The Langelier Saturation Index

Saturation Index = pH + TF + CF + AF – 12.1

pH: measured from test kit
TF: temperature factor; measured at pool
CF: calcium hardness factor, measured from test kit
AF: alkalinity factor, measured at pool

Formula Result:
Between –0.5 and 0.5 = water is balanced
Above 0.5 = water is over-saturated (scale-forming)
Below –0.5 = water is under-saturated (corrosive)

Temperature F ° = TF	Calcium Hardness = CF	Total Alkalinity = AF
32° = 0.0	5 = 0.3	5 = 0.7
37° = 0.1	25 = 1.0	25 = 1.4
46° = 0.2	50 = 1.3	50 = 1.7
53° = 0.3	75 = 1.5	75 = 1.9
60° = 0.4	100 = 1.6	100 = 2.0
66° = 0.5	150 = 1.8	150 = 2.2
76° = 0.6	200 = 1.9	200 = 2.3
84° = 0.7	300 = 2.1	300 = 2.5
94° = 0.8	400 = 2.2	400 = 2.6
105° = 0.9	800 = 2.5	800 = 2.9
128° = 1.0	1000 = 2.6	1000 = 3.0
Saturation Index = pH + TF + CF + AF – 12.1

Example:

Test results:
pH reading is 7.0
Water Temperature is 84° (TF is .7 from chart)
Calcium Hardness reading is 300 ppm (CF is 2.1 from chart)
Alkalinity reading is 25 ppm (AF is 1.4 from chart)

Saturation Index =
(pH) 7.0 + (TF) 0.7 + (CF) 2.1 + (AF) 1.4 – 12.1 = -0.9

Since the equation solution equals –0.9, this indicates a corrosive water condition.

Ideal readings for proper water balance:
- pH 7.2 to 7.6
- Total Alkalinity 80 to 125 ppm (parts per million)
- Calcium Hardness 200 to 400 ppm (parts per million)