The quick answer: there is no honest single number such as "1 quart per 10,000 gallons". The acid needed to lower pH depends on:
- how far you are moving pH (7.8→7.5 needs much less than 8.2→7.5);
- your total alkalinity (TA), because TA is the pH buffer, so high-TA water needs a lot more acid;
- borates if you use them, which add more buffering;
- water temperature.
Enter those numbers in the calculator below. It accounts for them and gives the amount of 31.45% or 14.5% acid, or dry acid, for your pool.
Why charts get it wrong
Most "muriatic acid charts" assume one TA, usually around 100 ppm, and ignore everything else.
- If your TA is 60 ppm, the chart overdoses you and pH crashes.
- If your TA is 140 ppm, the chart underdoses you, and you're back at the store wondering why nothing moved.
That buffering is the whole story. TA is mostly bicarbonate. When you add acid, the bicarbonate soaks up most of it before pH moves. More bicarbonate means more acid for the same pH change.
What pH should you aim for?
- Acceptable range: 7.2 to 7.8.
- Sweet spot for most pools: about 7.4 to 7.6. That is comfortable for eyes and gentle on plaster, vinyl and equipment.
- Don't drop below about 7.2 on a dose. Below 7.0, water starts to etch plaster and can wrinkle vinyl liners.
Lowering pH is rarely urgent. pH 7.9 is not dangerous; it just raises the risk of scale and cloudy water over time. Take your time and get it right.
How to add muriatic acid safely
Muriatic acid is hydrochloric acid. Treat it with respect.
- Protect yourself. Wear goggles and gloves, and stand upwind. Open the jug outdoors, never in a closed shed.
- Pump on. Water must be circulating.
- Never mix acid and chlorine, and don't pour them at the same spot. Add one, wait at least 30 minutes, then the other. Acid plus chlorine releases chlorine gas.
- Pour slowly into the deepest part, or in front of a return jet, low and close to the water surface. Some owners pre-dilute it in a bucket of pool water. If you do, always add acid to water, never water to acid.
- Split big doses. If the calculator shows more than one safe addition, add the first part, circulate for 30 minutes, retest, and continue. Pool Calc shows the split automatically.
- Retest pH after 30 minutes before adding more.
Store acid away from chlorine. Acid fumes corrode metal and degrade chlorine nearby, so keep the containers apart and ventilated.
Acid also lowers alkalinity (and that's useful)
Every time you add acid, TA drops too. That is a fixed amount of chemistry:
31.45% muriatic acid lowers TA by 10 ppm for every 25.6 fl oz in 10,000 gallons (about 200 mL per 10,000 litres).
That's why pools with high TA "eat" acid. Their pH keeps climbing back, so you keep adding acid, and TA slowly comes down. If your TA is above your target, lowering TA once is usually the long-term fix for constantly rising pH. Our guide to lowering pool alkalinity explains the acid-plus-aeration method.
Muriatic acid strengths and dry acid
| Product | Strength | Volume compared with 31.45% |
|---|---|---|
| Muriatic acid (full strength) | 31.45% HCl (20° Baumé) | 1× |
| "Safer"/low-fume muriatic acid | ~14.5% HCl | 60.1 fl oz where 31.45% needs 25.6 fl oz |
| Dry acid (sodium bisulfate) | ~93% | 34.4 oz by weight where 31.45% needs 25.6 fl oz |
Dry acid is easier to store and handle. Its trade-off is sulfates, which build up over seasons. That matters mainly for plaster pools with very high sulfate levels, and for salt cell manufacturers who list a sulfate limit. For most pools, either acid is fine. Muriatic acid is usually cheaper per dose.
Always enter the strength from your label into the calculator. Products vary.
Why does my pH keep rising?
If you're adding acid every few days, find the cause. Common ones:
- High TA. Bicarbonate keeps releasing carbon dioxide (CO₂), and pH climbs as it does.
- Aeration: waterfalls, spillover spas, fountains, SWG cells (their hydrogen bubbles) and heavy splashing.
- New plaster, which pushes pH up for months.
- Salt water generators, which tend to raise pH over time.
Lowering TA to the lower part of its range (often around 60–80 ppm, depending on your pool and chlorine source) slows the rise a lot. Adding borates (about 50 ppm) adds buffering that resists pH rise. See pool pH too high for the full checklist.
Common mistakes
- Dumping a whole quart because a chart said so. Measure, calculate, split.
- Testing pH right after a big chlorine shock. At very high FC, phenol red pH tests read falsely high, or turn purple and orange. Wait until FC is back to normal levels, or treat the reading with caution.
- Chasing pH before TA. If TA is high, fix TA first; otherwise you'll fight pH all summer.
- Acid near the skimmer with metal equipment downstream. Pour in the deep end instead.
Let the app do the arithmetic
Pool Calc uses your TA, borates and pool volume to calculate acid for the pH you want. It splits large doses into safe additions and reminds you to wait before adding chlorine. Your log then shows how fast pH drifts, so you can see when TA needs attention.
One pool? Every feature, free. No account, no ads, and iCloud backup is included.
Safety
- Keep chemicals out of reach of children and pets, and keep them out of the water until it is back in range.
- Never mix chemicals. Never add acid and chlorine together: mixing them releases toxic chlorine gas.
- Add chemicals to water, never water to chemicals.
- Store chemicals sealed, cool and dry, apart from each other.
- Read and follow the product label. It always comes first.