Calculator of various indexes via chemical composition of the aqueous solution. It outputs hardness, alkalinity and corrosion levels.

# Parameter Value Units
1 pH
2 Electric Conductivity (EC) µS/cm @ 25ºC
3 Temperature ºC
4 Bicarbonates mg HCO3-/L
5 Carbonates mg CO32-/L
6 Chlorides mg Cl-/L
7 Sulphates mg SO42-/L
8 Magnesium mg Mg2+/L
9 Calcium mg Ca2+/L
8 TDS factor
# Index Used parameters Value Assessment
1 Hardness (mg CaCO3/L) Ca2+, Mg2+
2 Alkalinity (mg CaCO3/L) HCO3-, CO32-
3 Langelier Simplified Index (LI) pH, CE, T, HCO3-, CO32-, TDS, Ca2+
4 Langelier Saturation Index (LSI) pH, CE, T, HCO3-, CO32-, TDS, Ca2+
5 Ryznar Stability Index (RSI) pH, CE, T, HCO3-, CO32-, TDS, Ca2+
6 Puckorius Scaling Index (PSI) pH, CE, T, HCO3-, CO32-, TDS, Ca2+
7 Larson-Skold Index (Lal) HCO3-, CO32-, Cl-, SO42-


TDS factor

TDS stands for TOTAL DISSOLVED SOLIDS, which is the measure in en mg/l or ppm of the total quantity of dissolved solids. This measure can be realized directly or can be related to electric conductivity, as is the case here.

The relationship between TDS and electrical conductivity of 1 mg/l of TDS is approximately equal to 2 µS/cm in most cases, but that depends of the number of chemical species. You have to choose the TDS factor value as a function of the number of different compounds dissolved.

Thus, choosing the TDS factor depending on the characteristics of the solution:

  • For solutions with a low amount of dissolved salts: Permeate (0.47).
  • For solutions with a moderate amount of dissolved salts: Remineralized (0.56).
  • For solutions with a big amount of dissolved salts: Bicarbonate (0.7).


Hardness is a property of water related to the amount of metallic non-alkali cations diluted, basically alkali earth metals such as calcium, Ca2+, and magnesium, Mg2+.

The effects of hardness can be observed in the precipitation of soaps, preventing them to perform its cleaning function, and in calcium carbonate, CaCO3, scaling that takes place inside hot water tanks or inside hot water piping.

Water types can be classified depending on hardness value:

Water type Soft Moderately hard Hard Very hard
Hardness (mg CaCO3/L) 0-50 50-150 150-300 >300


Alkalinity is a measure of the capacity of a given solution to neutralize acids in the equivalene point of carbonate or hydrogencarbonate.

Alkalinity is equal to the stoichiometric sum of the bases in solution. In nature, hydrogencarbonate alkalinity tends to represent the biggest part of it all due to the frequent presence and dissolution of carbonate rocks and carbon dioxide of the atmosphere.

It gets measured in mg CaCO3/l and it is the sum of the amount of carbonate and hydrogencarbonate present in the solution.

You can classify the type of water regarding alkalinity as follows:

Water type Very low Low Medium High Very high
Alkalinity (mg CaCO3/L) <12.3 12.3-41 41-98.4 98.4-147.6 >147.6

Langelier simplified Index (LI)

Water equilibrium is very important to ensure surface, equipment, accesories and piping protection. The corrosive or scaling tendency of some waters brings issues to many processes.

A scaling water can reach a point where it starts to scale inside the piping until it completely blocks the flow.

A corrosive water can promote chemical corrosion and later perforation of equipment and metallic piping.

The Langelier simplified Index is a simple method to gauge whether the water is corrosive, scaling or balanced.

  • If LI <0 - corrosive water
  • If LI =0 - balanced water
  • If LI >0 - scaling water

A treatment to modify its chemical composition can be done to corrosive waters to avoid corrosion issues. Examples of this are carbonic acidity neutralization, To avoid corrosion in corrosive waters.

To avoid scaling, you can do a chemical process to reduce pH and/or water alcalinity.

Langelier Saturation Index (LSI)

The Langelier Saturation Index (LSI) of water is useful to determine the corrosive or scaling tendencies of domestic or industrial water. It is a tool to determine the required index to balance water and its saturation levels, the goal being reducing the corrosive or scaling tendencies.

From the LSI calculation you can know the water tendency as follows:

  • LSI > 0 - Scaling tendency, meaning it is oversaturated and tends to precipitate CaCO3.
  • LSI = 0 - Water is considered to be saturated of CaCO3; no CaCO3 precipitation nor dissolution is expected, it is balanced.
  • LSI < 0 - Corrosive tendency, meaning CaC03 will dissolve in this water.

The ideal LSI range is -0.3 to +0.3.

Ryznar Stability Index (RSI)

The role of this index is to know if the water is scaling or corrosive.

Ryznar Stability Index (RSI) is a Langelier Saturation Index (LSI) modification to better assess the tendency of calcium carbonate scaling. Ryznar noted that very hard and soft waters can display the same LSI value.

In that regard, he included the role of hardness, separating types as follows:

RSI assessment
>9.0 intolerable corrosion
7.5-9.0 heavy corrosion
7.0-7.5 corrosion
6.0-7.0 light scaling or corrosion
5.0-6.0 light scaling
4.0-5.0 heavy scaling

Puckorius Scaling Index (PSI)

The practical index for scaling, developed by Puckorius, is similar to Ryznar's, with differences in the use of pH to estimate if the water has scaling or corrosive tendencies, or if it is balanced otherwise.

Depending on the value:

  • PSI < 4,5 - Scaling tendency
  • 4,5 < PSI < 6,5 - Optimal range (no corrosion/scaling)
  • PSI > 6,5 - Corrosive tendency

Larson-Skold Index (Lal)

This index just tells the corrosion degree of a given water.

Larson takes into account the chlorides, sulphates and total alkalinity. It is useful for balanced waters and also purified waters, especially for cold water (<20ºC). It is not a useful metric for very high or very low alkalinity.

  • LaI < 0.8 - Non corrosive water
  • 0,8 < LaI < 1.2 - Significant corrosion
  • LaI > 1.2 - High corrosion