LSI Calculator Guide: Understanding the Langelier Saturation Index

The Langelier Saturation Index (LSI) tells you whether your pool water is balanced, corrosive, or scale-forming. Understanding LSI helps prevent expensive damage to pool surfaces, equipment, and plumbing—problems that cost far more to fix than they do to prevent.

What is the Langelier Saturation Index?

Developed by Dr. Wilfred Langelier in 1936, the LSI predicts whether water will deposit calcium carbonate scale or dissolve it. For pools, this translates directly to surface and equipment longevity.

Think of LSI as a balance scale:

• Balanced water (LSI ≈ 0) neither deposits nor dissolves calcium
• Positive LSI means water is supersaturated—it will deposit scale
• Negative LSI means water is undersaturated—it will dissolve calcium from surfaces

The LSI Formula

LSI combines five water chemistry factors:

Each factor is derived from lookup tables based on your actual water readings. Here's what each represents:

Factor	Based On	What It Measures
pH	pH reading	Acidity/alkalinity of water
TF (Temperature Factor)	Water temperature °F	How temperature affects calcium solubility
CF (Calcium Factor)	Calcium hardness ppm	Amount of dissolved calcium
AF (Alkalinity Factor)	Total alkalinity ppm	Water's buffering capacity

Factor Lookup Tables

Temperature Factor (TF):

Temperature °F	TF Value
32°F	0.0
50°F	0.3
60°F	0.4
70°F	0.5
77°F	0.6
84°F	0.7
95°F	0.8
105°F	0.9

Calcium Factor (CF):

Calcium Hardness ppm	CF Value
25 ppm	1.0
50 ppm	1.3
75 ppm	1.5
100 ppm	1.6
150 ppm	1.8
200 ppm	1.9
300 ppm	2.1
400 ppm	2.2
600 ppm	2.4
800 ppm	2.5

Alkalinity Factor (AF):

Total Alkalinity ppm	AF Value
25 ppm	1.4
50 ppm	1.7
75 ppm	1.9
100 ppm	2.0
125 ppm	2.1
150 ppm	2.2
200 ppm	2.3
300 ppm	2.5
400 ppm	2.6

What Negative LSI Does to Your Pool

Corrosive water (negative LSI) actively seeks calcium to reach equilibrium. Since your pool water is in constant contact with plaster, grout, and metal, that's where it finds calcium.

Signs of Corrosive Water Damage

• Etched plaster: Rough, pitted surface that feels like sandpaper
• Faded plaster: Loss of color, especially in colored plaster finishes
• Exposed aggregate: Pebble or quartz aggregate becomes visible
• Metal corrosion: Green staining from copper, rust from iron
• Heater damage: Shortened heat exchanger life
• Deteriorating grout: Tile grout becomes soft and washes away

What Positive LSI Does to Your Pool

Scale-forming water (positive LSI) has more calcium than it can hold in solution. The excess precipitates out onto surfaces.

Signs of Scale Buildup

• White crusty deposits: On tile, waterline, and spillways
• Cloudy water: Calcium precipitating in suspension
• Reduced flow: Scale buildup inside pipes and equipment
• Heater inefficiency: Scale insulates heat exchangers
• Salt cell scaling: Calcium deposits on chlorine generator plates
• Rough surfaces: Scale deposits create texture on smooth surfaces

How to Adjust LSI

Each factor in the LSI formula can be adjusted to bring water into balance:

To Raise LSI (Fix Corrosive Water)

• Raise pH: Add soda ash (sodium carbonate)
• Raise alkalinity: Add baking soda (sodium bicarbonate)
• Raise calcium: Add calcium chloride

To Lower LSI (Fix Scaling Water)

• Lower pH: Add muriatic acid or dry acid
• Lower alkalinity: Add muriatic acid (will also lower pH)
• Lower calcium: Partial drain and refill with softer water

For chemical dosing amounts, use our chemical dosage calculator.

Temperature's Impact on LSI

Water temperature significantly affects LSI. The same water chemistry can be balanced in winter and scaling in summer—or vice versa.

This is why pools in hot climates often need lower pH targets (7.2-7.4) to compensate for high temperatures pushing LSI positive.

LSI and Salt Pools

Salt chlorine generators create localized high-pH conditions at the cell, which can cause scaling even when bulk water LSI is balanced. Salt pool operators should:

• Target the lower end of the balanced range (-0.3 to 0)
• Keep calcium below 400 ppm
• Inspect and clean cells regularly
• Consider reverse polarity cells that self-clean

Learn more about salt pool chemistry in our salt calculator.

LSI and CYA (Cyanuric Acid)

Cyanuric acid (stabilizer) affects how chlorine and pH interact, and some argue it should be factored into LSI calculations. The traditional LSI formula doesn't include CYA, but high CYA levels can mask pH effects.

For pools with CYA above 50 ppm, some professionals use a "corrected alkalinity" that subtracts one-third of CYA from total alkalinity before calculating LSI. Our CYA calculator can help you understand your stabilizer levels.

When to Check LSI

Calculate LSI:

• Seasonally: At minimum, check in peak summer and winter
• After significant water addition: Refills change calcium and alkalinity
• After chemical adjustments: Verify changes achieved desired result
• When troubleshooting: Scale or etching problems
• New pool startup: Critical for protecting fresh plaster

Common LSI Mistakes

Ignoring Temperature

Many pool operators calculate LSI once and assume it stays constant. Temperature changes throughout the year—and even throughout the day—affect LSI significantly.

Chasing Perfect Zero

LSI doesn't need to be exactly 0. The -0.3 to +0.3 range is stable. Constantly adjusting to hit zero wastes chemicals and time.

Fixing the Wrong Factor

If LSI is negative, raising pH is the easiest fix. But if your pH is already 7.8+, the real problem is low calcium or alkalinity. Use the formula to identify which factor is actually out of range.