What is total alkalinity in a hot tub?

Total alkalinity is the water's ability to resist a change in pH. It acts as a buffer against everything that would otherwise move pH: sanitizer, sweat, rain, fill water minerals and the treatments you add. The usual target is 80 to 120 ppm.

Why does my hot tub pH keep changing?

Almost always because total alkalinity is too low. Without enough buffer, every small input swings pH sharply, so a correction holds for a day or two and then moves again. Raise alkalinity into range first, let the water settle, and pH frequently corrects itself.

Should you adjust alkalinity or pH first?

Alkalinity first, always. It buffers pH, so correcting pH before alkalinity is settled means the water undoes the adjustment. The full order is alkalinity, then pH, then calcium hardness, then sanitizer.

What happens if hot tub alkalinity is too high?

The buffer becomes so strong that pH stops responding to adjustment. pH tends to lock high, which makes sanitizer far less effective, and minerals come out of solution, which shows as cloudy water and scale forming around the waterline.

Does rainwater affect hot tub alkalinity?

Considerably. Rainwater arrives with almost no alkalinity and almost no calcium hardness, so a tub filled from a rainwater tank needs correcting on every fill rather than occasionally. Rain landing in an uncovered tub also dilutes the buffer over a wet season.

Hot Tub Alkalinity, and Why It Holds pH Steady

By AlumiTubsAugust 24, 2026

Total alkalinity is the water's ability to resist a change in pH. It is not the same as pH, and it is the reason pH either stays where you put it or drifts every few days. Get alkalinity right and pH becomes stable. Leave it wrong and you will be correcting pH forever.

This is why every balancing sequence starts here. Adjusting pH before alkalinity is settled means each correction gets undone by the water itself.

Not sure where your water is? Check your readings.

What alkalinity actually does

Think of alkalinity as a buffer sitting between your water and anything that would change its pH.

Everything that goes into a hot tub nudges pH one way or the other: sanitizer, sweat, rain, the minerals in your fill water, the treatments you add to fix something else. Alkalinity absorbs those nudges. When there is enough of it, pH barely moves. When there is too little, every small input swings pH sharply.

The usual target is 80 to 120 ppm. Some manufacturers publish a slightly wider band, so it is worth knowing your own tub's figure, but 80 to 120 is where most hot tub water is stable.

When alkalinity is too low

Below range, the water has nothing holding pH steady. The symptoms are recognizable once you know what you are looking at.

pH that will not stay put. You correct it, it reads fine, and two days later it has moved again. This is the classic signature
pH swinging in both directions rather than drifting consistently one way
Corrosive water, because low alkalinity usually arrives alongside low pH, and acidic water attacks surfaces, fittings and heating components
Skin and eye irritation, since the water is now sitting outside the comfortable range more often than not
Sanitizer that seems to vanish, because unstable pH makes it work inefficiently

What to do: raise alkalinity into range first, then let the water circulate and settle before you look at pH at all. Very often pH corrects itself once the buffer is back, and the pH adjustment you were about to make turns out to be unnecessary.

Raise it in stages rather than all at once. A large single correction overshoots easily, and alkalinity is slower to bring back down than it is to push up.

When alkalinity is too high

Above range, the buffer becomes so strong that pH stops responding to you.

pH that resists adjustment. You add a treatment, retest, and the reading has barely moved
pH locked high, which makes sanitizer far less effective
Cloudy water, because high alkalinity encourages minerals to come out of solution
Scale forming on surfaces and around the waterline

What to do: lower alkalinity into range before touching pH. As with raising, work in stages and retest between them. If the tub is filled from a supply that is naturally high in alkalinity, expect this to be a recurring adjustment on every fill rather than a one-time fix.

Why the order matters

The sequence is always the same, and alkalinity is always first.

Total alkalinity, typically 80 to 120 ppm. It buffers everything after it
pH, generally 7.4 to 7.6 depending on your tub and its interior material
Calcium hardness, typically 150 to 250 ppm
Sanitizer, which only performs properly once the first three are right

Skipping to pH is the most common mistake in hot tub water care, and it is self-defeating. Without the buffer in place, the pH you set will not hold, so you correct it again, and each correction adds more to the water. Over a few weeks that shows up as rising total dissolved solids and water that behaves unpredictably.

Correct alkalinity, wait, retest. Then move on.

How to test it properly

Test strips read alkalinity reliably enough for routine use. A few habits improve the result.

Test mid-tub rather than beside an inlet, where the reading may not represent the whole body of water
Hold the strip flat after dipping and do not shake water off it
Wait the time on the packaging before matching against the chart
Retest fifteen to twenty minutes after adding anything, so the water has circulated
Keep strips cool, dry and out of sunlight, and check the expiry date, since degraded strips read low

On a tub without a circulation pump, stir thoroughly with a paddle before testing.

What moves alkalinity

Your fill water. The biggest single factor. Municipal supplies vary widely, well water more so
Rainwater, which arrives with almost no alkalinity at all and needs correcting on every fill
Rain landing in an uncovered tub, which dilutes the buffer over a wet season
Sanitizer, which nudges alkalinity as it works
Bather load, since sweat and body products both add to the water
Topping up, which changes the balance in proportion to how much you add

Any top-up from a different source is worth a retest. Half a tub of well water added to a municipal fill is a different chemistry from either alone.

Alkalinity and the other readings

The four parameters are connected, which is why a single reading rarely tells the whole story.

Alkalinity and pH. The closest relationship. Alkalinity determines whether pH is stable, and pH tells you where the water currently sits. Low alkalinity means pH readings that cannot be trusted to hold.

Alkalinity and calcium hardness. Both contribute to whether water is scale-forming or aggressive. High alkalinity alongside high calcium is the combination most likely to produce scale and cloudiness together.

Alkalinity and sanitizer. Indirect but real. Alkalinity stabilizes pH, pH governs how well sanitizer works, so unstable alkalinity means sanitizer that performs inconsistently and appears to disappear.

More on each in pH and why it affects your skin, calcium hardness and scale, and sanitizer, and why it stops holding.

If you have an AlumiTub

The published target is 80 to 120 ppm, adjusted first, and it applies to every fill regardless of source.

Two situations need more attention than most. Rainwater arrives with almost no alkalinity and almost no calcium hardness, which makes it chemically aggressive toward a marine-grade aluminum interior until it is corrected, so it needs balancing on every fill rather than occasionally. Water drawn through a softener has the same problem for the same reason, and a softener strips calcium by design, so use an unsoftened tap where you can.

The rest of the published AlumiTub targets are pH 7.4 to 7.8, calcium hardness 150 to 250 ppm, and bromine 2 to 3 ppm, which is lower than the 3 to 5 ppm typical of a conventional spa. Use bromine rather than chlorine, and avoid salt chlorine generators and anything containing copper or silver. Good Clean Living Water Care is built around those numbers.