How to raise calcium hardness in a pool
Low calcium hardness doesn't make swimmers sick, but over time it lets the water dissolve calcium out of plaster, grout and concrete. Raising it is straightforward with calcium chloride. The main things to get right are the target for your surface and adding the product safely.
Is it too low?
| Pool surface | Common target range |
|---|---|
| Plaster, pebble, quartz | 250–450 ppm |
| Vinyl liner, fiberglass, painted | 150–250 ppm |
Low hardness is mainly a concern for cement-based surfaces. Signs include etched or rough plaster, pitted grout and a rising hardness reading as the water slowly takes calcium from the walls. In vinyl and fiberglass pools there's less to protect, but very soft water can still be aggressive to metal parts and heaters.
Calcium works together with pH, alkalinity and temperature. The LSI calculator shows whether your water as a whole tends to dissolve or deposit calcium. Low calcium can be partly offset by running pH and alkalinity a bit higher, but for plaster, raising calcium is usually the better fix.
Why calcium hardness is low
- Soft fill water, common in some regions and with rainwater or softened water.
- A recent drain and refill.
- Heavy rain diluting the pool.
How much calcium chloride to add
Calcium chloride comes in two common forms, and the dose differs:
- Flake (dihydrate), about 77% calcium chloride.
- Anhydrous pellets, about 94% or more.
Check the label for your product's strength.
Example: a 10,000-gallon plaster pool reads 150 ppm and the target is 250 ppm. The calcium hardness calculator gives about 12 lb of flake calcium chloride, or about 9.84 lb of anhydrous pellets. A 20,000-gallon pool would need twice as much.
The calculator page shows the formula: hardness is reported as calcium carbonate, so the dose converts from that to calcium chloride and then divides by the product's purity.
How to add it safely
- Fill a bucket about three-quarters full with pool water.
- Add the calcium chloride slowly, stirring with a plastic or wooden paddle. Don't add more than the bucket can dissolve; split large doses across several buckets.
- With the pump running, pour the solution slowly around the pool, ideally in front of a return jet.
- Add about half the total, circulate for several hours, retest, then add the rest.
Don't pour it into the skimmer, and don't let undissolved product sit on the floor of a vinyl or plaster pool.
Avoid cloudy water and scale
Adding a lot of calcium to water with high pH and alkalinity can push it over the edge into scaling, which shows up as a white haze in the water. To prevent it:
- Bring pH to about 7.2–7.4 before adding calcium.
- Check the LSI with the new calcium level before adding it all. Aim for between −0.3 and +0.3.
- Add in portions, not all at once.
If the water does cloud, lower pH a little and keep the filter running. It usually clears over a day or two.
Other ways calcium goes up
Cal-hypo chlorine adds about 0.71 ppm of calcium hardness for each 1 ppm of chlorine. In a pool with soft water, using cal-hypo for part of the season raises hardness gradually. The chlorine cost calculator shows how much over a month.
Frequently asked questions
Can I use any calcium chloride?
Use a product sold for pools. De-icing products may contain other salts or additives that you don't want in the water.
Can calcium hardness be too high?
Yes. Above about 500 ppm, scale becomes more likely. See how to lower calcium hardness.
Is calcium hardness the same as total hardness?
No. Total hardness includes magnesium. Pool guidelines use calcium hardness, so use a test that reads calcium specifically.