How to lower pH in a pool
Add acid: muriatic acid or dry acid (sodium bisulfate). How much depends on your alkalinity and stabilizer as much as on the pH reading, so the calculator uses all three.
Add this much acid
Other acids for the same change
| Muriatic acid 31.45% | – |
| Muriatic acid 14.5% ("half-strength") | – |
| Dry acid (sodium bisulfate 93%) | – |
How to add it
- Goggles and gloves on. Pump running.
- Pour slowly and low into the deep end, in front of a return jet. Never into the skimmer.
- Brush the spot if you have plaster. Retest pH after 30+ minutes.
Chemical safety: read the product label first and follow it where it differs from this page. Add acid to water, never water to acid. Never mix chlorine products with acid or with each other, not even in a bucket. Add one chemical at a time with the pump running, wait at least 30 minutes (acid and chlorine: until the next test), and retest before topping up.
What high pH does to your chlorine
Free chlorine in water is split between two forms: hypochlorous acid (HOCl), the fast germ-killer, and hypochlorite ion (OCl⁻), which is far weaker. pH decides the split. Slide the pH and watch the fast-acting share change.
This is why CDC warns that chlorine’s ability to kill germs drops as pH rises, especially above 8.0, and why pool pros keep pH in the mid 7s rather than chasing 7.0: lower pH is harder on plaster, metal and eyes.
Shares from the HOCl dissociation constant (pKa ≈ 7.5 at 25 °C). In stabilized pools most chlorine is also bound to CYA, which shrinks the working share further.
Why pH keeps rising and how to slow it
Pool water holds more dissolved carbon dioxide than air wants it to. As CO₂ escapes, pH rises. Anything that splashes or bubbles speeds that up: waterfalls, spillovers, jets aimed up, swimmers, salt cells making hydrogen bubbles, and fresh plaster curing.
Total alkalinity sets how much CO₂ there is to lose. Many owners find that letting TA settle to 60–80 ppm (with the rest of the water balanced, see the LSI calculator) cuts acid use sharply. Stabilizer also counts toward TA, which is why the calculator subtracts part of it.
pH quick reference
| pH | What it means |
|---|---|
| below 7.0 | Corrosive; etches plaster, pits metal. Outside CDC range. Raise it. |
| 7.0–7.3 | Chlorine very effective; OK short-term, watch low-calcium water. |
| 7.4–7.6 | The usual target zone. |
| 7.7–7.8 | Upper edge of CDC range; lower it soon. |
| above 7.8 | Chlorine weakening, scale and cloudiness likely. Add acid. |
Questions pool owners ask
How much muriatic acid do I add to lower pH?
It depends on pool size, how far you want pH to move, and your total alkalinity. As a guide, lowering pH from 7.8 to 7.4 in 10,000 gallons with 100 ppm alkalinity and no stabilizer takes about 12 fl oz of 31.45% muriatic acid. Higher alkalinity or stabilizer means more acid; this calculator models both.
What pH should a pool be?
CDC recommends keeping pool and hot tub pH between 7.0 and 7.8. Most pool owners aim for 7.4–7.6: comfortable for eyes, gentle on equipment, and chlorine still works well. Above 8.0 chlorine loses much of its strength and calcium scale forms more easily.
Why does my pool pH keep going up?
The usual cause is carbon dioxide gassing out of the water, which raises pH. It happens faster with high total alkalinity, waterfalls, spa spillovers, bubblers and salt cells. Lowering alkalinity towards 60–80 ppm slows the rise a lot.
How do I add muriatic acid to a pool safely?
Wear goggles and gloves, work upwind and never add water to acid. With the pump running, pour the acid slowly into the deep end in front of a return jet, low to the water surface. Don’t add it through the skimmer. Retest after at least 30 minutes of circulation.
Is dry acid better than muriatic acid?
Dry acid (sodium bisulfate) is easier and safer to store and handle, but costs more per dose and slowly builds sulfates, which can harm some plaster and salt-cell components over time. Muriatic acid is cheaper and leaves nothing behind except a little salt.