How it works
From reading to amount
Test readings are in ppm, which is milligrams per liter. Alkalinity and calcium hardness are reported "as calcium carbonate", so each product's amount follows from its molar mass:
- Alkalinity up: baking soda (sodium bicarbonate). 84 g raises alkalinity by 50 ppm in 1,000 liters.
- Alkalinity down: muriatic acid, then aeration. The acid lowers both alkalinity and pH; aerating (jets pointed up, a fountain) brings pH back up without bringing alkalinity back.
- pH up: soda ash (sodium carbonate). pH down: muriatic acid (31.45%, the usual strength).
- Calcium up: calcium chloride, about 95% pure. Stabilizer up: cyanuric acid.
- Calcium or stabilizer down: no chemical does it. Replace part of the water: to go from 100 to 50 ppm, replace half.
Why pH needs your alkalinity
pH is buffered: alkalinity (carbonate) and stabilizer (cyanurate) soak up acid before pH moves. The calculator models both, so higher alkalinity or CYA means more acid for the same pH drop. For a pH drop from 7.8 to 7.5 in 10,000 gal with 30 ppm CYA:
| Alkalinity | 60 ppm | 80 ppm | 100 ppm | 120 ppm |
|---|---|---|---|---|
| Muriatic acid | 6.6 fl oz | 8.2 fl oz | 9.8 fl oz | 11.4 fl oz |
The pH figure is an estimate (±20%): borates, water temperature and test-kit error all shift it. That's why the result says to add 75% first and retest.
Worked example
18,000 gallon pool, alkalinity 60 to 90 ppm
- Raise: 90 − 60 = 30 ppm
- Baking soda: 1.4 lb per 10,000 gal per 10 ppm
- 18,000 ÷ 10,000 × 3 × 1.4 lb = 7.6 lb
- At typical prices: $5.32–$26.60
Quick reference: per 10,000 gallons
| To | Add | Amount |
|---|---|---|
| Raise alkalinity 10 ppm | Baking soda | 1.4 lb |
| Lower alkalinity 10 ppm | Muriatic acid 31.45% | 25.6 fl oz |
| Raise calcium hardness 10 ppm | Calcium chloride | 15.6 oz |
| Raise stabilizer 10 ppm | Cyanuric acid | 13.4 oz |
| Lower pH 7.8 → 7.6 (TA 80, CYA 30) | Muriatic acid 31.45% | 5 fl oz |
| Raise pH 7.2 → 7.4 (TA 80, CYA 30) | Soda ash | 11.1 oz |
Scale by your gallons: a 20,000 gal pool takes twice as much. Usual ranges: pH 7.2–7.8 and stabilizer at most 90 ppm (CDC Model Aquatic Health Code); alkalinity 60–120 ppm for most pools; calcium 200–400 ppm plaster, 150–250 ppm vinyl or fiberglass.
Mistakes to avoid
- Fixing pH before alkalinity. Get alkalinity into range first; pH is easier to hold after.
- Adding everything at once. One chemical at a time, pump running, and retest before the next.
- Pouring water into acid. Always add acid to water. Never mix acid with chlorine: it releases chlorine gas.
- Too much stabilizer. Only dilution brings it down. Add in steps and retest after a week.
- Calcium chloride straight into a vinyl pool. It heats up as it dissolves; add it the way the label says.
Questions people ask
How much muriatic acid to lower pH?
It depends on your alkalinity and stabilizer. At alkalinity 80 and CYA 30, lowering pH from 7.8 to 7.5 in 10,000 gal takes about 8.2 fl oz of 31.45% acid. Pick pH above to get yours.
How much baking soda to raise alkalinity?
1.4 lb raises 10,000 gallons by 10 ppm. Grocery-store baking soda is the same chemical as pool alkalinity increaser and usually cheaper.
How much stabilizer do I add?
13.4 oz raises 10,000 gallons by 10 ppm. Most pools aim for 30–50 ppm; saltwater pools often 60–80. The CDC sets 90 ppm as the maximum.
How do I lower stabilizer or calcium?
Drain and refill. To halve the level, replace half the water. Test your fill water first; hard tap water adds calcium back.
How long to wait between chemicals?
Let each one circulate before the next: about 30 minutes for acid or soda ash, 6 hours for baking soda, a day for calcium. Stabilizer takes up to a week to show on a test.