How to lower pH in a pool
To lower pH from 7.9 to 7.5 in a 15,000-gallon pool (alkalinity 100 ppm, stabilizer 40 ppm), add about 17 fl oz of 31.45% muriatic acid.
Always follow your product’s label directions. Pool and spa chlorine, bromine, shock and algaecides are EPA-registered pesticides: use products labeled for pool or spa use, and don’t swim or soak until levels are within the limit on your product’s label. If this page and a label disagree, the label wins.
On this page (8 sections)
Dry acid works too: about 1.45 lb for the same change.
Calculator: muriatic or dry acid dose
The exact dose depends on your alkalinity and stabilizer, so a fixed "ounces per 0.1 pH" rule is often wrong. Enter pH now, the pH you want (7.4–7.6 is a good target; never below 7.2), total alkalinity (TA), stabilizer (CYA), borates if you added them, and your pool volume. Choose the acid you have. The result is rounded down and split into steps of no more than 0.4 pH, with the total if you need more than one step.
Before you trust a high reading, check free chlorine (FC). Above 10 ppm FC, the pH test's indicator can be bleached and read wrong, so wait until chlorine drops. And if your kit shows its top color (usually 8.2), your real pH may be higher: dose the first step and retest instead of guessing the total. For more on the product itself (strengths, pouring, 14.5% acid), see the muriatic acid calculator. The target range and what pH does are on pool pH levels.
Same pH drop, different acids
| Total alkalinity | Muriatic 31.45% | Muriatic 14.5% | Dry acid 93.2% | TA removed |
|---|---|---|---|---|
| 80 ppm | 9.8 fl oz | 23 fl oz | 13.2 oz | 3.9 ppm |
| 100 ppm | 11 fl oz | 27 fl oz | 15.7 oz | 4.6 ppm |
| 120 ppm | 13 fl oz | 31 fl oz | 1.1 lb | 5.3 ppm |
Rounded down. Dry acid amounts are by weight (oz and lb), liquid acid by volume (fl oz).
Why pH rises (aeration, high TA, fresh plaster, SWG)
Pool water holds more dissolved carbon dioxide than the air above it. Whenever that CO2 escapes, pH goes up, even though alkalinity stays the same. Most rising-pH problems are some version of this:
- Aeration. Waterfalls, fountains, spillovers, jets aimed at the surface, and heavy swimming all speed up CO2 loss.
- High total alkalinity. More carbonate in the water means more CO2 waiting to escape. PHTA notes that lower TA slows the rise of pH, which is why many salt and liquid-chlorine pools run TA at 60–90 ppm.
- Fresh plaster. New plaster, pebble, and other cement finishes release alkaline compounds as they cure, pushing pH up for the first weeks to months.
- Salt water generators (SWG). The cell produces fine hydrogen bubbles that carry CO2 out of the water, so salt pools usually need acid every week or two.
- Chlorine choice. Liquid chlorine and cal-hypo (calcium hypochlorite) raise pH a little when you add them. Trichlor tablets do the opposite, which is why tablet pools more often have low pH.
Hot tubs follow the same chemistry with more aeration. If that is your problem, see hot tub pH too high.
Step by step
- Test pH, TA, and CYA with a fresh sample, and make sure free chlorine is below 10 ppm.
- Enter your numbers in the calculator and measure the first step, never more than 0.4 pH at a time.
- Turn off waterfalls and other aeration features, and keep the pump running.
- Wear gloves and eye protection. Pour the acid slowly, low over the water, in front of a return jet in the deep end. For vinyl and above-ground pools, pre-dilute it in a bucket of pool water (acid into water).
- Wait about 30 minutes with the pump running, then retest pH.
- If pH is still above your target, add the next step and retest again.
- Check pH again the next day. If it climbs back quickly, lower alkalinity or reduce aeration.
Went too far and pH is now below 7.2? Don't add more acid to "balance" anything: raise it with aeration or soda ash, as in how to raise pH.
Safety
Never mix acid and chlorine, and never use the same scoop or bucket for both: together they release chlorine gas. Add acid to water, never water to acid. Keep swimmers out until the acid has circulated and you have retested. Store acid away from chlorine. Poison Control: 1-800-222-1222. Always follow the product label.
Lower pH without lowering TA too much
People worry that acid will "crash" alkalinity. A normal pH correction barely changes TA: in the table above, going from 7.9 to 7.5 removes only 3.9–5.3 ppm of TA. The alkalinity drop becomes large only when you add much more acid than pH alone would need.
The opposite problem is more common: TA is high and pH keeps rising every few days. Lowering pH over and over only treats the symptom. The fix is to lower TA on purpose with repeated acid doses and aeration in between, which brings TA down while pH comes back up. That process, with doses per round, is in how to lower alkalinity in a pool. Once TA is lower, pH rises more slowly and you add less acid overall.
Dry acid vs muriatic acid
- Muriatic acid (hydrochloric, 31.45% or 14.5%) is the cheapest per unit of acid and leaves only chloride, the same thing that's in salt. Downsides: strong fumes, corrosion of nearby metal, and heavy jugs that are awkward to pour.
- Dry acid (sodium bisulfate, 93.2%) is a granular product that is easier to store, measure, and handle, with no fumes. It leaves sulfate in the water. Sulfate leaves only when water does, so years of dry acid raise total dissolved solids (TDS). It is popular for hot tubs, where water is changed often anyway.
For the same job they are interchangeable: the calculator converts between them. Pick muriatic for a large outdoor pool on a budget. Pick dry acid if handling a fuming liquid is a problem for you.
Natural methods: what works and what doesn't
- Reduce aeration: works. Turn off water features and point returns downward. It doesn't lower pH, but it slows the rise so each acid dose lasts longer.
- Lower alkalinity: works. Covered above. It's the long-term fix for a pool that always drifts up.
- Borates: help slow the rise. At 30–50 ppm they buffer against rising pH (PHTA). Adding them raises pH at first, so you'll need acid afterward. See borax in a pool.
- Vinegar: not practical. Household vinegar at 5% acidity carries about 3.2 mol of acid per gallon, compared with about 37.7 mol in a gallon of 31.45% muriatic acid. So you would need about 12 gallons of vinegar to do what one gallon of muriatic acid does, and you would add organic material that uses up chlorine.
- Rain and fresh water: unreliable. Rain often lowers pH a little, but it also dilutes everything else. Topping up with tap water only helps if your tap water has lower pH and alkalinity.
FAQ
How to bring pH down in pool quickly?
Use muriatic acid, poured in front of a return jet with the pump running and water features off. Dose up to 0.4 pH at a time, retest after about 30 minutes, and repeat. There is no faster safe method.
What causes pool pH to rise?
Mainly carbon dioxide escaping from the water, driven by aeration, high total alkalinity, salt cell bubbles, and new plaster. Liquid chlorine and cal-hypo also raise pH slightly when added. Lowering alkalinity and reducing aeration slow the rise.
Sources
- CDC, Model Aquatic Health Code 2024, Annex (pH ranges and chlorine effectiveness): cdc.gov (PDF)
- PHTA technical bulletin, Boric acid and sodium borates: phta.org
- Taylor Technologies, K-1005 instruction manual: PDF
- CDC, pool chemical injuries: archive.cdc.gov