Calcite Saturation Index calculator
For pools and hot tubs. Enter your readings to see whether your water will deposit scale or dissolve calcium.
Water won't damage surfaces or leave deposits.
To balance it
- Inside your range, there is nothing to fix.
Enter pH, alkalinity, and calcium to see your water balance.
What you need
Readings from any test kit or strips: pH, total alkalinity, calcium hardness, and water temperature. Add cyanuric acid (CYA) and salt if you have them, because both change the result. Choose Hot tub for spa temperatures and ranges.
What the result means
- Balanced
- Water won't damage surfaces or leave deposits.
- Slightly Corrosive
- Water is slightly aggressive and may slowly affect surfaces.
- Slightly Scaling
- Water may leave light scale deposits over time.
- Corrosive
- Water may damage pool surfaces and equipment over time.
- Scale-Forming
- Water will deposit calcium scale on surfaces and equipment.
For pools, the balanced range is -0.5 to +0.5, and its top drops to +0.3 in water warmer than 85°F (29°C) or colder than 60°F (16°C). Hot tubs run lower: the top of the balanced range is +0.1 from 95°F (35°C), 0.0 from 100°F (38°C), and -0.1 from 104°F (40°C).
How it is calculated
The calculator uses the same formula as the PoolFu app: the Calcite Saturation Index with an ionic strength correction, after subtracting the alkalinity that cyanuric acid adds to your total alkalinity reading. The formula and worked examples are in the guide: Calcite Saturation Index (CSI), explained
Questions
Is CSI the same as LSI?
They measure the same thing: whether water is saturated with calcium carbonate. The CSI is the more precise form, because it accounts for the dissolved solids in the water, which matters most in salt pools.
Why does my salt pool show a negative CSI?
Salt pools usually run cyanuric acid at 60 to 90 ppm, which removes 20 to 30 ppm of carbonate alkalinity from the calculation, and salt raises ionic strength. A CSI of -0.3 to -0.5 in a salt pool at proper CYA is normal and balanced for the system.
Is it safe to swim when CSI is off?
A slightly corrosive or scale-forming pool is still safe to swim in. CSI is about protecting surfaces and equipment over months and years; focus on sanitizer and pH for safety.