Langelier Saturation Index Worksheet for Commercial Pools

The MAHC Annex gives the Langelier Saturation Index as LSI = pH + TF + CF + AF - TDSF, where three factors come from its Table 5.7.4.6 and the TDS factor is 12.1 or 12.2. The target is -0.3 to +0.3, and slightly positive is preferred. Below is the hand method, two worked examples, the order to adjust things in, and a monthly sheet for a 30-pool book.

What is the formula, and what goes in each box?

The Annex (5.7.4.6) defines the inputs:

• pH: the tested pool water.
• TF: temperature factor from the table, at the water temperature when you sampled.
• CF: calcium hardness factor from the table.
• AF: total alkalinity factor from the table.
• TDSF: 12.1 if TDS is below 1,000 ppm, 12.2 if above.

The Annex says to use the factors closest to your actual reading. Table 5.7.4.6:

Temp	TF	CH, ppm	CF	TA, ppm	AF
32 F	0.0	5	0.3	5	0.7
37 F	0.1	25	1.0	25	1.4
46 F	0.2	50	1.3	50	1.7
53 F	0.3	75	1.5	75	1.9
60 F	0.4	100	1.6	100	2.0
66 F	0.5	150	1.8	150	2.2
76 F	0.6	200	1.9	200	2.3
84 F	0.7	300	2.1	300	2.5
94 F	0.8	400	2.2	400	2.6
105 F	0.9	800	2.5	800	2.9
128 F	1.0	1,000	2.6	1,000	3.0

The CH table also has rows at 1,300 (2.7), 1,600 (2.8), 2,000 (2.9) and 2,500 ppm (3.0).

A useful pattern in the table: every CF equals log10 of the hardness minus 0.4, and every AF equals log10 of the alkalinity, rounded to 0.1. You can compute between-row values exactly from that, but keep the lookup version for any sheet an inspector or a board will read, because it follows the Annex steps.

What ranges does the code allow for the inputs?

• pH 7.0 to 7.8 (5.7.3.4.1, or whatever your state adopted). The Annex adjustment range for balance is 7.2 to 7.6.
• Total alkalinity 60 to 180 ppm (5.7.4.4.1).
• Calcium hardness no more than 2,500 ppm (5.7.4.4.3). The Annex adjustment ranges are 200 to 400 ppm for a masonry pool and 80 to 200 ppm for vinyl or fiberglass.
• Annex 5.7.4.4.3 also puts the range at 100 to 200 ppm for venues above 90 F.

The Annex says minor alkalinity and hardness deviations are not imminent health hazards but point to poor water balance management.

How does temperature alone move the result?

Example: a heated indoor pool at 84 F tests pH 7.6, calcium hardness 400 ppm, total alkalinity 100 ppm and TDS 800 ppm.

1. TF at 84 F = 0.7. CF at 400 ppm = 2.2. AF at 100 ppm = 2.0. TDSF = 12.1, since TDS is below 1,000.
2. LSI = 7.6 + 0.7 + 2.2 + 2.0 - 12.1 = 0.4.

That is above +0.3, so the Annex describes it as scale-forming. Now an outdoor pool filled in the spring at 66 F with the same chemistry:

1. TF at 66 F = 0.5.
2. LSI = 7.6 + 0.5 + 2.2 + 2.0 - 12.1 = 0.2.

Same water chemistry, a different side of the band. The 18 F difference moved the result by 0.2. That same outdoor pool at 84 F in July goes back to 0.4 with no chemistry change at all, so a pool balanced in May can drift out by midsummer.

The TDS factor is a step: 990 ppm and 1,010 ppm TDS differ by 0.1 in the result with no chemistry change. Do not chase that difference.

How do you correct alkalinity for cyanuric acid?

The Annex has two rules that are easy to mix up:

• Alkalinity test (5.7.4.4.1): CYA interferes with the test. Divide the CYA reading by 3 and subtract it from the measured total alkalinity.
• LSI at an outdoor pool using stabilized chlorine (5.7.4.6): it recommends deducting 30 percent of the CYA reading from the total alkalinity result.

Example: measured TA 120 ppm, CYA 60 ppm. By thirds: 120 - 60/3 = 120 - 20 = 100 ppm. By 30 percent: 120 - 0.30 x 60 = 120 - 18 = 102 ppm. Both land on the AF row for 100 ppm (2.0). The choice rarely matters with the coarse table, but it can: at TA 200 and CYA 90, the 30 percent correction gives 173, which lands on the 150 row (AF 2.2) while the uncorrected 200 gives 2.3, a 0.1 change. Pick one correction for your SOP and use it on every sheet. The Annex also lists stabilizer and TDS among the things that push LSI down.

What if the fill water is hard and the calcium cannot be lowered?

Example: an outdoor pool at 66 F filled from a hard source tests pH 7.5, calcium hardness 800 ppm, measured TA 120 ppm, CYA 60 ppm and TDS 1,400 ppm.

1. TA corrected by 30 percent = 102 ppm, so AF = 2.0. CF at 800 ppm = 2.5. TF = 0.5. TDS is above 1,000, so TDSF = 12.2.
2. LSI = 7.5 + 0.5 + 2.5 + 2.0 - 12.2 = 0.3.

It sits on the edge of the band and will go over as the water warms: at 84 F, TF = 0.7 and the result is 0.5. The Annex advice for this case is that lowering calcium is not always possible, so lower the pH or the total alkalinity, with pH as the more important parameter, and that lowering alkalinity usually lowers pH too.

Dry-acid dose from the Annex (5.7.4.4.1): 1.6 lb sodium bisulfate per 10,000 gallons lowers alkalinity by 10 ppm. To drop 25 ppm in a 60,000 gallon pool: 1.6 lb x (60,000 / 10,000) x (25 / 10) = 1.6 x 6 x 2.5 = 24 lb. Retest and adjust pH afterward, and follow the product label. Raising alkalinity takes 1.4 lb of baking soda per 10,000 gallons for each 10 ppm.

LSI result	What is out of range	Adjust, in this order
Below -0.3	pH under 7.2	Raise pH to 7.2 to 7.6 first
Below -0.3	pH fine, TA under 60 ppm	Raise TA toward 60 to 180 ppm, then recheck pH
Below -0.3	pH and TA fine, CH low	Raise CH toward the range for the surface
Above +0.3	pH over 7.6	Lower pH first
Above +0.3	pH fine, TA high	Lower TA with acid, retest pH
Above +0.3	Hard fill water, CH cannot come down	Hold pH at the low end of 7.2 to 7.6, keep TA at the low end, retest monthly

How often do you test, and what goes in the log?

• Total alkalinity weekly (5.7.5.5), calcium hardness monthly (5.7.5.6), saturation index monthly (5.7.5.8), TDS quarterly (5.7.5.9).
• Water temperature recorded with each chlorine and pH test in heated venues (5.7.5.10).
• The daily record list in 6.4.1.6 includes temperature (item 8), calcium hardness (10), total alkalinity (11) and saturation index (12).
• The Annex says a generally accepted TDS limit is 1,500 ppm above the TDS of the fill water.

How does this run across 30 pools?

Group the pools by fill-water source. MAHC 5.7.4.5 requires fill water from a public water system, one meeting the AHJ's requirements, or one the AHJ approved, and the Annex recommends testing it yearly. The Annex's example tests (bacteria, nitrates, iron, manganese and similar) do not include hardness or alkalinity, so add those yourself. Pools that share a hard source share the same calcium problem and the same fix. Test a sample of the source once per group and record it.

For outdoor pools, redo the sheet when the water temperature moves by 10 F or more: each step in the table is about 0.1. A one-page monthly sheet for 30 pools needs these columns: pool, fill group, temperature, pH, CH, TA after correction, TDS, LSI, and a flag for results outside the band or inputs outside the code ranges. Review group by group: three pools in a hard-water group all at +0.4 is one conversation, not three. Poolean builds monthly logs from each pool's readings and test photos, and you can print them, save a PDF or download a CSV. Book a 30-minute call to see how.

Related reading: commercial pool pH range, cloudy commercial pool water and the water test log template.

The short version

• LSI = pH + TF + CF + AF - TDSF, using the Annex Table 5.7.4.6 factors and TDSF 12.1 or 12.2.
• Keep it within -0.3 to +0.3, and slightly positive if you can.
• Temperature alone moves the result by 0.2 between 66 F and 84 F, so recompute as the season changes.
• Fix pH first, then alkalinity, then calcium. Correct alkalinity for CYA the same way on every sheet.
• Group pools by fill-water source and review each group monthly.