Calcium Hardness - Complete Guide

Calcium Hardness In Swimming Pools - Ultimate Guide

Calcium hardness is a vital component of your pool’s chemistry. When the calcium is unbalanced, your pool can develop other chemical imbalances in a type of chain-reaction that ultimately becomes damaging to your pool’s surface and equipment.

In this ultimate guide to pool calcium hardness, we’ll talk about what calcium hardness is and why it is important in swimming pools.

We’ll then cover ideal levels of calcium hardness for pools and how you can test these levels, as well as what can happen when your pool does not meet these levels. We also include tips for returning your calcium hardness levels to normal.

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What Is Calcium Hardness?

Calcium hardness refers to the total amount of dissolved calcium in your pool’s water. Calcium is present in most water sources to some extent, and tends to be found in higher concentrations in bore water.

Many homes contain water softeners, which are machines that work to remove excess minerals such as calcium from your water. Soft water is often considered to be more pleasant for indoor bathing, and also works to prevent calcium scale in the home.

It is important to note that total hardness in pools is technically not the same as calcium hardness. This measurement includes calcium but also includes magnesium, which is considered by many to be irrelevant.

Currently, most test kits measure the total hardness of your water. It is more difficult to find test kits that specify the calcium content of your pool.

Why Is Calcium Hardness Important In Pools?

Calcium hardness is important in pools because it affects all aspects of your water quality.

Inadequate or excess calcium hardness can lead to chemical ineffectiveness, water cloudiness and impacts on pH & alkalinity, all of which can ruin the swimming experience and damage your equipment.

It can leach calcium from concrete and plaster surfaces, and it can also cause issues with gel coat oxidation (a white, chalky residue) in fibreglass pools.

Ideal Calcium Hardness Levels For Swimming Pools

Ideal levels of calcium hardness in a pool range from 200-400 parts per million (ppm).

Below this range, your pool’s water develops slightly corrosive properties and begins to seek out minerals from other sources. Above this range, your pool water contains too many minerals, and you are likely to notice changes in water clarity as well as scaling.

How To Test For Calcium Hardness

There are two options when it comes to testing calcium hardness at home. There’s either liquid reagent test kits that can measure calcium hardness, or testing strips that measure total hardness (not calcium specifically).

While the liquid kits will be more accurate, the testing strips are good enough for most situations. The non-calcium component of your total hardness is likely to be stable over time.

Liquid reagent test kits should include a vial for pool water, and may include up to four testing solutions. Testing solutions are only capable of returning a range of values, but this is enough to identify and adjust your calcium levels.

To test your calcium hardness levels, fill the vial up with the appropriate amount of pool water. Then, add drops of the testing agents as is outlined in your test kit instructions.

Once you have mixed these solutions as recommended, you will compare the colour of your test water to the chart provided in order to determine your pool’s calcium content.

Imbalanced Calcium Hardness

Calcium Hardness Is Too Low

When calcium hardness is too low, water begins to develop corrosive properties. Your pool may experience drops in pH and especially total alkalinity, and you may notice other side effects of this such as etching and deterioration of your pool’s surface.

Corrosive water tries to draw minerals out from other sources in an attempt to achieve equilibrium. As it does this, calcium can be leached from your pool’s lining, leading to damage.

Calcium Hardness Is Too High

When calcium hardness is too high, there are too many minerals present in your pool’s water. The most common sign of high calcium is cloudy water, which occurs when the calcium starts reacting with the alkalinity in your water and forming solid calcium carbonate.

As you would expect, this cloudiness usually occurs when the total alkalinity is high and the pH is high (ie. there is a lot of the carbonate form of alkalinity dissolved in the water).

High calcium hardness and related buildup of calcium scale can cause all kinds of damage to your pool’s equipment.

As the scale builds up on machinery, sensitive areas like your chlorinator and pump impeller can get scale on them. This is especially common in narrow areas of your pool’s tubing, such as in water return lines.

Fixing Calcium Hardness

How To Raise Calcium Hardness

Raising the calcium hardness level in your pool is much easier than lowering it. In order to raise your hardness levels, you will simply need to add more calcium containing compounds to your pool.

The recommended way of doing this is by using products intended for this purpose, which are typically known as calcium hardness increasers.

These raise the calcium hardness content of your pool without affecting other properties of your pool’s water.

When measuring out a dose of hardness increaser for your pool, be sure to only use as much as your pool needs. Accidentally adding too much calcium to your pool will create different problems, and this condition can be more difficult to treat.

Use the calcium calculator to work out how much hardness increaser to dose into your pool.

How To Lower Calcium Hardness

There are two main methods for lowering calcium hardness in your pool. These include performing a partial drain and refill of your pool’s water, or using a product known as a calcium remover. In some cases, a combination of both of these approaches may offer the best solution.

We recommend that you begin lowering your calcium levels by performing a partial drain and refill of your pool’s water. By draining high calcium water from your pool, and then adding fresh, low calcium water, you lower the overall concentration.

This step can be repeated if necessary, but it is important that you do not drain your inground pool more than 8 centimetres or so below the normal water line at a time to avoid pool-popping.

Use the calcium calculator work out how much water to drain and refill.

Once you have drained and replaced part of your water, you should rebalance your pool’s pH and total alkalinity. Once you have corrected these levels, re-test your calcium hardness.

As mentioned earlier, the pH and alkalinity can interact with calcium hardness, so it’s better to rebalance them first before checking the calcium concentration.

You may also use a calcium hardness remover. Although the chemicals in calcium removing products do not actually remove these calcium particles from your pool, they prevent dissolved calcium from depositing on your pool’s surfaces in the form of scale.


Pool water hardness is a crucial part of your pool’s chemistry. If your calcium levels become imbalanced and are not remedied, the effects can ultimately become damaging to your pool’s lining and equipment.

Do you have any questions about calcium hardness or why it is important in pools? Get in touch with us in the comments, we’d love to help!

Louis from Pool Advisor


A chemical engineer by trade, Louis is committed to debunking myths in the pool industry by explaining the underlying chemistry and making it accessible to all.