How to Raise Calcium Hardness in a Pool (6 Steps)



Is your calcium hardness too low?

Low calcium hardness will make your water corrosive, causing it to leach your pool surfaces and equipment and will eventually cause spot etching.

This article will walk you through how to lower calcium hardness in your pool, step-by-step.

Quick answer: You can raise your calcium hardness using either calcium chloride or calcium hypochlorite, both of which come as granules that are dissolved in a bucket before being added to your pool.

When is Calcium Hardness Too Low?

If you have a plaster or fiberglass pool, a reading below 300 parts per million (ppm) is typically less than ideal.

In reality, though, calcium hardness is much more forgiving than most other parameters, and you’ll likely only experience issues below 200 ppm.

For a saltwater pool, a reading of 200 ppm may even be considered ideal because scaling is more likely to occur inside a saltwater generator.

If you have a vinyl pool, you don’t need to worry about your calcium hardness being too low because vinyl liners are immune to the effects of unsaturated water. It’s a non-issue.

Why do these minimums seem so… vague?

Well, your water begins to seek out more calcium when the CSI (Calcium Saturation Index) is negative. The higher this negative value, the more “hungry” your water becomes, and the more calcium it needs.

But the CSI of your water is always changing. This makes calcium hardness thresholds inherently flawed, and it’s why these numbers can only ever be considered general guidelines.

What Chemicals Can You Use for This?

Most pool owners use calcium chloride.

This white granular or flakey substance will significantly raise the calcium level in your water without affecting pH, alkalinity, or any other parameter. It’s the primary ingredient of ‘calcium hardness increaser’.

It also comes in liquid form, known as calcium chloride dihydrate, which can be poured directly into your pool instead of being pre-dissolved.

Calcium can also be added to your pool through a particular type of chlorine; calcium hypochlorite, often called cal-hypo.

This is usually a granular or powdery substance that’s used for sanitizing or shocking a pool, but it adds nearly as much calcium to your water as it does free chlorine, so it can work as a source of calcium if need be.

How to Raise Calcium Hardness (7 Steps)

The process for lowering calcium hardness is largely the same whether you’re using calcium chloride or calcium hypochlorite.

Here’s what you’ll need for this:

  • A pool test kit
  • Calcium chloride OR calcium hypochlorite
  • A bucket of water
  • A stir stick to mix the water

1. Test Your Calcium Hardness

Like every chemical adjustment in pool maintenance, this all starts with testing.

Before adding any substance to your water, you should get an up-to-date reading of your current calcium hardness level using a test strip or liquid test kit.

A liquid kit will always give you a more accurate reading, but traditional test strips work in a pinch.

2. Decide on Your Target Level

Fiberglass and concrete/plaster pools should stay between 300-500 parts per million (ppm), and slightly lower at between 200-400 ppm if you have a saltwater generator.

In the unlikely event your calcium hardness tends to drop over time, you should target the lower end of those ranges — otherwise, going straight down the middle of the range often makes the most sense.

Note: It’s always easier to go up than it is to go down.

3. Calculate the Required Dosage

If you’re only looking to raise your calcium hardness level by itself, especially if you need to do it significantly or quickly, use calcium chloride. This is going to be the best approach for most people.

However, for smaller increases, you can use calcium hypochlorite (a type of chlorine) to raise or maintain your free chlorine level at the same time — though you may still need to spread this out over days or weeks.

You can get the exact dosage for either substance using our calculator below:

4. Mix the Granules in a Bucket

If you’re using the liquid variant of calcium chlorine, known as calcium chloride dihydrate, you can ignore this step as it’s already pre-dissolved.

Otherwise, for granular calcium chloride or calcium hypochlorite, you’ll need to mix the granules in a bucket of water to pre-dissolve before adding them to your pool.

If you happen to be using a combination of both substances, do not mix them together in the same bucket. Mixing pool chemicals is rarely a good idea.

5. Pour it Into Your Pool

With your pre-mixed solution in hand, pour around the perimeter of the pool and ideally toward the deeper end.

If possible, focus on pouring in front of active return jets to help the solution circulate and distribute. Keeping your pump running for a few hours after application will also help.

6. Test the Water Again

The only thing left to do is to check your work.

Perform another test using the same testing kit or method, and make sure your new calcium hardness reading is where expected.

If it’s still a little too low, follow steps 2-5 again.

All good? Then you’re done!

How Do You Prevent Low Calcium Hardness?

Calcium hardness doesn’t evaporate or degrade over time so it will stay in the water until the water itself is removed.

If you lose a lot of water through splash out or backwashing, you may find your calcium hardness slowly declining. Even then, the water you use to top up your pool will contain some level of calcium, so you’re always replacing at least some calcium.

As a result, any loss of calcium usually happens so slowly that it’s easy to manage with semi-regular testing.

Simply test your calcium hardness every month or so, adding more calcium chloride or calcium hypochlorite whenever it gets close to your minimum level.

The Bottom Line

Low calcium in a pool isn’t a common problem, but it is easily fixable.

All it takes is a sprinkle of the right substance and a bit of patience, and your calcium hardness will be right back where it needs to be.

Categories: Pool Care, Pool Chemistry