Quick answer: 1.4 lb of baking soda per 10,000 gallons raises total alkalinity by 10 ppm (16.8 g per 1000 L). To lower it by 10 ppm, use 25.6 fl oz of 31.45% muriatic acid per 10,000 gallons — that is four-fifths of a quart, not a full one. Target is 80–120 ppm, measured after subtracting a third of your cyanuric acid reading.
What alkalinity actually does
Total alkalinity is not "how alkaline the water is" — that is pH. Alkalinity is the water's capacity to absorb acid without the pH moving. It is a shock absorber, and pH is the ride.
That relationship explains almost every alkalinity problem people actually have:
| Carbonate alkalinity | Behaviour | What you notice |
|---|---|---|
| Below 60 ppm | Almost no buffering | pH bounce — wild swings from small additions; etching and corrosion |
| 60–80 ppm | Light buffering | Workable for salt pools; pH still needs watching |
| 80–120 ppm | Correct range | pH holds steady for days at a time |
| 120–150 ppm | Heavy buffering | pH creeps upward; acid demand rises |
| Above 150 ppm | Over-buffered | Persistent high pH, cloudy water, scale on tile and cell plates |
If your pH will not stay put, fix alkalinity first. Chasing pH while alkalinity is out of range is the single most common mistake in pool chemistry — you are adjusting the ride while ignoring the suspension. Get alkalinity into range and pH usually settles on its own.
The correction almost every calculator misses
If your pool is outdoors and stabilised, your alkalinity test is reading high — and not by a trivial amount.
An alkalinity test is an acid titration: it measures everything in the water that neutralises acid. Cyanuric acid — the stabiliser protecting your chlorine from sunlight — forms cyanurate ions that do exactly that. So they get counted as alkalinity even though they contribute nothing to carbonate buffering.
The correction used across the industry is:
carbonate alkalinity = total alkalinity − (⅓ × cyanuric acid)
| TA reading | CYA | Cyanurate share | True carbonate alkalinity | Verdict |
|---|---|---|---|---|
| 110 ppm | 0 ppm | 0 | 110 ppm | In range |
| 110 ppm | 30 ppm | 9.9 | 100.1 ppm | In range |
| 110 ppm | 60 ppm | 19.8 | 90.2 ppm | In range — do nothing |
| 110 ppm | 90 ppm | 29.7 | 80.3 ppm | Bottom of range |
| 100 ppm | 100 ppm | 33.0 | 67.0 ppm | Actually low — raise it |
Look at the last two rows. A pool reading a comfortable 100–110 ppm can have genuinely low carbonate alkalinity once stabiliser is accounted for. Dose from the raw number and you will add acid to a pool that needed bicarbonate — then wonder why the pH keeps crashing.
The doses, and where they come from
These figures are not copied from a chart. They follow from equivalent weights, which is why they can be trusted at any volume and any dose size.
Alkalinity is reported as ppm of calcium carbonate. CaCO₃ has a molar mass of 100.09 g and supplies two equivalents, so one equivalent of alkalinity = 50.04 g as CaCO₃. Any chemical's dose is then just its own equivalent weight over that number:
| Chemical | Equivalent weight | Ratio to CaCO₃ | Dose | Effect |
|---|---|---|---|---|
| Sodium bicarbonate (baking soda) | 84.01 | 1.6787 | 1.401 lb (635.4 g) | TA up, pH barely moves |
| Soda ash (sodium carbonate) | 53.00 | 1.0590 | 0.884 lb (400.9 g) | TA up, pH up sharply |
| Muriatic acid 31.45% (20°Bé) | 36.46 | 0.7286 | 25.56 fl oz (0.756 L) | TA down, pH down |
| Muriatic acid 14.5% (10°Bé) | 36.46 | 0.7286 | 59.83 fl oz (1.769 L) | TA down, pH down |
| Dry acid (sodium bisulfate, 93.2%) | 120.06 | 2.3991 | 2.148 lb (974.4 g) | TA down, pH down |
The bicarbonate figure of 1.401 lb is worth pausing on: it lands almost exactly on the industry's familiar "1.4 pounds per 10,000 gallons per 10 ppm". That agreement is a useful check that the derivation is sound — and it means the numbers above can be trusted where the rules of thumb run out.
The "add a quart of acid" myth. A quart is 32 fl oz, but the correct dose for a 10 ppm drop in 10,000 gallons is 25.6 fl oz of 31.45% acid. Following the quart advice overshoots by 25%. It also matters which acid you have: much retail muriatic is now the gentler 14.5%, which needs 2.34× as much. Check the label — the two strengths are packaged almost identically.
Why you cannot lower alkalinity on its own
Acid does not distinguish between alkalinity and pH. Every drop you add lowers both. So the common situation of high alkalinity but acceptable pH has no direct fix — add enough acid to correct alkalinity and pH ends up too low.
The way around it is aeration, and it exploits an asymmetry worth understanding: aerating water drives off dissolved carbon dioxide, which raises pH — but it does not put alkalinity back. That gives you a ratchet:
| Step | Action | pH | Alkalinity |
|---|---|---|---|
| 1 | Add acid | 7.6 → 7.0 | 150 → 120 |
| 2 | Aerate (fountains, jets, spa blower) | 7.0 → 7.4 | 120 → 120 (unchanged) |
| 3 | Add acid again | 7.4 → 7.0 | 120 → 100 |
| 4 | Aerate again | 7.0 → 7.4 | 100 → 100 |
Each cycle strips alkalinity while returning pH to where it started. It takes days rather than hours, and there is no faster method — anything promising to drop alkalinity without touching pH is selling you acid plus patience under another name.
Do not chase pH back up with soda ash between cycles. Soda ash raises alkalinity too, which undoes the work. Aeration is free; let it do the lifting.
Targets by pool type
| Pool type | Target TA | Why |
|---|---|---|
| Plaster / concrete / gunite | 80–100 ppm | Plaster supplies its own calcium; lower TA limits scaling |
| Vinyl liner | 100–120 ppm | No calcium from surfaces, so more buffer is helpful |
| Fibreglass | 100–120 ppm | Inert surface; buffering has to come from the water |
| Salt chlorine generator | 70–90 ppm | Cells push pH up constantly; less buffer means less fighting it |
| Spa / hot tub | 80–120 ppm | Aeration drives pH up fast; steady buffer needed |
The salt pool figure surprises people who assume more buffer is always better. A salt cell generates hydroxide continuously as it makes chlorine, pushing pH upward all season. High alkalinity makes that drift harder to counteract, not easier, so salt pools are deliberately run leaner.
Dosing safely
- Get the volume right. Every dose scales linearly with volume, so a 20% volume error is a 20% dosing error. Work it out from dimensions with the pool volume calculator rather than trusting the builder's round number.
- Pump running, always. Chemicals added to still water sit in a layer and can damage surfaces underneath.
- Half at a time. Add no more than half the calculated dose, wait, retest, then decide. Overshooting alkalinity is far more tedious to fix than sneaking up on it.
- Bicarbonate: broadcast slowly across the surface. It dissolves readily and will not damage a liner.
- Acid: dilute into a bucket of water first — acid into water, never water into acid — and pour into a deep area away from fittings, lights and the skimmer. Never pour acid into the skimmer.
- Wait six hours minimum before retesting; overnight is better. One full turnover is the real requirement.
Safety. Muriatic acid causes severe burns and its fumes damage lungs and metal. Wear eye protection and gloves, work upwind, and never mix it with chlorine products — that combination releases chlorine gas. Store acid and chlorine well apart. If you are uneasy handling it, dry acid is meaningfully safer to work with, at a modest cost premium.
Common mistakes & pro tips
| Mistake | What happens | Fix |
|---|---|---|
| Ignoring cyanuric acid | Add acid to a pool that needed bicarbonate | Subtract ⅓ of the CYA reading first |
| "Just add a quart of acid" | Overshoots by 25% | 25.6 fl oz per 10k gal per 10 ppm at 31.45% |
| Not checking acid strength | 14.5% acid under-doses by more than half | 14.5% needs 2.34× the volume of 31.45% |
| Chasing pH while TA is wrong | Endless correction, never stable | Fix alkalinity first, then pH |
| Soda ash between aeration cycles | Undoes the alkalinity reduction | Let aeration raise pH — it is free |
| Retesting after an hour | Partly-mixed reading tempts overdosing | Six hours minimum, overnight preferred |
| Guessing pool volume | Every dose wrong by the same margin | Calculate from actual dimensions |
Pro tip. Test alkalinity before the pool has run for the day, at the same time each week. Alkalinity is stable enough that a consistent testing routine tells you the trend, and the trend is far more useful than any single reading — it tells you whether your pool is gaining or losing buffer, which is what you actually need to manage.
How to use this calculator
- Choose raise or lower.
- Enter your pool volume in any unit.
- Enter your current and target alkalinity in ppm.
- Enter cyanuric acid if you know it — the calculator will show your true carbonate alkalinity and dose from that instead.
- Pick the chemical from the dropdown. Doses appear in both metric and US units, along with a suggested first (half) dose.
Frequently asked questions
How much baking soda do I need to raise pool alkalinity?
About 1.4 lb of sodium bicarbonate per 10,000 gallons raises total alkalinity by 10 ppm — roughly 16.8 g per 1000 L. The figure comes from chemistry rather than a rule of thumb: one mole of bicarbonate supplies one equivalent of alkalinity, so 84.0 g of bicarbonate delivers 50.04 g measured as calcium carbonate.
What should pool alkalinity be?
Aim for 80–120 ppm in most pools. Plaster and concrete sit comfortably at 80–100; vinyl and fibreglass tolerate 100–120. Salt pools do better at 70–90, because salt cells drive pH upward and lower alkalinity slows that drift. What matters is carbonate alkalinity after subtracting the cyanuric acid contribution — not the raw test number.
Does cyanuric acid affect the alkalinity reading?
Yes, and most calculators ignore it. Cyanurate ions neutralise acid during the titration, so a stabilised pool reads higher than its true carbonate alkalinity. Subtract about ⅓ of the CYA reading. A pool testing 110 ppm TA with 60 ppm CYA has a true carbonate alkalinity near 90 ppm — on target rather than high. Chasing the raw number would have you adding acid you do not need.
How much acid lowers pool alkalinity?
Roughly 25.6 fl oz of 31.45% muriatic acid per 10,000 gallons lowers TA by 10 ppm. That is four-fifths of a quart, not a full quart — the common "add a quart" advice overshoots by about 25%. If you have the weaker 14.5% acid now sold in many areas, you need about 59.8 fl oz for the same effect.
Can I lower alkalinity without lowering pH?
Not directly — any acid lowers both. The workaround is aeration. Add acid to bring pH to 7.0–7.2 (dropping alkalinity too), then aerate with fountains, jets or a spa blower. Aeration drives off CO₂ and raises pH back toward 7.4 without restoring alkalinity. Repeat until TA reaches target. Slow, but it is the only way to move the two independently.
Why does my pool pH keep drifting?
Alkalinity is the buffer that holds pH steady, so drift usually means alkalinity is out of range. Below ~60 ppm there is almost no buffering and pH swings sharply — "pH bounce". Above ~150 ppm the buffer is so strong that pH creeps upward and resists correction. Fix alkalinity first and pH normally settles by itself.
Does baking soda raise pH as well as alkalinity?
Only slightly — bicarbonate sits near pH 8.3 in solution, so it nudges pH gently while raising alkalinity substantially. That makes it the right choice when alkalinity is low but pH is acceptable. To raise pH significantly, soda ash is the stronger option: it lifts pH sharply and also raises alkalinity, needing only 0.88 lb per 10,000 gallons for a 10 ppm rise.
How long should I wait before retesting alkalinity?
At least six hours of pump running; overnight is better. The water needs a full turnover to distribute the chemical, and testing a partly-mixed pool gives a misleading reading that tempts overdosing. Add no more than half the calculated dose at a time, wait, retest, repeat.