LSI Calculator (Langelier Saturation Index)

The formula is pH, plus a temperature factor, plus a calcium factor, plus a carbonate alkalinity factor, minus a constant for dissolved solids. The calcium and alkalinity factors are logarithms, which is what makes the arithmetic behave the way it does.

Run the numbers from a normal starting point — pH 7.5, calcium 300, alkalinity 100 — and a pattern falls out that is worth committing to memory:

Changing pH by 0.3 moves LSI by 0.30. Doubling calcium hardness moves it by 0.30. Doubling alkalinity moves it by 0.30. All three are the same, because log10(2) is 0.301 and pH enters the formula directly.

Smaller changes scale the way you would expect from a log: calcium 300 to 400 is worth 0.12, alkalinity 100 to 120 is worth 0.08. And temperature from 60°F to 90°F is worth 0.36 — the largest single move on that list, and the one nobody adds on purpose.

Every guide tells you to fix LSI with pH. The reason usually given is that pH moves the index one for one, which is true but incomplete — doubling your calcium does exactly as much.

The real reason is reversibility. Moving pH by 0.3 is an afternoon, costs a few dollars of acid or a fountain left running, and is undone just as easily if you overshoot.

Doubling calcium hardness is bags of calcium chloride, and there is no product that takes calcium back out. The only way down is replacing water. Alkalinity sits in between: raising it is a bag of bicarb, lowering it is the acid-and-aerate cycle over several days.

So the order is not about which input is most powerful. They are equally powerful. It is about which mistakes you can walk back.

Cyanuric acid registers on a total alkalinity test, and a good part of what the test reports on a stabilized pool is cyanurate rather than carbonate. Only the carbonate part does the buffering that LSI is built on.

So feeding raw alkalinity into the formula overstates it. The fix is to subtract roughly a third of your CYA reading and use that figure instead — a pool testing 100 ppm alkalinity with 60 ppm of stabilizer has a carbonate alkalinity nearer 80.

Twenty points of alkalinity is worth about 0.1 on the index, so this is not a rounding difference. A pool reading -0.2 by the uncorrected math is really at -0.3 and heading the wrong way. It bites hardest where it is least expected, because salt pools deliberately run 60 to 80 ppm of CYA — and so does any pool left on tablets long enough.

This calculator asks for CYA and applies the correction. If another one does not ask, the number it gives you is too kind by roughly the amount of stabilizer you are carrying.

Temperature is the input people forget, and it is the biggest one on the list.

Water balanced at 0.00 in a 60°F pool reads +0.36 at 90°F — past the +0.3 line and into scaling, with nothing added and nothing tested differently. That is the whole explanation for why the heater and the salt cell scale before the walls do: the water touching them is hotter than the water in the pool.

It also explains the seasons. The same chemistry that is comfortably balanced in July can go mildly aggressive in November, which is why cold water is harder on plaster than people expect and why a pool closed on summer numbers is not closed on the right ones.

Check it three or four times a year rather than weekly — at opening, at peak summer temperature, and at closing. Those are the points where the answer changes without you doing anything.