Pool LSI Calculator: Langelier Saturation Index for Salt Pools

pH

Water temperature°F

Calcium hardness (CH)ppm

Total alkalinity (TA)ppm

Cyanuric acid (CYA)ppm

Total dissolved solids (TDS), 0 if not measuredppm

Salt reading, 0 if not a salt poolppm

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The Langelier Saturation Index puts one number on whether your water leans toward etching what it touches or toward dropping scale on it. Enter pH, water temperature, calcium hardness, total alkalinity, cyanuric acid, any TDS reading you have and your salt reading; the calculator returns the index and, beside it, every factor that went into it — the temperature factor, the calcium factor, the alkalinity factor, the TDS factor and the carbonate alkalinity they were looked up against. Nothing is held back inside the arithmetic, so the whole sum can be checked by hand.

Worked example: a salt pool with no TDS reading

The readings below are the ones this calculator starts on, set down as a worked example rather than as advice for any particular pool. A salt pool tests at pH 7.5, 80°F, 300 ppm calcium hardness, 100 ppm total alkalinity, 40 ppm cyanuric acid and 3,200 ppm of salt, with no TDS reading of its own.

• Carbonate alkalinity is 86.8 ppm: the kit's 100 ppm, less 0.33 of the 40 ppm of cyanuric acid in the water.
• The looked-up factors are TF 0.7 from the 84°F row, CF 2.1 from the 300 ppm row and AF 1.9 from the 75 ppm row.
• Nothing was entered for TDS, so the salt reading stands in for it, and 3,200 ppm lands on Pentair's 3,000 ppm row: a TDS factor of 12.35.
• 7.5 + 0.7 + 2.1 + 1.9 − 12.35 leaves −0.15, which the calculator reports as balanced.

Put the very same water on the single 12.2 that the MAHC Annex gives above 1,000 ppm — guidance for public pools — and it reads 0.00 instead. That second figure has a result row to itself, because the whole of the 0.15 between the two answers is the choice of TDS factor and nothing else about the pool.

Worked example: corrosive water

A second set of readings, and a different verdict: pH 7.2, 66°F, 100 ppm calcium hardness, 70 ppm total alkalinity, no cyanuric acid at all, and a measured TDS of 500 ppm.

There is no cyanuric acid to take out, so carbonate alkalinity is simply the 70.0 ppm the kit read, and the row says as much instead of printing a factor. TF is 0.5 from the 66°F row, CF 1.6 from the 100 ppm row and AF 1.9 from the 75 ppm row, and 500 ppm of TDS keeps the lower TDS factor, 12.1. So 7.2 + 0.5 + 1.6 + 1.9 − 12.1 comes to −0.90, and the calculator reports the water as corrosive. On what a reading that low does, the MAHC Annex, guidance for public pools, is blunt: "This AQUATIC VENUE water can cause etching, pitting, dissolving, and staining of walls, grouting, and plumbing."

The formula and the table

The sum itself is one line of the MAHC Annex, guidance for public pools: "The LSI formula is: LSI = pH + TF + CF + AF – TDSF". The pH goes in as tested. TF, CF and AF are read out of a table of rows, and the same Annex says which row to take: "Use the TF, CF, and AF factors closest to your actual reading."

Closest leaves one case open. Where a reading sits exactly halfway between two rows, the calculator takes the higher one; that is this site's convention, and it is applied the same way anywhere a figure is looked up in a published table. It is why 80°F is read at the 84°F row rather than the 76°F one, why 250 ppm of calcium hardness is read at 300 ppm, and why pH 7.5 takes the correction factor printed against pH 7.6.

The table printed under the calculator is that same Annex table, 5.7.4.6, guidance for public pools, reproduced whole so the chosen row can be found by eye. Temperature runs from 32°F to 128°F, calcium hardness from 5 to 2,500 ppm and alkalinity from 5 to 1,000 ppm. A reading beyond either end of a column is not extrapolated: the end row is used, and a result row names the column it happened in.

TDS and salt pools

The last term is the only one that comes from somewhere other than that table. The MAHC Annex, guidance for public pools, puts it as two figures: "If the TDS of the water is less than 1,000 ppm (mg/L), use the factor of 12.1." and "If the TDS of the water is greater than 1,000 ppm (mg/L), use the factor of 12.2."

Two figures cover a wide span of water. PHTA's water balance bulletin, discussing indexes that treat 12.1 as a fixed constant rather than letting it move with TDS, says where that shows up: "This last error is especially apparent in salt pools". The bulletin ties the 12.1 value to 500 ppm of TDS, and a pool carrying 3,200 ppm of salt sits a long way past that.

So from 1,000 ppm upward this calculator reads the factor off Pentair's IntelliChlor guide instead, which runs from 12.19 at 1,000 ppm to 12.44 at 5,000 ppm and picks a row on the same rule: "Note: Use the reading closest to your actual reading in choosing the factor." Below 1,000 ppm the 12.1 figure stands. Above it, a result row still prints what the identical water would score on the single 12.2 of the MAHC Annex, guidance for public pools, so the swap hides nothing.

With no TDS reading entered, the salt reading is used in its place. The Annex, guidance for public pools, is plain that the two are not separate things: "The sodium chloride will contribute to TDS". For the same reason a TDS reading that comes back below the salt reading is not taken at face value either — the calculator uses the salt figure and adds a row asking for a retest, since salt is part of the total rather than something alongside it.

A chlorine pool with a real reading under 1,000 ppm simply stays on 12.1. At pH 7.4, 84°F, 250 ppm calcium hardness, 90 ppm total alkalinity, 30 ppm cyanuric acid and 800 ppm of TDS, carbonate alkalinity works out at 80.7 ppm and the index at 0.00 — the middle of the range, on the lower of the two published factors.

Alkalinity for the index

What belongs in the index is carbonate alkalinity, not the number on the test kit. Cyanuric acid registers on an alkalinity test without buffering the water the way carbonate does, so it has to come back out first. The MAHC Annex, guidance for public pools, does that with a flat fraction: "it is recommended that 30% of the CYA reading be deducted from the total alkalinity test result to get a more accurate reading of the LSI."

This calculator subtracts PHTA's factor for your pH instead, a figure that climbs from 0.23 at pH 7.0 to 0.36 at pH 8.0. At pH 7.6 it is 0.33, so 40 ppm of cyanuric acid takes 13.2 ppm off a 100 ppm reading and leaves the 86.8 ppm in the first worked example. The pH and alkalinity calculator sets that table out and explains why the correction moves with pH at all.

What counts as balanced

The verdict row is judged against one sentence of the MAHC Annex, guidance for public pools: "Water balance should fall within the range of –0.3—+0.3, as determined by the Langelier Saturation Index." Inside it, the row reads balanced; under it, corrosive; over it, scale-forming.

PHTA's water balance bulletin, industry guidance, carries the top of its range further out — the minimum and maximum for proper balance being "considered by the APSP to be -0.3 to +0.5". The calculator prints both: its verdict on the narrower range, and a second row saying where the same number sits on the wider one. A pool reading +0.4 is therefore told outright that it is scale-forming on one published range and inside the other, which is more use than either figure on its own.