Free LSI Calculator

Understanding the Langelier Saturation Index (LSI)

The Langelier Saturation Index (LSI) is a widely-used measure of water balance in swimming pools and spas. Developed by Dr. Wilfred Langelier in 1936, the LSI predicts whether water will scale, corrode, or remain balanced based on its calcium carbonate saturation level.

What is LSI?

LSI is a calculated number that tells you if your pool water is aggressive (corrosive), balanced, or scale-forming. It considers six critical water parameters:

• Temperature: Warmer water holds less dissolved calcium, increasing LSI
• pH: Higher pH increases LSI significantly
• Total Alkalinity: Buffers pH changes and affects carbonate levels
• Calcium Hardness: The primary mineral that scales or dissolves
• Cyanuric Acid (CYA): Stabilizer that affects carbonate alkalinity
• Total Dissolved Solids (TDS): All dissolved minerals in the water

LSI Ranges Explained

The LSI scale indicates your water's tendency to scale or corrode:

Purple Zone (LSI ≥ +0.31)

Scale-Forming: Water is oversaturated with calcium carbonate and will precipitate calcium onto surfaces, equipment, and plumbing. You may see cloudy water, rough pool surfaces, white deposits on tiles, and reduced equipment efficiency.

Action Required: Lower pH, reduce alkalinity, or partially drain and refill to reduce calcium hardness. Avoid adding calcium-based chlorine products.

Green Zone (0.00 to +0.30)

Balanced (Ideal): Water is perfectly saturated with calcium carbonate. This is the target range where water will neither scale nor corrode. Your pool surfaces, equipment, and plumbing are protected.

Maintenance: Monitor regularly and make minor adjustments to maintain this range as temperature and pH fluctuate.

Yellow Zone (-0.30 to -0.01)

Slightly Corrosive (Acceptable): Water is slightly undersaturated but close enough to balanced. pH will naturally rise toward the ceiling, which may bring LSI into the green zone.

Watch: Monitor pH rise and ensure it doesn't push LSI into purple when pH reaches its ceiling. Consider slight adjustments to calcium or alkalinity.

Red Zone (LSI ≤ -0.31)

Aggressive/Corrosive: Water is hungry for calcium and will dissolve it from any source available: plaster, grout, metal equipment, heaters, and heat exchangers. You may see etching on plaster, pitting on concrete, staining, and shortened equipment life.

Urgent Action: Raise calcium hardness, increase pH slightly, or increase alkalinity. Add calcium chloride to bring calcium levels up quickly.

Understanding pH Saturation (pHs)

The pHs value shown in the calculator is the "saturation pH" - the pH at which your water would have an LSI of exactly 0.0 (balanced) given your current temperature, calcium, alkalinity, and TDS. When your actual pH equals pHs, your water is perfectly balanced. When pH is higher than pHs, LSI becomes positive (scale-forming). When pH is lower than pHs, LSI becomes negative (corrosive).

pH Drift and Management

pH naturally tends to rise in pools due to several factors:

• Aeration and splashing (waterfalls, jets, fountains)
• Carbon dioxide escaping from the water
• Addition of chlorine (especially liquid chlorine)
• Bather load and organic contamination breaking down

The goal is to balance your chemistry so that even as pH drifts upward, your LSI stays in the acceptable range (green or yellow zone). If pH rises significantly above pHs, you'll move into the purple (scale-forming) zone and should add acid to bring pH back down.

Carbonate Alkalinity vs Total Alkalinity

Total Alkalinity (TA) measures all alkaline substances in your water, including carbonates, bicarbonates, and cyanurates. However, only carbonate alkalinity actually affects LSI. When you add cyanuric acid (CYA) to stabilize chlorine, it contributes to total alkalinity but not to carbonate alkalinity.

Our calculator automatically adjusts for this by subtracting approximately 33% of your CYA level from total alkalinity to get carbonate alkalinity, which is then used in the LSI formula.

How to Use This LSI Calculator

1. Test Your Water: Use a reliable test kit or take a sample to a pool store for accurate readings of all six parameters.
2. Enter Current Values: Input your test results into the calculator fields. Use the help text for guidance on typical ranges.
3. Review Your LSI: The calculator will show your LSI value, color zone, and status immediately.
4. Check pH Ceiling: Compare your current pH to the pH ceiling. If they're close, your pH won't rise much more. If there's a gap, pH will naturally increase.
5. Read Recommendations: The calculator provides specific guidance based on your LSI zone and water parameters.
6. Make Adjustments: Follow the recommendations to bring your water into balance.
7. Retest and Recalculate: After making adjustments, wait 24 hours, retest, and recalculate to verify your water is balanced.

Common LSI Balancing Strategies

Lowering High LSI (Purple Zone)

• Lower pH: Add muriatic acid or dry acid to reduce pH. This is the quickest way to lower LSI.
• Reduce Alkalinity: Acid will lower both pH and alkalinity. Target alkalinity in the 60-80 ppm range if calcium is high.
• Avoid Calcium-Based Products: Use liquid chlorine instead of cal-hypo. Don't add calcium chloride.
• Partial Drain and Refill: If calcium hardness is extremely high (>500 ppm), dilution may be necessary.

Raising Low LSI (Red Zone)

• Add Calcium Hardness: Use calcium chloride to quickly raise calcium levels. Add 10 lbs per 10,000 gallons to raise calcium by approximately 100 ppm.
• Raise pH Slightly: If pH is below 7.4, raise it to 7.4-7.6 using soda ash (sodium carbonate).
• Increase Alkalinity: Add sodium bicarbonate (baking soda) to raise alkalinity. Target 80-120 ppm.
• Use Calcium-Based Chlorine: Cal-hypo will add both chlorine and calcium with each dose.

Maintaining Balanced LSI (Green/Yellow Zone)

• Test Weekly: LSI can shift as pH rises or temperature changes.
• Adjust pH as Needed: Keep pH in the 7.4-7.6 range for most pools.
• Balance Calcium and Alkalinity: Higher calcium allows lower alkalinity, and vice versa. Find a combination that keeps LSI balanced even when pH rises to the ceiling.
• Consider Temperature: Summer heat raises LSI, so you may need to lower pH or alkalinity seasonally.

Factors That Affect LSI

Temperature

For every 10°F increase in water temperature, LSI increases by approximately 0.1. This is why pools tend to scale more in summer and hot tubs with very high temperatures (100-104°F) are especially prone to scaling. Conversely, cold water is more corrosive.

pH

pH has the most dramatic effect on LSI. A 0.2 increase in pH raises LSI by approximately 0.2 as well. This is why pH management is critical—even small changes significantly impact water balance.

Calcium Hardness

Higher calcium increases LSI, making water more scale-forming. Lower calcium decreases LSI, making water more aggressive. The ideal range depends on your other parameters, but generally 200-400 ppm works for most pools. Plaster pools often target higher calcium (300-400 ppm) to protect the surface.

Total Alkalinity

Alkalinity acts as a pH buffer and contributes carbonate ions that affect LSI. Higher alkalinity increases LSI. However, very low alkalinity causes pH to be unstable and hard to maintain, so there's a practical lower limit (typically 60-80 ppm).

Cyanuric Acid (CYA)

CYA is essential for outdoor pools to prevent chlorine degradation from UV sunlight. However, CYA contributes to total alkalinity readings without contributing to carbonate alkalinity, which is why our calculator adjusts TA based on CYA levels. High CYA (>100 ppm) can make pH testing less accurate and chlorine less effective.

Total Dissolved Solids (TDS)

TDS has a minor effect on LSI compared to other parameters. Higher TDS slightly lowers LSI. Typical non-salt pools have TDS around 1,000-2,000 ppm. Very high TDS (>3,000 ppm) can cause water clarity issues and reduced chlorine efficiency, but the impact on LSI is small. Salt pools typically have TDS of 3,000-4,000 ppm from the salt alone.

Special Considerations

Saltwater Pools

Saltwater chlorine generators produce high-pH chlorine, causing pH to rise constantly. Saltwater pools typically require regular acid additions to maintain balanced LSI. Target the lower end of the pH range (7.2-7.4) to account for natural pH rise. Salt itself contributes to TDS but doesn't directly change LSI significantly.

Plaster Pools

Newly plastered pools require special startup chemistry to cure properly and prevent scaling or etching. Follow the Startup procedure carefully. After startup, plaster pools benefit from slightly higher calcium (300-400 ppm) and maintaining LSI in the 0.0 to +0.2 range to protect the surface without causing scaling.

Vinyl and Fiberglass Pools

These pools don't have calcium-rich surfaces to protect, so LSI is less critical. However, metal components (ladders, rails, heaters, pumps) can still corrode in aggressive water. Target LSI of -0.1 to +0.2, which is forgiving for these pool types while protecting equipment.

Hot Tubs and Spas

High temperatures (100-104°F) dramatically increase LSI, making spas prone to scaling even with moderate pH and calcium levels. Keep pH on the lower end (7.2-7.4), use lower calcium (150-250 ppm), and consider lower alkalinity (60-80 ppm) to compensate for the temperature effect. Test LSI frequently.

Common Mistakes and Misconceptions

Mistake 1: Focusing Only on pH

Many pool owners only adjust pH without considering LSI. pH of 7.5 can be corrosive in one pool and scale-forming in another, depending on calcium, alkalinity, and temperature. Always consider the complete picture.

Mistake 2: Ignoring Temperature Changes

Summer heat and winter cold significantly affect LSI. What was balanced in spring may be scale-forming in July. Recalculate LSI seasonally and adjust accordingly.

Mistake 3: Adding Chemicals Without Testing

Guessing or using outdated test results leads to imbalanced water. Test before adding chemicals, especially when making LSI-related adjustments.

Mistake 4: Using Inaccurate Test Kits

Cheap test strips or old reagents give unreliable results. Use a quality test kit (Taylor, LaMotte) or professional testing for accurate calcium, alkalinity, and pH readings.

Mistake 5: Over-Correcting

Make gradual adjustments and retest. Large chemical additions can overshoot the target and create new problems. Be patient.

Benefits of Maintaining Balanced LSI

Protects Pool Surfaces

Balanced water prevents etching and damage to plaster, pebble, tile, and grout. Your pool will look better and last longer.

Extends Equipment Life

Corrosive water damages heaters, heat exchangers, pumps, and metal fittings. Scaling water clogs plumbing and reduces flow. Balanced water keeps equipment running efficiently for years.

Improves Water Clarity

High LSI can cause cloudy water from calcium precipitation. Low LSI can cause staining from metal corrosion. Balanced LSI helps maintain crystal-clear water.

Reduces Chemical Costs

Balanced water requires fewer corrective chemical additions. You'll spend less time and money chasing chemistry problems.

Better Swimmer Comfort

Balanced water feels better on skin and eyes. Corrosive water can be harsh and irritating. Scale-forming water can leave deposits on skin and swimwear.

Professional Results

Pool service professionals use LSI to provide superior water balance. Using this calculator puts the same professional-grade analysis in your hands.

When to Recalculate LSI

• Weekly: During swimming season or active use
• After Chemical Adjustments: Wait 24 hours for water to mix, then retest and recalculate
• Seasonal Changes: Spring opening, summer heat waves, fall closing
• After Heavy Rain: Dilution from rainwater changes chemistry
• After Drain and Refill: New water has different chemistry than old water
• When Problems Appear: Scaling, staining, cloudy water, or equipment issues

Additional Resources

For more information on pool water chemistry, explore our other free tools:

• Pool Volume Calculator: Calculate your exact pool volume for chemical dosing
• Pool Resources: Articles, guides, and videos on pool maintenance and water chemistry