Free Calculator · Published October 2026
Langelier Saturation Index Calculator: Scale or Not
Enter pH, temperature, calcium hardness, alkalinity, and TDS. See whether this approximation points to scale or to dissolving calcium carbonate.
Built by SubcontractorHub — the software plumbing contractors use to quote, finance, and run every job.
Book a DemoA Langelier Saturation Index Calculator Is an Index, Not a Rate
pH alone is the well water pH page. Blue-green stains and neutralizer media are acidic well water. This Langelier saturation index calculator uses the rest of the mineral line.
The field or lab pH you will subtract from pHs. Say which one you used.
Converted to Celsius inside the formula. Cold well water is not room temperature.
Important: All calculator results are ballpark estimates
All calculations are estimates based on historical information and should be verified by the user. This tool is provided as a free service for planning purposes only and is not a substitute for a professional assessment, a supplier quote, or applicable local code requirements.
What the Langelier Saturation Index Calculator Is Doing
pH minus a calculated saturation pH
The index is measured pH minus pHs. pHs comes from temperature, calcium hardness as CaCO3, alkalinity as CaCO3, and TDS, using the approximation printed in the page comment and in the factors under the result. A positive Langelier saturation index means this math says calcium carbonate tends to come out of solution. A negative index means it tends to dissolve. Zero is the balance line of the formula, not a promise that a copper pipe is safe.
Why the units matter
Calcium as the ion is not calcium hardness as CaCO3. Grains per gallon are not milligrams per liter. The water hardness converter is the place to move between ppm and grains before you type a number here. Alkalinity has to be as CaCO3 too. If the lab sheet uses a different convention, convert it first or the index is someone else's water.
Worked examples
Same formula as the form. The band is the planning label, not a code section.
| Case | pHs | LSI |
|---|---|---|
| Low pH, cool, low hardness | 9.21 | -2.81 |
| Mid pH, moderate minerals | 7.86 | -0.26 |
| Higher pH and hardness | 7.19 | 1.01 |
What to do with the sign
A clearly negative result belongs next to the corrosion conversation: pH, copper stains, and whether a neutralizer goes ahead of a softener and a UV. The acid neutralizer sizing calculator is that equipment question. A clearly positive result belongs next to scale: heaters, valves, and membranes. This page does not pick the media and does not estimate a dose. Copper in water is the stain page when the symptom is blue-green, which can be corrosion even when a single pH reading looked acceptable.
Temperature and the season
The formula uses Celsius inside the logarithm, after you type Fahrenheit. A cold well and a hot heater loop are not the same index even when pH, hardness, alkalinity, and TDS match. Run the number at the temperature of the water you are judging. A heater that scales can sit on water that looked balanced at the well head. Do not average the two temperatures into one typed value and call it the house.
A softener does not replace the index
Softening swaps calcium and magnesium for sodium or potassium. That changes the calcium hardness you would type next time, and it can change how scale behaves in a heater. It does not, by itself, tell you the saturation pH. Rerun the five inputs on the treated water if you want the index after the softener. A negative result is still a reason to look at corrosion and at a neutralizer ahead of equipment that fouls at low pH. A positive result is still a reason to look at scale. Neither sign is a media dose.
What the bands are for
Below −0.5 this page says the approximation points to the corrosive side of calcium carbonate. Above +0.5 it points to the scale-forming side. Between those labels the result is near the zero line of the formula. The cut points are planning labels so a dealer can sort a stack of lab sheets. They are not a plumbing code, not a pipe-life estimate, and not a reason to skip a coupon test or a manufacturer limit. If a later lab changes one input, throw the old label out and run the math again.
What to write on the job ticket
pH, temperature, calcium hardness as CaCO3, alkalinity as CaCO3, TDS, the pHs, and the LSI, plus whether the pH was a field meter or a lab. A water treatment dealer who saves the five inputs can rerun the index when a later test changes one of them. SubcontractorHub is a place to keep that sheet with the proposal. The index is still an approximation.
Frequently Asked Questions
What does a Langelier saturation index calculator tell you?
It estimates whether calcium carbonate tends to dissolve or to deposit, from pH, temperature, calcium hardness, alkalinity, and total dissolved solids. LSI is the measured pH minus the calculated saturation pH. It is a planning index, not a corrosion rate and not a promise about a particular pipe.
Which units does the formula expect?
Calcium hardness and alkalinity are milligrams per liter as calcium carbonate. TDS is milligrams per liter. Temperature is entered in Fahrenheit and converted to Celsius inside the formula. A lab that reports calcium as the ion, not as CaCO3, is a different number. Do not type that ion into this field.
Is a negative LSI the same as acidic well water?
Low pH and a negative LSI often travel together, and they are not the same measurement. pH is one input. LSI also uses hardness, alkalinity, TDS, and temperature. A neutralizer sized only from pH can still leave a water that this index calls unbalanced, or the reverse. Read both.
Does the index size a neutralizer or a softener?
No. It does not output calcite, soda ash, or resin. A negative result is a reason to look at corrosion and at the acid-neutralizer conversation. A positive result is a reason to look at scale and at softening or antiscalant where a membrane is involved. The equipment page is a different calculator.
How should the bands be used?
This page labels below −0.5 and above +0.5 as the corrosive side and the scale side of this approximation, and the middle as near the zero line. Those cut points are planning labels, not a plumbing code. A coupon test, a pipe inspection, or a manufacturer limit overrides the label.
Keep the Five Inputs on the Job
pH, temperature, hardness, alkalinity, and TDS should sit next to the equipment you quoted.
Get a Free DemoAll calculations are estimates based on historical information and should be verified by the user. This tool is provided as a free service for planning purposes only and is not a substitute for a professional assessment, a supplier quote, or applicable local code requirements. It does not predict a corrosion rate or size a neutralizer.