Pool Water Chemistry Calculator — Free LSI & Dosing Tool for UK Engineers
Calculate the Langelier Saturation Index and get chlorine dosing recommendations for any pool or hot tub. Free, instant, no signup required.
Enter your readings
All fields required except TDS (defaults to 1,000 mg/L if left blank).
How the Langelier Saturation Index Works
The Langelier Saturation Index (LSI) predicts whether your pool water will deposit scale or dissolve calcium from surfaces. It combines five measurements — pH, temperature, calcium hardness, total alkalinity, and total dissolved solids — into a single number.
An LSI between −0.3 and +0.3 means balanced water. Below −0.3 the water is corrosive: it attacks grout, plaster, and metal fittings. Above +0.3 it's scale-forming: calcium deposits build up on heat exchangers, tiles, and pipework.
This calculator uses the standard Langelier formula: LSI = pH − pHs, where pHs (the saturation pH) is derived from temperature, calcium hardness, alkalinity, and TDS factors. The formula is:
pHs = (9.3 + A + B) − (C + D)
- A = (log₁₀[TDS] − 1) / 10
- B = −13.12 × log₁₀(°C + 273) + 34.55
- C = log₁₀[Calcium hardness] − 0.4
- D = log₁₀[Total alkalinity]
Chlorine Dosing Guidance
When free chlorine is below 1.5 mg/L, the calculator recommends dosing amounts for three common UK chemical products. Dosing rates are based on standard concentrations:
- Sodium hypochlorite (14–15% w/v) — the most common liquid chlorine for UK service engineers. ~7.2 mL per m³ per 1 mg/L chlorine increase.
- Calcium hypochlorite (65%) — granular chlorine. ~1.5 g per m³ per 1 mg/L increase. Adds calcium — check hardness.
- NaDCC granules (56%) — stabilised chlorine. ~1.8 g per m³ per 1 mg/L increase. Adds cyanuric acid — monitor stabiliser levels.
These are starting-point calculations. Always dose in stages with the pump running and retest after 30 minutes of circulation. Site-specific factors (bather load, sunlight exposure, water temperature) affect consumption rates.
When to Use This Calculator
Run the LSI calculation at every service visit where you test alkalinity and calcium hardness — at minimum, monthly. Use the chlorine dosing section whenever free chlorine is below the PWTAG guideline range of 0.5–3.0 mg/L.
For a breakdown of all PWTAG water quality parameters and testing frequencies, see our complete guide to swimming pool water testing.
Automate your water chemistry workflow
PoolRound will log readings on-site, calculate LSI automatically, and generate PWTAG-compliant service reports — no manual formulas.