Free tool · Pool water
Pool LSI calculator
A free pool LSI calculator. Enter your test-kit readings to see the Langelier Saturation Index, what it means for the water, and what to add for your pool's gallons. No sign-up. It runs the same math as the water balance card in our app.
Calculator
Enter your readings
pH, alkalinity and calcium are required. Leave the rest blank and the calculator assumes 80°F water, no stabilizer and 1,000 ppm TDS, like the app does.
Worked example
One pool, step by step
A 15,000-gallon chlorine pool tests at pH 7.8, 86°F, calcium 200, alkalinity 120, CYA 50 and free chlorine 1. TDS is not tested, so it counts as 1,000.
- Carbonate alkalinity = 120 − 50 × 0.35 = 102.5 ppm.
- Temperature factor at 86°F = 0.72.
- Calcium factor = log10(200) − 0.4 = 1.90.
- Alkalinity factor = log10(102.5) = 2.01.
- LSI = 7.8 + 0.72 + 1.90 + 2.01 − 12.19 = +0.24. That is balanced.
What the calculator says to add
- 44 fl oz Liquid chlorine 12.5%. FC 1 is low. Brings it to about 4 ppm.
- 27 fl oz Muriatic acid 31.45%. pH 7.8 is high. Brings it to about 7.5.
- 7.5 lb Calcium chloride 77%. Calcium 200 is low. Brings it to about 300. Capped for one visit; retest next time.
Balanced water can still need chemicals. Here chlorine, pH and calcium are each out of range.
For comparison, a pool at pH 7.6, 84°F, calcium 300, alkalinity 90, CYA 60 and TDS 1,000 comes out at +0.03: balanced.
The math
How the LSI calculator works
LSI = pH + temperature factor + calcium factor + alkalinity factor − TDS factor. Zero means the water is saturated with calcium carbonate: it neither dissolves surfaces nor leaves scale.
| Factor | How it is figured | What it tells you |
|---|---|---|
| pH | Used as read. | The biggest lever. A 0.1 change in pH moves the LSI by 0.1. |
| Temperature | From a standard table: 60°F = 0.4, 76°F = 0.6, 84°F = 0.7, 94°F = 0.8. Values in between are interpolated. | Warm water scales more. The same pool reads lower in winter. |
| Calcium hardness | log10(CH) − 0.4. 200 ppm = 1.90, 400 ppm = 2.20. | Doubling calcium adds about 0.3. |
| Alkalinity | log10 of carbonate alkalinity. Carbonate alkalinity = TA − CYA × a pH factor (0.23 at pH 7.0, 0.33 at 7.6, 0.36 at 8.0). | CYA reads as alkalinity on a test kit. Without this correction, a stabilized pool looks more scaling than it is. |
| TDS | 12.10 under 1,000 ppm; 12.19 at 1,000; 12.29 at 2,000; 12.35 at 3,000; 12.41 at 4,000. | More dissolved solids lower the LSI a little. In a salt pool, TDS is about the salt reading plus 1,000. |
Next steps
What to do at each LSI range
| LSI | What the water does | What to do |
|---|---|---|
| Below −0.30 Corrosive | Pulls calcium out of plaster and grout, etches surfaces, and attacks metal such as heater parts. | Raise alkalinity first with baking soda; it steadies pH. Then raise pH with soda ash, and calcium with calcium chloride. |
| −0.30 to +0.30 Balanced | Neither etches nor scales. | Keep chlorine in range and retest at the next visit. |
| Above +0.30 Scale-forming | Leaves calcium scale on tile and inside heaters, and can cloud the water. | Lower pH and alkalinity with acid. High calcium only comes down with a partial drain. |
How the app flags it. In 4Home Pros, an LSI past ±0.5 raises an alert for the owner, and past ±0.8 the alert is marked Act now.
Dose rates
What the doses are based on
Rates per 10,000 gallons, scaled to your pool:
- Liquid chlorine 12.5%: 10.7 fl oz per 1 ppm (cal-hypo 2.0 oz, trichlor 1.5 oz, dichlor 2.4 oz). Free chlorine never targets less than 7.5% of CYA.
- Muriatic acid: 26 fl oz per 10 ppm of alkalinity, or 12 fl oz per 0.2 of pH. You get one acid dose, sized to the larger need.
- Baking soda: 1.5 lb per 10 ppm of alkalinity. Soda ash: 6 oz per 0.2 of pH.
- Calcium chloride: 1.25 lb per 10 ppm. Stabilizer: 13 oz per 10 ppm. Salt: 0.0834 lb per 1 ppm.
- Most per visit: 32 fl oz acid, 3 lb baking soda, 5 lb calcium chloride, 40 oz stabilizer, and a 10 ppm chlorine rise.
Don't know the gallons? Length × width × average depth × 7.5, in feet. For a round or oval pool, use 5.9.
In the app
Do this at every stop, not just once
In 4Home Pros pool service software, this calculator is part of the visit checklist. Your tech enters the test-kit readings, sees the LSI, and ticks what went in. The pool's gallons stay saved on the site. It's on the free plan. You can even log the visit by chat with our AI connector.
Other calculators are worth a look too. Orenda has its calculator on the web and in a free app[1]. Skimmer's LSI calculator, powered by Orenda, works inside Skimmer[2] on its Scaling Up plan[3]. See how we compare with Skimmer.
FAQ
Questions
What is a good LSI for a pool?
Between −0.30 and +0.30 counts as balanced. Below that, water is corrosive and can etch plaster and attack metal. Above it, water is scale-forming and can leave scale on tile and in heaters.
Does cyanuric acid (CYA) change the LSI?
Yes, indirectly. A test kit counts part of the CYA as alkalinity. The calculator subtracts that share (about a third at pH 7.6) to get carbonate alkalinity. Skip the CYA and the LSI reads too high.
How do I use it for a salt pool?
Choose Salt pool and enter the salt reading. If TDS is blank, the calculator uses salt + 1,000 ppm. Salt pools also get lower targets for alkalinity (60–80) and higher targets for CYA (60–80).
Why does my LSI drop in winter?
Temperature is part of the formula. The factor is 0.7 at 84°F and 0.4 at 60°F, so the same water reads about 0.3 lower when it cools down.
Is this the same math as the 4Home Pros app?
Yes. Same formula, tables, targets, dose rates and per-visit limits. In the app, the owner can change the targets and how wide "balanced" is (±0.3 by default).
Get the LSI and doses at every stop, free.
Free for your first 50 job sites. No card. Same account on iPhone, Android and the web.