Why calcium hardness matters
Calcium hardness (CH) is the amount of dissolved calcium in the water, reported as ppm of calcium carbonate. Water always tries to reach a balance with the surfaces it touches. If it holds too little calcium, it pulls calcium out of plaster, grout and concrete, leaving etching and rough patches, and it is more aggressive to metal parts in heaters. If it holds too much, the extra calcium comes out as white scale on tiles, in salt cells and inside heaters, and can make water cloudy.
Australian Standard AS 3633, as quoted by SPASA, gives a calcium hardness range of 80 to 500 ppm. Concrete and plaster pools are usually kept toward the middle of that range, while vinyl and fibreglass pools, which do not contain calcium, can run lower. Your pool builder or surface manufacturer may give a specific target; use it. Calcium hardness is one of the inputs of the water balance (LSI) calculator, which shows whether your water is likely to etch or scale.
Calcium chloride (g) = ppm increase × litres ÷ 1,000 × 1.109 ÷ purity
Rule of thumb: 1.2 lb of 77% calcium chloride raises 10,000 gal by 10 ppm
Share of water to replace to lower CH = (current − target) ÷ (current − fill water)
Worked examples
- 15,000 gallons, 150 → 250 ppm: 18.0 lb (8.18 kg) of 77% calcium chloride, or 14.8 lb (6.70 kg) of 94% anhydrous.
- 50,000 litres, 100 → 250 ppm: 10.8 kg (23.8 lb) of 77% calcium chloride.
- Lowering 600 → 400 ppm with 100 ppm tap water: replace (600 − 400) ÷ (600 − 100) = 40% of the water. With 50 ppm rainwater-like fill it would be 36%.
Adding calcium chloride safely
- Dissolve it first in a clean plastic bucket of pool water, adding the flakes to the water a little at a time. It gets hot as it dissolves, especially the anhydrous kind; do not hold the bucket against your body.
- Pour the solution slowly around the deep end with the pump running.
- For large increases, add it in two or three portions over a day or two, so the water does not cloud.
- Retest after 24 hours of circulation.
Cal-hypo chlorine also adds calcium (about 0.7 ppm of hardness for every 1 ppm of chlorine), so pools chlorinated with cal-hypo often need little or no calcium chloride and should be tested more often.
Lowering calcium hardness
There is no chemical that safely takes calcium out of a residential pool in practice. Dilution is the standard fix: drain part of the water and refill with softer water. The calculator shows what share to replace. Test your fill water first: if your tap water is already hard, dilution works slowly and you may need to rely on keeping pH and alkalinity a little lower to avoid scale.
Common mistakes
- Testing hardness with total hardness strips. Total hardness includes magnesium; use a calcium hardness test.
- Dumping flakes straight into the pool. They can sit on the floor, heat up and mark the surface.
- Ignoring the other factors. High pH, high alkalinity and warm water all push scale formation; balance them together.
Estimates for general information. Follow the product label and add large doses in stages.
Calcium chloride dose chart
77% calcium chloride needed to raise calcium hardness by 10, 50 and 100 ppm.
| Pool size | +10 ppm | +50 ppm | +100 ppm |
|---|---|---|---|
| 5,000 gal | 9.6 oz (273 g) | 3 lb (1.36 kg) | 6.01 lb (2.73 kg) |
| 10,000 gal | 1.2 lb (545 g) | 6.01 lb (2.73 kg) | 12.02 lb (5.45 kg) |
| 15,000 gal | 1.8 lb (818 g) | 9.01 lb (4.09 kg) | 18.03 lb (8.18 kg) |
| 20,000 gal | 2.4 lb (1.09 kg) | 12.02 lb (5.45 kg) | 24.03 lb (10.9 kg) |
| 25,000 gal | 3 lb (1.36 kg) | 15.02 lb (6.81 kg) | 30.04 lb (13.63 kg) |
| 30,000 gal | 3.61 lb (1.64 kg) | 18.03 lb (8.18 kg) | 36.05 lb (16.35 kg) |
| 10,000 L | 5.1 oz (144 g) | 1.59 lb (720 g) | 3.17 lb (1.44 kg) |
| 25,000 L | 12.7 oz (360 g) | 3.97 lb (1.8 kg) | 7.94 lb (3.6 kg) |
| 50,000 L | 1.59 lb (720 g) | 7.94 lb (3.6 kg) | 15.87 lb (7.2 kg) |
| 75,000 L | 2.38 lb (1.08 kg) | 11.9 lb (5.4 kg) | 23.81 lb (10.8 kg) |
Frequently asked questions
How much calcium chloride raises hardness by 10 ppm?
About 1.2 lb (545 g) of 77% calcium chloride per 10,000 US gallons, or 144 g per 10,000 litres.
What should calcium hardness be in a pool?
Australian Standard AS 3633 gives 80–500 ppm. Plaster and concrete pools usually aim for the middle of the range; vinyl and fibreglass can run lower. Follow your builder's target.
How do I lower calcium hardness?
Replace part of the water with softer water. Share to replace = (current − target) ÷ (current − fill water hardness).
Does cal-hypo raise calcium hardness?
Yes, by about 0.7 ppm for each 1 ppm of chlorine it adds, so it builds up over a season.
How much calcium chloride for a 20,000 gallon pool?
About 2.4 lb (1.09 kg) of 77% calcium chloride per 10 ppm, so about 24 lb (10.9 kg) to raise hardness by 100 ppm.
Sources
- SPASA Australia: Water balance (AS 3633 calcium hardness 80–500 ppm).
- Dose from molar masses: calcium chloride 110.98 g/mol, calcium carbonate 100.09 g/mol.
Last reviewed 28 October 2026. Results are estimates; see our disclaimer.