Pool Water Balance Calculator: Saturation Index, Alkalinity and Calcium

Will your water scale or etch

Water that is short of calcium carbonate dissolves it back out of plaster, grout and the inside of a heater. Water that holds too much drops it on the walls and the cell. The saturation index is the one number that tells you which way yours leans. It is a model, not a measurement, and it is only as good as the six readings you put into it.

Your test results

All six are needed. We do not fill in a missing one, because every published default we checked turned out to be somebody else’s pool.

Water temperature

Total alkalinity ppm

Calcium hardness ppm

Cyanuric acid ppm

Total dissolved solids ppm

That dissolved solids number came off my salt chlorine generator.

Total dissolved solids is not the same as the salt number on a chlorine generator. A generator reads salinity off conductivity; total dissolved solids counts everything in the water, your calcium and alkalinity included. Do not type one in place of the other and do not add them together.

Do you use a borate product?

Worked example, from the paper this calculator follows: pH 7.5, total alkalinity 100 ppm, calcium hardness 300 ppm, 84 degrees Fahrenheit, total dissolved solids 1,000 ppm, no cyanuric acid and no borates. The saturation index is +0.13. That is Table 5 of the published method, recomputed here rather than copied. Type your own readings and this example disappears.

+0.13 Example only. This is not your water. It is the worked case described above. Enter your own six readings and this replaces itself with your result.

Updates as you type. Nothing to submit.

Move one reading

One thing at a time. Change two and you cannot tell which one did what, and neither can we. Dose, wait a full turnover, retest, and come back with the new number.

What to change

Pool volume gallons

What this will not tell you

How much acid to add to reach a pH. Acid demand is not a straight line from any reading you can hand us, and every published table we checked hides its own assumptions. Titrate it with your kit and dose against that.
Anything at all if you use borate. Borate shows up in a normal alkalinity test and makes this index read high, which is the direction that tells you the water is fine while it quietly etches your plaster. We found no borate correction we could trace to a primary source, so we stop rather than guess.
An index when cyanuric acid has eaten all your carbonate alkalinity. At 100 ppm alkalinity and 300 ppm stabiliser there is essentially nothing left to measure, and the logarithm the index is built on has no answer. One widely used calculator quietly substitutes a hundredth of a part per million and prints a number anyway. We refuse.
An index outside the ranges the published tables cover: pH 7.0 to 8.2, 32 to 113 degrees Fahrenheit, and 500 to 5,000 ppm of dissolved solids. Past those edges the equation is being extrapolated, and we will not do that silently.
A salt reading treated as total dissolved solids. A chlorine generator measures conductivity and calls it salt; dissolved solids is a different quantity that already includes your calcium and alkalinity. Adding the two together double counts.
Whether your plaster is etching. The index tells you which way the water leans, not what your surface is doing. On a vinyl or fibreglass pool there is no cement for undersaturated water to dissolve, so we show the number and withhold the etching verdict.
A dose for two things at once, or the index you would get afterwards. Move one reading, wait a turnover, test again, and type the number you actually measured.
How to lower alkalinity or calcium. Alkalinity only comes down through acid that moves pH at the same time, and the two cannot be separated in a single dose. Calcium only comes down by replacing water.

What this refuses to calculate, and why, is set out in the methodology.