Understanding Calcium Hardness

February 3, 2017

Calcium hardness is one of the most important factors in water chemistry, and swimming pool chemistry especially. It is one of the six LSI factors, and because it does not fluctuate much, we love using calcium hardness as a foundation for water balance. Let's discuss why.

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You can find a plethora of information online about calcium hardness, carbonate scale and other related topics, so we thought we would narrow your search and summarize the basics for you. You can also read more from this blog about scale, protecting plaster, the LSI and the buffering capacity of calcium in relationship to total alkalinity.

What is Calcium Hardness?

Calcium hardness is the measurement of how much calcium (or lime) is dissolved in water. The terms "hard water", "soft water" and overall "total hardness" refer to the level of calcium hardness along with other minerals like magnesium. Test kits in the pool business can negate other minerals and just test for calcium hardness.

Plaster etching

To understand calcium hardness, know that water will seek balance on its own. In other words, if your fill water is "soft" (low calcium hardness), but wants to be higher, it will draw calcium from anywhere it can—namely, the plaster surface of your pool. Calcium being drawn from the plaster into solution is known as etching. Etching is permanent damage to your pool surface. Having an LSI reading below -0.3 indicates your water is likely to etch plaster because it is hungry for more calcium.

Etching can be avoided by doing a proper startup and giving the water the level of calcium it needs. You can add calcium carbonate to the water, for example.


Measure calcium hardness in your tap water

Here's a scenario that we have heard often:

"Our plaster surface is etched all over the place. It's sharp and abrasive, and we don't know why. It has been getting worse over the years. We tested our fill water for everything including pH and calcium hardness. The calcium was 140ppm out of the faucet. But when we tested the calcium in our pool, it remains pretty constant around 250."

In this scenario, the pool's fill water has low calcium hardness compared to the pool itself. Remember, water will seek balance on its own...and in this case, it wanted more calcium. So it pulled calcium from the plaster into solution to get the level up to around 250. That extraction of calcium from the surface is what caused the etching to occur.

Before building a pool, test the fill water. It can help you and the pool owner decide what type of chemicals to use before you start. If calcium will be needed, consider using calcium hypochlorite (cal hypo), or be sure to budget for adding calcium carbonate on startup and in the future. Otherwise, you're destined to have a calcium-hungry pool surrounded by calcium-rich plaster. Etching is inevitable.

Carbonate scale formation

scale on salt generatorOn the other side of the LSI spectrum from etching is scale. Scaling occurs when there is more calcium in solution than the water wants, so it falls back out of solution. The accumulation of calcium carbonate on surfaces is called scale.

Scale can vary in texture and thickness, but it's almost all calcium carbonate (lime). The good news is, with a good chelating agent and LSI balance in your water, scale is manageable (unlike etching).

Water seeks balance on its own, but it is interesting that the balanced calcium levels can vary depending on what else you have in the water. For example, some pools may want to stay at 200ppm, and others want 250 or even 300ppm. It depends on other factors like water temperature, the type of chlorine you use, and your CYA levels.

Salt pools and scale

Did you read our article on scale? In there we discuss two factors that accelerate calcium falling out of solution: temperature and pH. Because of these two factors, the first two places in a pool system that develop scale are the salt chlorine generator and the pool heater.

Salt pools have a salt cell, which carry a charge to create chlorine from salt water. The cell generates heat. Increased water temperature is the main reason why scale forms on salt cells and heat exchangers. This can be avoided with regular dosing of SC-1000, and managing calcium levels in your pool.

But let's not forget that salt and sodium hypochlorite (bleach) also raise pH, and high pH drives calcium out of solution too. If you increase any of these factors, the LSI value rises:

And if you increase these factors, the LSI value lowers:


Calcium is your best friend for water balance. Because of how consistent and stabile it is, calcium hardness helps keep the LSI in range. Look at calcium as a solid foundation to build your chemistry strategy upon. Unlike total alkalinity—which fluctuates often—calcium hardness is remarkably stable. Once it reaches its happy level of saturation, it pretty much stays there. This has amazing benefits for pH control, if we are willing to be open minded and think differently.  Calcium is the most misunderstood chemistry in the pool business.

Use our LSI calculator and see for yourself.


All information provided is intended for educational purposes and is not implied to replace consultation with a qualified pool professional. It is recommended that all information from this or any other source is to be performed assuming individuals performing these functions will consult local state and federal requirements before you act upon it in any way. While this site attempts to provide information that may be relevant to you, no guarantees are made that some relevant information will not be missed. We recommend you consult a local pool professional before acting. 

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