock with chlorine granulate.

However, it would take too long for the calcium hardness to be optimal again. If the calcium level in your swimming pool is too high, I recommend using liquid chlorine for the pool shock.

How often should you measure calcium hardness in the pool?

Regular control of the water parameters ​​is the first step to long-term clean water in your pool.

But how often do you have to measure the calcium hardness in your pool?

Unlike the pH level – which should be tested 2 to 4 times a week – it is sufficient if the calcium hardness is tested at least once a month and, if necessary, optimized.

If the pool is used excessively and the number of bathers increases, the water hardness measurement should be carried out more regularly.

As already shown above, the titrimetric and photometric methods are appropriate.

You can also use the Langelier Index water conditioning formula.

The LI level indicates how corrosive the water is:

  • LI level = ph level – temperature (K) – calcium hardness factor (C) – alkalinity (A)
  • LI level = pH level + temperature (K) + calcium hardness factor (C) + alkalinity (A) -12.1

In practice, a level of 0 to 0.3 is considered ideal.

  • If the level is less than zero, the water is corrosive.
  • If the level is above zero, there will be limescale deposits in the pool.

But you don’t have to make it that difficult for yourself as a pool owner.

The Langelier index was only mentioned here for the sake of completeness.

Measure the water levels ​​regularly and keep a good water balance in the pool, then the determination of the Langelier index is unnecessary.

Keep the calcium hardness level in the pool optimal

Succeeding optimization of the calcium hardness in your pool is more complicated than it is the case with other parameters ​​in the water.

Therefore, the simplest method is to keep the concentration of the calcium hardness level in your swimming pool at an optimum between 200 and 400 ppm right from the start.

  • Maintain water balance: The calcium hardness interacts with the pH level and the alkalinity. Keep all levels ​​in the green area. Then you will not have any problems with the calcium hardness in your pool.
  • Use pool chemicals correctly: When pouring chemicals in your pool, pay attention to which agents you pour into the pool. Some products contain a high proportion of calcium, such as chlorine granules.
  • Test the pool water regularly: A continuous water test shows you when the calcium hardness is not optimal. You can take simple steps with a timely assessment – such as a partial water change – to lower the calcium hardness.
  • Eliminate problems early: Regular water controls allow issues with water hardness to be eliminated quickly and in a targeted manner. Do not wait long because a too high – and too low – calcium hardness level in the water damages your pool.

Keeping the calcium hardness in the pool water optimal is not hard.

Do not put the necessary corrections on the back burner, and then nothing can go wrong.

The pool is an object that has to be constantly monitored and maintained. If your pool maintenance is neglected, it can be expensive.

Buy the right pool chemicals and store them just in case.

Do you have any further questions about calcium hardness in the pool? Contact me at @contactswimfool on Twitter with your question – I’ll be happy to help.

I wish you good luck balancing the pool calcium hardness level!

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Hi. I'm Max Berg. I've been in the pool industry since 2015 and have always felt drawn to water. I'm the author behind swimfool.com, where I share my years of experience in pool maintenance and give helpful tips on keeping a swimming pool or hot tub clean. My tips reduce the costs of water treatment and protect the environment.

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