How to Lower (or Raise) Pool pH Safely

In most pools, pH has one habit: it climbs. Bringing it back down is probably the chemical job you’ll do most often, and the dose depends on more than the pH reading alone.

CDC recommends keeping pool pH between 7.2 and 7.8. Above that, chlorine loses much of its germ-killing strength, badly so above 8.0, and calcium scale forms more easily on tile and in heaters. Below it, water stings eyes, corrodes metal parts and can etch plaster.

Why it keeps rising

Pool water usually holds more dissolved carbon dioxide than it would in balance with the air. As that CO₂ escapes, pH goes up. Anything that churns the surface speeds this along: waterfalls, spa spillovers, return jets aimed upward, and the fine bubbles from a salt chlorine generator. High total alkalinity keeps a larger supply of CO₂ ready to escape. New plaster pushes pH up for months while it cures.

If you’re adding acid every few days, the lasting fix is usually lowering alkalinity, not adding acid more often. The alkalinity calculator lays out the acid-and-aeration method.

How much acid?

Alkalinity buffers pH changes, so the same pH drop takes more acid in a high-alkalinity pool. Here’s how much 31.45% muriatic acid it takes to move a 10,000-gallon pool from pH 7.8 to 7.4, at three alkalinity levels (CYA 30 ppm):

These figures come from the carbonate-chemistry model in our pH calculator. They’re estimates: real water also contains things the model leaves out, such as borates. A printed chart that ignores alkalinity can be off by close to a factor of two across normal pools.

For a fuller example, take an 18,000-gallon pool at pH 7.9, with alkalinity at 110 and CYA at 40. Bringing it to 7.5 takes about 24 fl oz of 31.45% muriatic acid. The same job takes about 57 fl oz of a 14.5% “half-strength” acid, or about 33 oz (by weight) of dry acid. Alkalinity also drops a little, from 110 to about 105.

pH calculatorGet an acid or soda ash dose from your pH, alkalinity and pool size.

Adding acid

You have two choices. Muriatic acid (hydrochloric acid) is usually sold at 31.45%, or at about 14.5% in lower-fume versions. Dry acid is sodium bisulfate granules. Both lower pH and alkalinity. Dry acid is easier to store and handle, but it adds sulfate to the water over time.

1. Run the pump.
2. Measure the acid outdoors into a plastic container, never metal.
3. Pour it slowly into the deep end, away from the skimmer, with the container held close to the water surface. Dissolve dry acid in a bucket of pool water first, adding the acid to the water.
4. Wait at least 30 minutes, ideally a few hours, then retest before adding more.

Add about three-quarters of the calculated amount first. Home test kits read pH in steps of about 0.2, so it’s easy to overshoot. Some kits also offer an “acid demand” test, which measures your own water’s response directly.

Raising pH

Low pH is less common, and there are three ways to raise it, each with a trade-off.

• Aeration costs nothing and adds nothing. Aim return jets up at the surface, run water features, or add an air bubbler. pH rises over a day or two and alkalinity doesn’t change.
• Soda ash (sodium carbonate, sold as “pH up”) works fast, but it raises alkalinity too. In our model, taking a 15,000-gallon pool from pH 7.0 to 7.4 with alkalinity at 80 takes about 41 oz (2.6 lb) of soda ash, and alkalinity ends up near 99. Dissolve it in a bucket of pool water and add it in portions, because large doses can cloud the water briefly.
• Borax raises pH with less effect on alkalinity, but it adds borates, which change how the water buffers from then on.

If both pH and alkalinity are low, raise alkalinity first with baking soda. pH often comes up partway on its own.

Once pH is steady in range, chlorine works the way it should, which matters most when you’re shocking the pool.