How do you test for calcium hardness in your pool?

How do you test for calcium hardness in your pool?

Calcium is crucial for your pool’s water chemistry, and maintaining its proper levels is essential. Unbalanced calcium hardness is not just bad for water balance, but it is also the most significant factor that can damage pool equipment. 

Here, we will teach you about the proper method to evaluate the calcium hardness in your pool and some valuable tips about this subject. 

How to test for calcium hardness in your pool

Using an appropriate pool test kit is the most accurate and easiest measure to test calcium hardness in your pool. The pool test strips don’t work here because they measure total hardness (a mix of calcium and magnesium) instead of calcium hardness (only calcium) in water. 

There are no test strips available on the market that measure pure calcium hardness. Drop test kits allow you to measure as low as ten ppm (parts per million) in increments. 

So test kits are also a better choice for making minor adjustments. 

Here’s a step-by-step process for measuring calcium hardness using a test kit. We will use the Taylor liquid test kit for this guide. However, you can find the appropriate instructions on the packaging label/manual of the kit you choose.

  1. Fill a clean, blank test vial (often included with the kit) with water from a well-circulated area of your pool.

  2. Add 20 drops of R10 reagent (calcium buffer) from the kit into this vial. Shake gently to mix well.
  3. Add five drops of R11 reagent (calcium indicator), and the color of the solution will slowly turn pink.

  4. Add R12 (hardness reagent) slowly to the solution, counting each drop until the color changes from pink to blue.

  5. Multiply the number of drops of R12 reagent added to the mix and multiply it by 10. The resulting number is the calcium hardness of your pool water. For example, if you used 14 drops of R12, the calcium hardness of your pool water is 14 times 10 equals 140 ppm.

Quick Tip: You can also use a digital photometer to measure calcium hardness. However, they are less accurate, pricey, and rarely used. 

What is the optimal range for calcium hardness in a swimming pool?

The optimal levels of calcium hardness in a swimming pool should range between 200 to 400 ppm. Here’s the exact range for different types of pools based on their design:

  • Concrete Pool: 200 - 400 ppm.
  • Fiberglass Pool: 150 - 250 ppm.
  • Vinyl Liner Pool: 150 - 250 ppm.

How often and why should you test calcium hardness in your pool water?

The calcium hardness in a pool should be checked monthly or at least once every six months. Factors such as the TDS (Total Dissolved Solids), changes in alkalinity or pH, water temperature, and the addition of chemicals can change the calcium hardness. 

Signs such as scaling, pitting, cloudy water, and clogged filters indicate unbalanced calcium levels. 

Here’s what happens if the calcium hardness of water is too low:

  • Etching of plaster (as water starts drawing calcium).
  • Pitting of concrete surfaces.
  • Corrosion of metal items.

Here’s what happens when the calcium hardness is too high:

  • Scaling on pool walls and equipment.
  • Cloudy pool water (especially if the pH/alkalinity is high)
  • Clogged filters and reduced circulation
  • Heater inefficiency


You should test the calcium hardness of pool water at least once every month, and using pool test kits is the most accurate method for it. The ideal levels fall between 200-400 ppm, and they should always be maintained. 

Too low calcium hardness can cause etching and corrosion of surfaces and equipment. Whereas very high levels can result in scaling and a clogged circulation system. 


Does baking soda increase calcium hardness in a pool?

No. Baking soda is used to raise alkalinity in pool water and will not affect calcium hardness.

Do you need to test calcium hardness in a vinyl pool?

Yes. You should test calcium hardness in a vinyl pool and ensure it is always between 150 - 250 ppm.

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