Pool alkalinity calculator
Total alkalinity (TA) is the water's buffer: it keeps pH from swinging. Enter your volume and readings to see how much baking soda raises it, or how much acid lowers it.
What level to aim for
A common target range is 60–120 ppm, and many pool owners aim for 80–100 ppm. Lower alkalinity in that range (around 60–80 ppm) suits saltwater pools and pools that are mainly chlorinated with liquid chlorine, whose pH tends to rise. Pools using trichlor tablets, which are acidic, often do better nearer 100–120 ppm. If alkalinity is too low, pH swings easily and can drop low enough to etch plaster. If it's too high, pH keeps drifting up and scale forms more easily.
The formulas
Acid (equivalents) = ppm decrease × pool liters ÷ 1,000 ÷ 50.0
Alkalinity is reported "as calcium carbonate", where 50 mg corresponds to one milli-equivalent. Baking soda (sodium bicarbonate) weighs 84 g per equivalent, so raising alkalinity by 1 ppm takes 84 ÷ 50 = 1.68 mg of baking soda per liter. Each equivalent of acid removes one equivalent of alkalinity. For 31.45% muriatic acid, one equivalent is about 100 mL.
| Change in 10,000 gallons | Amount |
|---|---|
| Raise TA by 10 ppm | 1.4 lb of baking soda |
| Lower TA by 10 ppm | 25.6 fl oz of 31.45% muriatic acid, or about 2.1 lb of dry acid |
Worked examples
Raising: 15,000 gallons from 70 to 90 ppm
20 ppm × 56,781 liters ÷ 1,000 × 1.68 ≈ 1,906 g, or about 4.2 lb of baking soda.
Lowering: 15,000 gallons from 120 to 90 ppm
30 ppm needs about 115 fl oz (3.4 L) of 31.45% muriatic acid. That much acid at once would push pH far too low, so it's done in steps over several days, as described below.
How to lower alkalinity
Acid lowers both pH and alkalinity. Aeration raises pH but leaves alkalinity alone. Alternating the two lowers alkalinity while pH ends up back where it started:
- Add acid to bring pH down to about 7.0–7.2. Don't go below 7.0.
- Aerate: point returns upward, run any waterfalls or fountains, and leave the cover off. pH climbs back over a day or two as CO₂ escapes.
- Repeat until alkalinity reaches your target. Each round lowers it by roughly the amount the pH calculator shows.
How to raise alkalinity
Broadcast baking soda over the water surface with the pump running, preferably in the deep end. Add no more than about half the dose at once, wait a few hours, and retest. Water can look cloudy for a short time after a large dose. Baking soda raises pH only slightly. If both pH and alkalinity are low, soda ash raises both, with more effect on pH.
Frequently asked questions
Should I fix alkalinity or pH first?
Usually alkalinity, because it controls how stable pH is. The exception is a pH far out of range (below 7.0 or above 8.0): bring that back first, then work on alkalinity.
Is baking soda from the grocery store the same as pool alkalinity increaser?
Both are sodium bicarbonate. Check the label for 100% sodium bicarbonate with nothing added. Pool-branded products often cost more per pound for the same chemical.
Why does my alkalinity keep dropping?
Acid lowers it, so pools that need frequent acid for rising pH lose alkalinity over time. Trichlor tablets are also acidic and wear it down. Rain can lower it too.