LSI Calculator for Pools
Calculate your Langelier Saturation Index to protect your pool plaster and equipment from corrosive water and calcium scaling.
What Is the LSI? (Langelier Saturation Index Explained)
LSI stands for Langelier Saturation Index, named after Dr. Wilfred Langelier who developed the formula in 1936. Pool professionals adopted it as the most complete single measure of water balance available. Most pool owners check pH and chlorine and call it done. The Langelier Saturation Index looks at five factors simultaneously and tells you what the water is actually doing to your surfaces and equipment beneath the waterline.
Water can have a perfect pH of 7.4 and still be aggressively corroding your plaster, heater, and pump internals if calcium hardness and alkalinity are off. The LSI catches this.
The LSI Formula
LSI = Actual pH – pHs, where pHs = (9.3 + A + B) – (C + D)
- A = TDS correction: (log10(TDS) – 1) / 10
- B = Temperature: -13.12 x log10(temp in Kelvin) + 34.55
- C = Calcium hardness factor: log10(CH) – 0.4
- D = Alkalinity factor: log10(carbonate alkalinity)
The CYA Correction – What Most Calculators Miss
Cyanuric acid creates cyanurate alkalinity which does not participate in calcium carbonate equilibrium. Put total alkalinity straight into the formula without subtracting the cyanurate fraction and your LSI will be overstated – the water will look more balanced than it actually is.
Example: TA = 100 ppm, CYA = 50 ppm, pH = 7.4. CYA factor at 7.4 = 0.66. Cyanurate alk = 33 ppm. Carbonate alk = 67 ppm. Without correction: LSI = +0.17. With correction: LSI = -0.05. Looks fine without it but is actually corrosive.
LSI Values and What They Mean
| LSI value | Condition | What is happening | Action |
|---|---|---|---|
| Below -1.0 | Severely corrosive | Actively dissolving plaster, grout, metal | Fix today |
| -0.5 to -1.0 | Moderately corrosive | Plaster etching. Metal corroding | Fix within 1 to 2 days |
| -0.3 to -0.5 | Slightly corrosive | Slow surface degradation over months | Fix within a week |
| -0.3 to 0.0 | Slightly low, acceptable | Minimal impact. Many pros target this deliberately | Monitor closely |
| 0.0 to +0.3 | Balanced – ideal | Water stable. Equipment protected | Maintain |
| +0.3 to +0.5 | Borderline scale-forming | Calcium starting to precipitate | Correct soon |
| +0.5 to +1.0 | Scale-forming | Active deposits. Heater losing efficiency | Correct promptly |
| Above +1.0 | Severely scale-forming | Rapid scaling. Heater damage risk | Fix immediately |
What Drives Your LSI
pH
Goes directly into the formula. A 0.1 change in pH changes LSI by exactly 0.1 – the fastest and most sensitive lever. Target 7.4 to 7.6 for both LSI balance and chlorine effectiveness.
Calcium hardness
The foundation of LSI management. Unlike pH, CH stays where you put it. The single most common cause of corrosive LSI in residential pools is low calcium hardness. Water wants calcium – if you don’t provide it, it takes it from your plaster, grout, tile, and metal fittings. Target 200 to 400 ppm. Saltwater pools do well at 250 to 350 ppm.
Total alkalinity with CYA correction
Total alkalinity buffers pH. For outdoor pools with CYA, only the carbonate fraction counts. The calculator subtracts cyanurate alkalinity automatically. Target 80 to 120 ppm for chlorine pools. For SWG pools, 60 to 80 ppm is better – lower TA slows the pH drift between acid treatments.
Water temperature
Warmer water raises LSI. A pool balanced at 75°F in spring may be borderline scale-forming at 90°F in August with no chemistry changes. Check LSI at the start of each season.
TDS
Smallest effect of all – less than 0.05 across normal pool ranges. Most owners don’t have a TDS meter, which is why it is optional in this calculator.
Cyanuric acid (CYA)
Not in the original Langelier formula, which was designed for industrial water. The pool industry added the CYA correction because outdoor pools use stabiliser to protect chlorine from UV. High CYA means more of your total alkalinity is tied up as cyanurate and cannot contribute to carbonate equilibrium.
LSI for Saltwater Pools
Electrolysis constantly pushes pH up. Left unmanaged, pH drifts to 7.8 to 8.2 within days. At those levels LSI goes positive quickly. Run TA at 60 to 80 ppm and CH at 300 to 400 ppm. Check LSI at least twice a week with a running salt cell.
LSI for Spas and Hot Tubs
Hot water raises LSI by 0.3 to 0.5 compared to the same chemistry at pool temperature. Scale on the heater element is the top cause of spa heater failures. For spas, target 0.0 to +0.3. Drain and refill every 3 to 4 months as TDS builds fast in hot water.
How to Fix Corrosive Water (LSI Below -0.3)
- Add calcium chloride. 2 lbs per 10,000 gal raises CH by 16 ppm. Pre-dissolve in a bucket first. Calcium chloride is available at pool stores or as pure ice melt (90%+).
- Raise pH with soda ash if below 7.2. 6 oz per 10,000 gal moves pH by 0.1.
- Raise total alkalinity with baking soda if below 80 ppm. 1.5 lbs per 10,000 gal raises TA by 10 ppm.
How to Fix Scale-Forming Water (LSI Above +0.3)
- Lower pH with muriatic acid or dry acid. Add in the evening near a return jet. Never drop more than 0.5 units at once.
- Lower total alkalinity if above 120 ppm. Muriatic acid, pump off, near the deep end.
- Dilute calcium if above 400 ppm. Drain 20 to 25% and refill. No chemical removes calcium.
- Scale inhibitor as short-term protection. A pool sequestrant prevents deposits while you correct the chemistry.
Ideal LSI by Pool Type
| Pool type | Ideal LSI | Key notes |
|---|---|---|
| Plaster / marcite inground | -0.3 to +0.3, near 0 | Plaster vulnerable to both corrosion and scale |
| Fibreglass | -0.3 to +0.3 | Osmotic blistering more likely with corrosive water |
| Vinyl liner | -0.3 to +0.3 | Vinyl unaffected but all equipment is |
| Saltwater / SWG | -0.3 to +0.3, aim 0 to +0.2 | Run TA at 60 to 80 ppm. Check more often. |
| Spa / hot tub | 0.0 to +0.3 | Slightly positive protects heater elements |
| Indoor pool | -0.3 to +0.3 | No UV, no CYA needed. Simpler calculation. |
What Test Kit Do You Need?
The Taylor K-2006 covers pH, calcium hardness, total alkalinity, CYA, and chlorine – everything you need for a complete LSI calculation. Most basic test strips only cover pH, chlorine, and sometimes TA. TDS is optional and a TDS meter costs $15 to $30 if you want it.
Frequently Asked Questions
What does LSI stand for?
LSI stands for Langelier Saturation Index, named after Dr. Wilfred Langelier who developed the formula in 1936. It is the industry standard measure of overall pool water balance.
What is a good LSI for a pool?
Between -0.3 and +0.3, with 0 being perfect. Most pool professionals aim for -0.1 to +0.2. For spas, target 0.0 to +0.3.
What does a negative LSI mean?
Negative LSI means the water is corrosive. It will dissolve calcium from plaster, grout, and metal. At -0.8 and below, damage is measurable within a single season.
How do I raise my pool LSI?
Raise calcium hardness (fastest and most stable), raise pH, raise total alkalinity, or raise water temperature. Calcium hardness is the best long-term lever because it doesn’t fluctuate like pH.
Do I need CYA in my LSI calculation?
Yes, for any outdoor pool using stabiliser. Without CYA correction, LSI comes out too high. The correction is pH-dependent and applied automatically in this calculator.
Why does my LSI change in summer without chemistry changes?
Water temperature. A pool balanced at 68°F in April will have an LSI roughly 0.3 to 0.4 higher at 88°F in July with identical chemistry.
What is the difference between LSI and regular water balance?
Range-based balance checks each parameter independently. LSI combines them into one number that accounts for how they interact. You can pass every individual test and still have a problematic LSI if the combination is off.
What happens if I ignore LSI for a full season?
At -0.8 for a season: etched plaster, thinning grout, pitted metal, possible heater failure. At +0.8 for a season: heavy scale on tile, rough plaster, reduced heater efficiency, filter fouling.