You will read, on almost every pool site, that baking soda raises alkalinity and soda ash raises pH. It's a useful rule and it will keep you out of trouble, but it isn't quite what's happening — and the way it's wrong explains most of the confusion around these two bags.
Both chemicals raise alkalinity. Pound for pound, soda ash raises it more. The difference that matters is what each does to pH on the way there, and that difference is large enough to make them genuinely different tools.
What a pound of each actually does
Same pool, same starting water — 10,000 gallons at pH 7.2, TA 70 ppm, CYA 40 — and one pound of each, run through the buffer model behind our calculators:
| 1 lb into 10,000 gallons | Alkalinity | pH | What that means |
|---|---|---|---|
| Baking soda sodium bicarbonate, NaHCO₃ | +7 ppm | +0.03 (to 7.23) | Moves alkalinity, leaves pH almost exactly where it was |
| Soda ash sodium carbonate, Na₂CO₃ | +11 ppm | +0.30 (to 7.50) | Moves alkalinity more — and pH about ten times as much |
Computed from the carbonate buffer model in our engine, not from a memorized figure. Note that every "6 ounces raises pH by 0.2" rule you'll read omits the starting pH, alkalinity and CYA — and the dose depends entirely on all three, which is why the calculators ask.
So the honest one-liner is: soda ash is the stronger base, and it raises both numbers. Baking soda is the one that moves alkalinity without dragging pH along. When you want only alkalinity — which is most of the time, because alkalinity drifts down while pH tends to drift up — that restraint is exactly the feature you're paying for.
Why baking soda can't push pH past 8.3
This is the part nobody explains, and it makes the rest obvious. Both chemicals dissolve into the same carbonate system your alkalinity reading measures. What separates them is how much base each one carries per molecule: bicarbonate carries one, carbonate carries two.
A solution of pure baking soda sits at about pH 8.3 — and that's not a coincidence, it's the equilibrium point of the bicarbonate system. Add baking soda to water that's below 8.3 and pH creeps up toward it. Add more and it creeps a little further. But it approaches 8.3 and stops, because you're adding the very thing that defines that equilibrium. You cannot overshoot with baking soda no matter how much goes in; that's a chemical guarantee, not a dosing tip.
Soda ash has no such ceiling. A soda ash solution sits around pH 11.3 — roughly a thousand times more basic — so it drives pH wherever the dose takes it, including well past where you wanted. That single fact is why one of these is forgiving and the other needs measuring.
A pool at pH 7 with TA 50 is the classic "everything's low" case, and it's tempting to reach for the cheap, safe bag. Doing it with baking soda takes about 4.2 lb per 10,000 gallons to bring TA to 80 — and leaves pH at 7.17. You've fixed the alkalinity and barely touched the problem you noticed. Soda ash fixes both at once here, which is what the pH calculator will tell you to use when it sees a low TA alongside a low pH.
So which do you need?
Read both numbers, not one. Your pH and alkalinity readings together pick the chemical — and in two of the four cases, the answer is something other than either bag:
| Your pH | Your TA | Use | Because |
|---|---|---|---|
| In range (7.2–7.8) | Low (under 60) | Baking soda | The one job it does perfectly — alkalinity up, pH essentially untouched. Dose it here |
| Low (under 7.2) | Low (under 60) | Soda ash | One chemical, both problems. Dose it here |
| Low (under 7.2) | In range or high | Neither — aerate | Soda ash would push TA higher than you want. Off-gassing CO₂ raises pH and leaves TA alone, for free — the pH calculator explains how |
| High (over 7.8) | Anything | Neither — acid | Both of these raise pH. You want it down; that's muriatic acid, and the dose depends on your TA |
Which to fix first when both are out is its own question, and the alkalinity calculator works through it — the short version is TA first when it's low, pH first when TA is high.
Same powder, three names, three prices
Neither of these is a pool chemical. Both are bulk commodities that predate swimming pools by a century, and the pool-branded versions are the same compound in a bag with a picture of water on it:
| Sold as | What's in the bag | Also found in |
|---|---|---|
| "Alkalinity Increaser", "Alkalinity Up", "Total Alkalinity Plus" | Sodium bicarbonate — baking soda, 100% of it | The baking aisle, and 12–50 lb agricultural or feed-grade sacks at a fraction of the pool-store price per pound |
| "pH Increaser", "pH Up", "Balance Pak 200" | Sodium carbonate — soda ash | The laundry aisle as "washing soda" (check it's 100% sodium carbonate, not a detergent blend) |
| "pH Increaser" — occasionally | Sometimes sodium bicarbonate instead | Read the ingredient line. Some products labeled for pH are actually baking soda, which will not do the job you bought them for |
Two practical notes. Check the ingredient line rather than the front of the bag — the marketing name tells you the intended job, not the compound, and as the last row shows they don't always agree. And buy the big bag: both keep indefinitely if they stay dry, and a pool that needs baking soda once will need it again.
A pound of pool-branded "alkalinity increaser" typically runs several times the price of the identical compound sold as baking soda, and a mid-size pool can want 2.8 lb of it to move TA from 70 to 90. It's the same sodium bicarbonate that costs a dollar a pound in the baking aisle and less in a feed sack. This is the clearest example of a pattern worth watching generally: the pool industry's margin is in relabeling, and the four chemicals a well-run pool actually needs are all commodities.
What about borax?
Borax (sodium tetraborate) is the third option people mention, and it behaves like a gentler soda ash: it raises pH, raises alkalinity slightly, and adds borates to the water. Those borates are the actual reason to use it — at 30–50 ppm they act as a second buffer that damps the pH rise a pool naturally drifts through, and they suppress algae mildly as a bonus.
It's a poor choice for a one-off pH correction, because the borate level it leaves behind is permanent until you dilute it out, and reaching a useful borate level needs a deliberate dose alongside acid to cancel the pH spike. If that appeals, treat it as its own project rather than a substitute for soda ash — the borate calculator does the paired borax-and-acid math.
Handling: the mistakes worth avoiding
- Pre-dissolve soda ash. Broadcast dry into the pool it creates a locally very high pH where it lands, which precipitates calcium and clouds the water — a common cause of "I balanced my pool and it went milky." Dissolve it in a bucket of water first and pour it slowly over a return jet. Baking soda is forgiving enough to broadcast, though dissolving is still tidier.
- Don't chase both numbers in one afternoon. Add one chemical, run the pump an hour to mix, then retest. The carbonate system takes time to settle, and a reading taken too early sends you chasing a number that was going to arrive anyway.
- Never mix either one with acid — or with anything else in a bucket. They're mild on their own; combined with acid the reaction is vigorous and pointless, since you'd be neutralizing the thing you just paid for.
- If the water clouds after dosing soda ash, that's the calcium precipitation above. It usually clears on its own as pH settles — cloudy water has four causes and this is the most self-inflicted one.
Beyond that, this is a two-bag decision. Keep baking soda on hand always; keep soda ash only if your pool actually runs low on pH, which many don't — most pools drift upward and need acid instead. The test dashboard takes both readings at once and tells you which of the two, if either, your water is asking for.