Calcium Hardness for Pools: A Beginners Guide



Are you trying to wrap your head around calcium hardness?

This is a complete beginner’s guide to calcium hardness in your pool, including what it is, why it matters, and much more.

Quick answer: Calcium hardness is a measure of the total calcium dissolved in your pool water. Keeping it balanced will prevent issues with your water, including cloudiness, scaling, and erosion.

What is Calcium Hardness?

Calcium hardness, often abbreviated to CH, measures how much calcium carbonate is dissolved in your pool water.

The more calcium dissolved in your pool, the higher your calcium hardness level, and vice-versa.

Note the emphasis on “dissolved”. Not all of this calcium in your pool can be dissolved because your water can only hold so much of it. This excess or “undissolved” calcium does not count towards your calcium hardness level.

The total amount of calcium that can be dissolved in your water is always changing, as it depends on things like your pH level, total alkalinity, and even the temperature of the water temperature.

Why is Calcium Hardness Important?

Calcium hardness helps to maintain balance in your pool, preventing water issues and potentially even structural damage.

As we just touched on, small changes in the water can quickly lower your water’s ability to hold calcium in a dissolved state, and this is a problem when you have a high calcium hardness level.

Any excess calcium will precipitate out of solution, often appearing as a subtle white film and eventually white flakes known as calcium scale.

Calcium scaling can lead to cloudy water and chalky stains on your pool walls, floors, and equipment. In severe cases, scaling can even restrict circulation as it clogs up your filters and plumbing lines.

On the other hand, when you a have low calcium hardness level, your water will begin to seek out (and suck out) calcium wherever it can.

This is called leaching and it wears on almost anything your water touches, including your plaster, concrete, tiles, and even grouting. This can lead to etching (erosion) on your surfaces, resulting in rough, pitted spots.

Note: When we talk about the water rejecting calcium or seeking out more of it, we’re actually referring to the saturation index, otherwise known as CSI or LSI in the world of pool chemistry.

What is the Ideal Calcium Hardness Level?

Technically, the ideal calcium hardness level is where your pool water no longer seeks out or rejects calcium. It’s neither under-saturated nor over-saturated.

In reality, this is a constantly moving target and not something that can be maintained in the long term. Instead, your calcium hardness should be close enough to ideal that the effects on your pool are negligible.

The good news is, there’s a wide margin of error here.

If you have a plaster or fiberglass pool, you’ll generally want to stay between 300 and 500 parts per million, though you can often get away with being slightly outside of this range.

If you have a vinyl pool, anything below 600 ppm is usually acceptable. Since vinyl liners don’t contain calcium for your water to leach, you really only have to worry about scaling caused by excessively high levels of calcium.

Finally, if you have a saltwater generator, aim toward the lower end of these ranges as salt pools tend to maintain a slightly higher pH level where scaling is more likely to occur.

How Does Calcium Hardness Influence Other Levels?

Calcium hardness doesn’t interfere with your pH, total alkalinity, cyanuric aid, or chlorine levels.

It mostly stays in its own lane, so to speak.

However, it does influence your CSI (Calcium Saturation Index), which tells you how over or under-saturated your water is.

Basically, if calcium hardness tells you how much calcium is in the water, CSI tells you how much of that calcium can be dissolved — though this generally isn’t a concern if your hardness is within or close to the ideal range.

Of course, it also contributes to your total dissolved solids (TDS), but that’s true of any dissolved substance in your water and it isn’t a particularly useful metric for pool maintenance.

What Causes Calcium Hardness to Change?

Calcium hardness shares a lot in common with cyanuric acid in the sense that it doesn’t tend to fluctuate very much on its own.

Instead, most of these changes happen when you “do stuff” to the water.

What Raises Calcium Hardness?

Your calcium hardness goes up when you add more calcium to the water, assuming it isn’t too saturated to be able to dissolve it.

This doesn’t always come from obvious sources.

For example, if you use calcium hypochlorite or “cal-hypo”, a type of chlorine commonly used for shocking a pool, you’re directly adding plenty of calcium to your water.

What’s more, if your local area supplies hard water, there’s a good chance you’re adding lots of dissolved calcium to your pool every time you top up, just through the source water alone.

Evaporation also has a small part to play; since calcium doesn’t evaporate, it gets left behind and becomes more concentrated over time.

What Lowers Calcium Hardness?

On the other hand, calcium doesn’t degrade in water so you rarely need to worry about your calcium hardness level getting too low.

Unless your pool is subject to frequent rainfall or top-ups (with soft source water) that dilute the calcium in the water, it’s considerably more likely to increase or stay roughly where it is.

How Do You Test Calcium Hardness?

You can test your calcium hardness level using almost any pool test kit you can get your hands on, especially those marked as 5-way tests.

Strips are the most widely used testing method, if not the most reliable.

They work by dipping a strip into the pool, waiting around 15 seconds, then comparing the colored tabs with the provided color chart. One of those tabs will represent your calcium hardness level.

Liquid testing kits are a big step up in accuracy but suffer from a slightly more complicated testing process, as well as a higher price tag.

These kits have multiple compartments to hold samples from your pool, with one dedicated to calcium hardness. A reagent is then added to the water to change its color, which can be measured using the provided chart.

How Do You Adjust Calcium Hardness?

You’ll need to readjust your calcium hardness level whenever it gets too high or possibly even too low.

Here’s how to do it effectively in both scenarios:

How Do You Raise Calcium Hardness?

The easiest way to introduce more calcium to your water, in turn raising your calcium hardness, is to add calcium chloride.

Calcium chloride comes in granular form, and it’s the primary ingredient in most calcium hardness increasers.

The granules are usually dissolved in a bucket of water to allow them to fully dissolve, before being poured into the pool. Calcium chloride also heats up as it dissolves in water so you may notice some light fizzling.

This substance can also be found in liquid form, otherwise called calcium chloride dihydrate. It has a lower concentration of calcium but it has the added convenience of pouring directly into your pool.

Finally, if you also need to significantly raise your chlorine level, you could use calcium hypochlorite to raise your calcium hardness by 0.8 ppm for every 1 ppm of free chlorine.

How Do You Lower Calcium Hardness?

Lowering calcium hardness isn’t as straightforward because there are no chemicals that directly remove calcium from the water.

While you may come across “calcium reducers” at your local pool store, they’re really just a special type of sequestrant.

These products bind with excess calcium to keep it in a dissolved state when it would otherwise form calcium scale. It doesn’t actually remove any calcium from the water, it just prevents it from causing trouble.

Even then, sequestrant breaks down over time, meaning it only works as a temporary solution while you work to lower your calcium hardness.

In fact, the most reliable method is to partially drain the pool and use soft fill water to dilute the calcium. For example, if you need to lower your calcium hardness by half, you would drain 50% of your water.

The only other alternative is to use reverse osmosis (RO), but this is both expensive and inefficient for lowering calcium hardness, especially given that it strips the water of everything—not just calcium carbonate.

How Do You Maintain Calcium Hardness?

Maintaining your calcium hardness level isn’t as difficult as maintaining your pH or total alkalinity.

So much so, that once balanced, you can safely test calcium hardness as infrequently as once per month.

As long as you remain mindful of what you’re putting into your water, including your source water, you shouldn’t see any significant drops or increases in calcium between each testing interval.

But there’s another layer to consider here, because a change in the conditions of your water can still lead to scaling or etching despite no change in your calcium hardness level.

As we touched on earlier, certain shifts will impact the saturation (CSI) of the water, making it more “hungry” or more “starved” for calcium.

Fortunately, as long as you keep your calcium hardness, pH, and total alkalinity all within a reasonable range, your CSI will remain relatively stable.

The Bottom Line

Calcium hardness measures the amount of dissolved calcium carbonate in your pool water, and keeping it balanced will prevent issues with your water clarity and pool surfaces.

Adjusting calcium hardness can be done by adding calcium chloride to raise it, or partially draining and refilling to lower it, and monthly testing will give you an indication of when these adjustments are necessary.

Finally, as is often the case in pool chemistry, maintaining healthy levels across the board is the best way to manage your calcium hardness.

Categories: Pool Care, Pool Chemistry