Guide
How to lower pH in a pool
Lower pool pH with muriatic acid or dry acid: doses per 10,000 gallons and litres at your alkalinity, the add-half-and-retest rule, and how to raise pH that is too low.
Updated · 4 min read
To lower pH, add acid, separately, with the pump running, then retest after 30 minutes. In a 10,000-gallon pool with total alkalinity (TA) at 80 ppm, bringing pH from 7.8 down to 7.5 takes about 8 fl oz of 31.45% muriatic acid. The amount depends on your TA as well as your volume, so for a change of 0.4 or more, add half, retest, then add the rest.
Where pH should be
Keep pH between 7.2 and 7.8. Aim for 7.6 in a pool dosed with liquid chlorine or tablets, and 7.5 in a salt pool or hot tub.
| Reading | What it means |
|---|---|
| Below 7.0 | Act now: hard on eyes, metal and plaster |
| 7.0 to 7.1 | Too low: raise it today |
| 7.2 to 7.8 | In range |
| 7.9 | Drifting high: lower it soon |
| 8.0 to 8.1 | Too high: lower it today, it can cloud the water and form scale |
| 8.2 or higher | Act now: 8.2 is the top of the test's color scale, so the real pH may be higher |
Salt pools push pH up as the cell runs. Add acid when a salt pool reaches 7.8, bringing it back to about 7.5.
Acid doses
Amounts for 10,000 US gallons and for 10,000 litres. Higher TA needs more acid for the same change.
| pH change | TA | 31.45% muriatic acid, per 10,000 gal | per 10,000 L | Dry acid, per 10,000 gal | per 10,000 L |
|---|---|---|---|---|---|
| 7.8 to 7.5 | 80 | 8.0 fl oz | 62 mL | 10.7 oz | 80 g |
| 7.8 to 7.5 | 100 | 9.6 fl oz | 75 mL | 13.0 oz | 97 g |
| 7.6 to 7.2 | 80 | 16.4 fl oz | 128 mL | 22.1 oz | 165 g |
| 8.2 to 7.6 | 80 | 11.3 fl oz | 89 mL | 15.2 oz | 114 g |
Muriatic acid comes in more than one strength. Weaker 14.5% acid takes about 2.3 times the volume of 31.45%: about 18.6 fl oz for the first row. Check the strength on the label before you measure.
How to lower pH, step by step
- Test pH and total alkalinity. The dose depends on both.
- Measure the dose for your pool. Scale the table to your volume; for a change of 0.4 or more, measure half.
- Add the acid separately, with the pump running. Pour it slowly in front of a return jet in the deep end, away from the skimmer. Wear gloves and eye protection.
- Wait 30 minutes, then retest pH. If it is still above your goal, add the rest and retest again.
- Wait another 30 minutes before any chlorine. Acid and chlorine are never added together.
Acid and chlorine release chlorine gas when they meet. Never mix them, or any two pool products, in a bucket or in the water, and store them apart.
Raising pH that is too low
Raise pH with soda ash, or with borax. In a 10,000-gallon pool at TA 80, going from 7.2 to 7.5 takes about 14.8 oz of soda ash (111 g per 10,000 L), which also raises TA by about 10 ppm, or 29.3 oz of borax (220 g per 10,000 L), which raises TA by about 6 ppm. Dissolve it in a bucket of pool water and add it separately, with the pump running; dumping it in dry can cloud the water.
If pH is low but TA is high, add nothing. Point the return jets up at the surface and run any waterfalls or spa jets: aeration raises pH without raising TA.
Why pH keeps coming back up
Total alkalinity holds pH in place. When TA is above its range (60 to 80 ppm for most pools), it keeps pushing pH up, and every acid dose is followed by another. Lowering TA fixes that; the alkalinity guide covers the acid-and-aeration method.
Metric and Australian notes
- The per-10,000-litre columns above work directly for metric pools.
- In Australia, muriatic acid is sold as "pool acid" (hydrochloric acid), often at a different strength from 31.45%. Check the label; weaker acid needs more.
- "Buffer" is sodium bicarbonate, which raises total alkalinity; it moves pH only slightly.
Questions people ask
How long after adding acid can I add chlorine?
Wait at least 30 minutes with the pump running. Never add acid and chlorine together, and never mix them anywhere, including in a bucket.
Why did my pH go back up?
Usually high total alkalinity. Lower TA over a few days with acid and aeration instead of chasing pH with acid every day.
My test reads 8.2. Is it really 8.2?
Treat it as 8.2 or higher. The test's color scale stops at 8.2, so the water may be above it. Add acid, wait 30 minutes, retest, and repeat if it still reads 7.8 or more.
Can I skip the retest?
The dose is an estimate, most accurate for small changes between 7.2 and 7.8 with TA around 80 to 120. Retesting after 30 minutes is what keeps you from overshooting.
Do borates change the acid dose?
Yes, a lot. Borates (from borax or a borate product) resist pH change too. In a 10,000-gallon pool at TA 80, taking pH from 7.8 to 7.5 with 50 ppm borates takes about 22.4 fl oz of 31.45% acid, nearly three times the dose without them.
For your own pool, enter pH, TA and the rest of your readings in the water check; it gives the acid dose in the units you buy.