Pool Calcium Hardness Calculator - Work Out How To Adjust Calcium Up Or Down

Is your calcium hardness outside the recommended range of 200-400 ppm? Use this pool calcium calculator to work out either how much calcium chloride to add to your pool to increase your hardness, or how much water to drain and refill to lower your hardness.

Pool Calcium Hardness Calculator

Generally, 200-400 ppm is recommended as the ideal range for calcium hardness in a swimming pool.

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How Much Calcium Increaser Should I Add To My Pool?

If you have low calcium in your pool, you will need to increase calcium hardness. To work out how much calcium increaser to add to your pool, you need to do the following calculation:

Calcium Dose = Pool Volume x (Target Calcium - Current Calcium)

For a 60,000 L pool with current calcium of 150 ppm and a target calcium of 250 ppm, the calculation looks like this:

Calcium Dose = 60,000 L x (250 ppm - 150 ppm)
Calcium Dose = 6,000,000 mg (CaCO3 equivalent) Calcium Dose = 6 kg (CaCO3 equivalent)

So for this example pool, a dose of 6 kg of calcium (as calcium carbonate) is required. However, calcium chloride (the main ingredient in calcium hardness increaser) contains only 36% calcium (the rest is chloride), whereas calcium carbonate contains 40% calcium.

Therefore the correct dose of calcium chloride (hardness increaser) is about 6.7 kilograms (6 kg * (0.4/0.36)).

Use the calculator above to work out the appropriate dose for your pool.

What Happens If I Put Too Much Calcium In My Pool?

Excess calcium in your pool (above 400 ppm) will increase the risk of scale forming on the surface of your pool. This is especially likely at elevated pH and alkalinity.

Once scale starts forming, it can be very difficult to get rid of it (particularly on some surfaces like pebblecrete), so it's better to keep your calcium levels within the recommended range as much as possible.

To lower your calcium hardness, you need to drain and refill some of the water in your pool.

As an example, if your current calcium concentration is 500 ppm and you want to lower it to 400 ppm, you will need to drain about 20% of your water.

Use this calculator to work out how much water to drain and refill in your pool.

How Do I Use This Pool Calcium Hardness Calculator?

To use this pool hardness calculator, you need to enter the following information about your pool:

  • Pool volume (in L)
  • Current calcium hardness (in ppm or mg/L)
  • Target calcium hardness (in ppm or mg/L)

The calculator will then tell you whether your calcium hardness needs to be increased or reduced.

If it needs to be increased, it will tell you how much calcium hardness increaser you need to add to your pool to get to your target level.

If it needs to be decreased, it will tell you how much water you need to drain and refill to lower your hardness to your target level.


Have any questions, feedback or ideas for how to improve this calculator? Or maybe you've spotted an error? Let me know over at the contact page!

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.