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Langelier Index
The Langelier Index is a powerful tool to calculate the water balance,
and to predict corrosion or scaling problems. Theoretically, a LI of zero
indicates perfect water condition for swimming pools. If LI>0, scaling
and staining of the water is present, and if LI<0 the water is corrosive
and highly irritating. A tolerance of ±0.4 is normally acceptable.
The Langelier formula is expressed as:
LI = pH + TF + HF + AF – 12.5
Where:
LI = Langelier Index (also called Saturation Index)
pH = pH of the water
TF = temperature factor
HF = hardness factor, log (Ca hardness, ppm as CaCO
3
)
AF = alkalinity factor, log (alkalinity, ppm as CaCO
3
)
To calculate the exact Langelier Index of your water please use the
WATER INDEX
reference tables.
For most pools, water is balanced if:
• The pH value is maintained within the recommended ranges of pH
7.2 - 7.6
• Ideally, the Alkalinity should be maintained within a range of
80 - 125 ppm
• The Calcium Hardness should be maintained within a range of
200 - 400 ppm.
To calculate your water balance, three parameters must be measured;
calcium hardness, alkalinity and pH. Find the hardness and alkalinity
factor in the reference tables below.
Thewater temperature is, in general, maintained between 24°C (76°F)
and 34°C (94°F). Assuming the temperature is kept within those
ranges, an average value or 0.7 may be used.
Water balance = pH+TF+HF+AF
Water Index Reference Table
Water Balance Condition
Recommendation
11.0-12.0
Corrosive
Increase pH and/or alkalinity
12.1-12.3
Acceptable Balance Retest water frequently
12.4-12.6
Ideal Balance
Maintain
12.7-12.9
Acceptable Balance Retest water frequently
13.0-14.0
Scale Forming
Reduce pH and/or alkalinity
Temperature
CalciumHardness
Alkalinity
°C
°F
TF
mg/L
(as CaCO₃)
HF
mg/L
(as CaCO₃)
AF
0
32
0
5
0.7
5
0.7
4
39
0.1
25
1.4
25
1.4
8
46
0.2
50
1.7
50
1.7
12
54
0.3
75
1.9
75
1.9
16
60
0.4
100
2.0
100
2.0
20
68
0.5
150
2.2
150
2.2
24
75
0.6
200
2.3
200
2.3
28
82
0.7
250
2.4
250
2.4
32
90
0.7
300
2.5
300
2.5
36
97
0.8
400
2.6
400
2.6
40
104
0.9
500
2.7
500
2.7
50
122
1.0
1000
3.0
1000
3.0
10
Photometers
10.35
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