Pool pH adjuster: how much acid or soda ash for your pool
The math for adjusting pool pH depends on three things: pool volume, current pH, and total alkalinity. Generic dose tables ignore TA, which is why most pH adjustments take 2–3 tries to get right.
Last updated: 2 June 2026
Generic pH adjustment tables give you a dose based on pool size alone. The Australian pool industry standard is to recommend “250 mL of muriatic acid per 50,000 L pool to lower pH by 0.2”. That’s roughly correct for a pool with TA of 80–100 ppm. It’s significantly off for a pool with TA at 60 ppm (the dose is too high, pH will drop further than intended) or TA at 160 ppm (the dose is too low, pH will barely move).
This guide covers the actual math for pool pH adjustment, accounting for total alkalinity, plus a dose reference table you can apply to your pool’s specifics.
Why total alkalinity changes the dose
Total alkalinity (TA) is the chemical buffer that prevents rapid pH changes. The higher the TA, the more chemical you need to move pH a given amount, because the alkalinity absorbs the change.
At low TA (60 ppm), the buffer is weak, small doses produce large pH swings. At high TA (160 ppm), the buffer is strong, even large doses produce small pH changes.
Most generic dose tables assume TA = 80–100 ppm. For pools outside that range, the dose is wrong.
The pH-down dose table (muriatic acid 32%)
| Pool size (L) | TA 60 ppm | TA 100 ppm | TA 140 ppm |
|---|---|---|---|
| Dose to drop pH by 0.2 (mL of 32% muriatic acid) | |||
| 20,000 | 80 mL | 110 mL | 150 mL |
| 30,000 | 120 mL | 165 mL | 225 mL |
| 50,000 | 200 mL | 275 mL | 375 mL |
| 70,000 | 280 mL | 385 mL | 525 mL |
| 100,000 | 400 mL | 550 mL | 750 mL |
For other TA values, interpolate. For other pH-drop amounts, scale linearly (0.4 drop = double the table value, 0.1 drop = half the table value).
The pH-up dose table (soda ash / sodium carbonate)
| Pool size (L) | TA 60 ppm | TA 100 ppm | TA 140 ppm |
|---|---|---|---|
| Dose to raise pH by 0.2 (grams of sodium carbonate) | |||
| 20,000 | 110 g | 140 g | 175 g |
| 30,000 | 165 g | 210 g | 265 g |
| 50,000 | 275 g | 350 g | 440 g |
| 70,000 | 385 g | 490 g | 615 g |
| 100,000 | 550 g | 700 g | 880 g |
Application procedure
For muriatic acid
- Calculate dose from the table above based on your pool’s volume and current TA.
- Pre-dilute the acid, pour into a 10L bucket half-filled with pool water. Never pour neat acid directly into the pool.
- Add the diluted solution slowly into the deep end of the pool with the pump running.
- Wait 4 hours.
- Re-test pH.
- If pH is still too high, repeat at half the original dose.
For soda ash
- Calculate dose from the table.
- Pre-dissolve in a bucket of warm pool water, soda ash takes 10 minutes to dissolve.
- Pour the dissolved solution through the skimmer with the pump running.
- Wait 4 hours.
- Re-test pH.
- If pH is still too low, repeat at half dose.
The half-dose principle
Always add less than calculated as a first dose. Two reasons:
- Pool chemistry isn’t perfectly predictable. The table is a starting point; your pool’s actual response may vary by ±20%.
- It’s much easier to add more acid than to fix over-acidified water. Same for soda ash, easier to add more than to fix over-alkalised.
Conservative approach: dose at 70% of calculated, wait 4 hours, re-test, add the remaining 30% if needed.
Common pH adjustment mistakes
- Adding acid to neat water. Always pre-dilute in pool water. Concentrated acid burning into pool plaster causes permanent damage.
- Pouring soda ash dry into the pool. The undissolved crystals settle on the pool floor and can etch plaster locally. Always pre-dissolve.
- Adjusting pH before adjusting TA. Always fix TA first if it’s out of range. Trying to manage pH with TA at 200 ppm or 40 ppm is a losing battle.
- Re-dosing within 2 hours. Chemistry needs time to circulate and reach equilibrium. Re-testing within 2 hours gives misleading readings.
- Ignoring water temperature. Cold water (below 18°C) takes longer to mix and respond. Allow extra time in winter.
Sources & references
- Trouble Free Pool, ABCs of Pool Water Chemistry
- ANSI/APSP-11, Water Quality Standards in Public Pools
- AS 3633-1989, Private swimming pools (Australian Standard)
Frequently asked questions
How much muriatic acid to lower pool pH by 0.2?
Depends on pool volume and total alkalinity. For a 50,000 L pool: ~200 mL at TA 60 ppm, ~275 mL at TA 100 ppm, ~375 mL at TA 140 ppm. Always pre-dilute in pool water before adding.
How much soda ash to raise pool pH by 0.2?
Depends on pool volume and total alkalinity. For a 50,000 L pool: ~275 g at TA 60 ppm, ~350 g at TA 100 ppm, ~440 g at TA 140 ppm. Always pre-dissolve in a bucket of warm pool water before adding.
Can I use baking soda to lower pool pH?
No. Baking soda (sodium bicarbonate) raises both pH and total alkalinity. To lower pH, use muriatic acid (hydrochloric acid 32%) or dry acid (sodium bisulfate).
How long after adding pH adjuster should I retest?
Wait at least 4 hours with the pump running. Cold water (below 18°C) may need 6–8 hours for full circulation and equilibration. Testing too early gives misleading readings.
Why won’t my pool pH stay down?
Almost always because total alkalinity is too high (above 140 ppm). High TA locks pH high, even large acid doses get absorbed by the buffer without moving pH. Lower TA first using the aeration method, then pH becomes manageable.
Does the Water Test Calculator app calculate pH doses?
Yes. Enter your current pH, target pH, pool volume, and total alkalinity, the app calculates the exact dose of muriatic acid or soda ash needed. No tables, no interpolation. Launching on iOS and Android in approximately 10 days.
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