Alkalinity, Calcium Hardness and Stabilizer Calculator

What these three numbers do

• Total alkalinity (TA) is the water's resistance to pH change, measured as ppm of calcium carbonate. Low TA lets pH swing; high TA makes pH drift upward and hard to bring down.
• Calcium hardness (CH) is the dissolved calcium. Water that is very low in calcium can etch plaster and grout over time; very high calcium can scale surfaces and heaters. Vinyl and fiberglass pools are less sensitive than plaster.
• Cyanuric acid (CYA, stabilizer) shields chlorine from sunlight in outdoor pools, but it also holds chlorine in reserve, so higher CYA means you need higher free chlorine. See free chlorine and CYA.

Targets vary with pool surface, chlorine source and climate, and product labels and pool professionals give different ranges. Use the range your pool builder, surface manufacturer or equipment manual recommends, and make changes in modest steps.

How the calculator works

All four calculations start from the same idea: ppm means milligrams per litre, and your pool holds gallons × 3.785 litres.

• Alkalinity is reported "as calcium carbonate", so the calculator converts your ppm change into chemical equivalents of calcium carbonate (50.04 mg per milliequivalent), then into the matching amount of product. Baking soda supplies one equivalent per 84.0 g. Hydrochloric acid removes one equivalent per 36.46 g of HCl.
• Calcium hardness is also reported as calcium carbonate (100.09 g per mole). Each mole of calcium chloride (110.98 g) adds one mole of calcium. The calculator divides by the product's purity: 77% for common dihydrate flake, 94% for anhydrous pellets.
• Cyanuric acid is measured as itself, so 10 ppm in 10,000 gallons is simply 378.5 g of pure stabilizer.
• Muriatic acid volume uses its density: about 1.16 g/mL for 31.45% acid and about 1.07 g/mL for 14.5% acid.

Quick reference per 10,000 US gallons

Worked example

Lowering alkalinity takes patience

Acid lowers both TA and pH. The usual approach is to add acid to bring pH down toward 7.0–7.2, then aerate the water (point returns at the surface, run waterfalls or fountains) so carbon dioxide escapes and pH rises again while TA stays lower. Repeat over several days. Never let pH fall below 7.0, the bottom of CDC's recommended range, and never pour acid near a skimmer that has chlorine tablets in it.

Why your result might differ

• Test error. Drop tests for TA and CH count drops, and each drop is typically 10 ppm (or 25 ppm on some kits). A one-drop miscount moves the answer by that much.
• Product strength. Check the label. Some calcium chloride is sold as 83–87%; some stabilizer is sold as a liquid with much less cyanuric acid per ounce.
• Fill water. If your tap water is hard or has high alkalinity, topping up raises those numbers over the season.

Common questions

Does baking soda raise pH?

A little. Baking soda mainly raises total alkalinity and nudges pH up only slightly. Soda ash (sodium carbonate) raises pH much more and is the usual choice when pH, not alkalinity, is the problem.

How do I lower cyanuric acid?

There is no practical chemical to remove it at home. Replace part of the water: draining and refilling 30% of the pool lowers CYA by about 30%, assuming the fill water has none.