Water Balance Flag (Saturation Index)

Excel Formulas › Pool & Spa Service

All versionsIF

The Langelier Saturation Index (LSI) tells whether pool water is balanced, scaling, or corrosive. Summing the index factors and flagging the result keeps water (and equipment) healthy.

Quick formula: a simple balanced/scaling/corrosive flag:

=IF(LSI > 0.3, "Scaling", IF(LSI < -0.3, "Corrosive", "Balanced"))

Sum the LSI factors (pH, temp, calcium, alkalinity, TDS) into an index; nested IF flags the water condition.

Functions used (tap for the full reference guide):

The example

LSI ≈ +0.1 → balanced.

	A	B
1	Item	Value
2	LSI value	+0.1
3	Flag	→ Balanced

The formula

The formula:

=IF(LSI > 0.3, "Scaling", IF(LSI < -0.3, "Corrosive", "Balanced")) // nested IF on the index

How it works

How it works:

Compute the LSI = pH + temp factor + calcium factor + alkalinity factor − TDS factor (− 12.1).
A value near 0 is balanced; positive scales, negative corrodes.
Use nested IF (or IFS) to flag Scaling / Balanced / Corrosive against thresholds.
Adjust pH and alkalinity first — they move the index most easily.

Educational use only — not chemical-safety or professional advice. Pool chemical dosing varies by product strength, water chemistry, and local rules. Always follow the product label, add chemicals to water (never water to chemical), test before and after, and never mix chemicals. Verify with a professional.

Try it: interactive demo

Live demo

Enter an LSI value.

LSI

Water:

Variations

Three-band IFS

Modern version:

=IFS(LSI>0.3,"Scaling",LSI<-0.3,"Corrosive",TRUE,"Balanced")

Distance from balanced

How far off:

=ABS(LSI)

Action priority

Fix pH/alk first:

=IF(ABS(LSI)>0.5, "Adjust now", "Monitor")

Pitfalls & errors

All factors. LSI needs pH, temp, calcium, alkalinity, and TDS.

Thresholds vary. ±0.3 is common; some use ±0.5.

Not advice. Use a proper LSI calculator/test for real adjustments.

Practice workbook

📊

Download the free Water Balance Flag (Saturation Index) practice workbook

A balance sheet with the IFS, distance, and priority variants, plus 4 challenges with answers. No sign-up required.

Frequently asked questions

How do I flag pool water balance in Excel?

Nested IF on the LSI: =IF(LSI > 0.3, "Scaling", IF(LSI < -0.3, "Corrosive", "Balanced")).

What is the LSI?

The Langelier Saturation Index — pH plus temperature, calcium, and alkalinity factors minus TDS — near 0 means balanced.

What do I adjust first?

pH and alkalinity — they move the index most easily.

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Function references: IF