Calcium hardness calculator
Updated:
Calcium hardness protects plaster and equipment. Enter your current and target calcium hardness and pool volume to get the exact amount of calcium chloride to add.
How much calcium chloride to add?
About 1.2 lb of 77% calcium chloride per 10,000 gallons raises calcium hardness by about 10 ppm, or about 16 oz of the 94% anhydrous kind. The calculator scales it to your volume and product. Dissolve it in a bucket of water first, since it gets hot as it dissolves, then pour it around the pool with the pump running.
Why calcium hardness matters
The MAHC Annex puts it plainly: too much calcium causes cloudiness and scale, and too little, especially with low pH or low alkalinity, makes the water aggressive enough to dissolve plaster and metal and stain the walls and floor. Plaster pools need more calcium than vinyl or fiberglass.
Calcium too high?
No chemical removes calcium. Lower it with a partial drain and refill, if your fill water is softer. When that is not practical, the MAHC Annex advises lowering pH or alkalinity to bring the saturation index (LSI) back into range, since those are the levers you can move.
How the dose is calculated
Calcium hardness is reported as calcium carbonate, and each product gets its own constant.
Calcium chloride needed
grams = 0.0055663 × (target − current) × gallons (77%) · 0.004470 (94%)
The constants come from each product’s molar mass and strength. For 10 ppm in 10,000 gallons that is 557 g of 77% product (1 lb 4 oz) or 447 g of 94% (16 oz).
Checking balance (LSI)
LSI = pH + TF + CF + AF − TDSF
The MAHC Annex formula. The temperature, calcium and alkalinity factors come from its Table 5.7.4.6, and the dissolved-solids factor is 12.1 below 1,000 ppm TDS or 12.2 above. A result from −0.3 to +0.3 is balanced.
Ideal ranges
| Level | Target | Notes |
|---|---|---|
| Most pools | 200–400 ppm | MAHC Annex 5.7.4.4.3 |
| Plaster, gunite or tile | 200–400 ppm | MAHC Annex 5.7.4.6 |
| Vinyl or fiberglass | 80–200 ppm | MAHC Annex 5.7.4.6 |
| Water above 90°F (hot tubs) | 100–200 ppm | MAHC Annex 5.7.4.4.3 |
| Upper limit, public pools | 2,500 ppm | MAHC 5.7.4.4.3 |
Frequently asked questions
Does cal-hypo add calcium?
Yes. Calcium hypochlorite shock adds calcium over time. If your calcium is already high, prefer liquid chlorine.
What’s LSI?
The Langelier Saturation Index combines pH, temperature, calcium hardness, alkalinity and dissolved solids into one number that shows whether water will deposit scale or dissolve surfaces. The MAHC Annex treats −0.3 to +0.3 as balanced, and prefers a small positive value to a negative one, because a thin scale layer does less harm than corrosion. The DoseMyPool app shows it for you.
Sources
- CDC, Model Aquatic Health Code, 5th edition (2024), Code Language. Section 5.7.4.4.3: 2,500 ppm maximum
- CDC, Model Aquatic Health Code, 5th edition (2024), Annex. Sections 5.7.4.4.3 (calcium hardness) and 5.7.4.6 (LSI formula, table and range)
Related calculators & guides
Get exact doses, saved and on schedule
DoseMyPool remembers your pool, gives the safe order to add things, and reminds you to test. Free on the App Store.