Calcite Saturation Index (CSI) for Pools and Hot Tubs

The Calcite Saturation IndexCalcite Saturation IndexA more precise form of the saturation index that also accounts for the dissolved solids in the water (ionic strength), which matters most in salt pools. (CSI) tells you whether your water will deposit calcium carbonate or dissolve it. Calcium carbonate is the mineral that forms scale. Water with a positive CSI tends to leave scale on tile, heaters, and jets. Water with a negative CSI tends to dissolve calcium from plaster, grout, and concrete. In a pool, water close to zero does neither; hot tubs need a negative CSI, explained below.

Try it with your own readings: Calcite Saturation Index calculator.

CSI and LSI

The Langelier Saturation IndexLangelier Saturation IndexThe original saturation index, from 1936. It predicts whether water will deposit scale (positive) or dissolve calcium (negative); zero means exactly saturated. (LSI) measures the same thing. Wilfred Langelier introduced it in 1936 for municipal water systems. The CSI is a more precise form of the calculation: it accounts for the dissolved solids in the water, its ionic strength, which matters most in salt pools.

Both read the same way. Zero means the water is exactly saturated with calcium carbonate, positive means it tends to deposit scale, and negative means it tends to dissolve calcium. PoolFu calculates the CSI.

How CSI Is Calculated

PoolFu uses this formula:

where:

• pH is your measured pH
• CH is Calcium HardnessCalcium HardnessThe amount of dissolved calcium in your water. Too low and water becomes corrosive; too high and you get scale buildup. in ppmppmThe standard unit for measuring chemical concentrations in pool water. 1 ppm equals about 1 drop in 13 gallons.
• CA is Carbonate AlkalinityCarbonate AlkalinityThe portion of total alkalinity that actually affects water balance. In stabilized pools, subtract about a third of your CYA from total alkalinity to get this number. in ppm, explained below
• I is the ionic strength of the water
• T is the water temperature in kelvin

Ionic strength is estimated from your calcium, alkalinity, and salt readings:

TA is your Total AlkalinityTotal AlkalinityA measure of your water’s ability to resist pH changes. Think of it as a buffer that keeps your pH stable. Measured in ppm. reading, and extra salt is your salt level minus 1.1678 times your calcium hardness, counted only when it is above zero.

How each reading moves CSI, using the first example pool below:

Carbonate Alkalinity and the CYA Correction

When you test total alkalinity, your kit measures everything that can neutralize acid. Not all of that alkalinity takes part in water balance.

Your TA reading includes:

• Carbonate alkalinity: the alkalinity that buffers pH and takes part in calcium carbonate chemistry
• CyanurateCyanurateThe form cyanuric acid takes when dissolved in water. It binds to chlorine, protecting it from sunlight but slowing its sanitizing speed. alkalinity: ions from your Cyanuric AcidCyanuric AcidAlso called stabilizer or conditioner. Protects chlorine from being destroyed by sunlight. Essential for outdoor pools, but too much reduces chlorine’s killing power. (stabilizer) that show up on the test but take no part in calcium carbonate chemistry
• Borate alkalinity: if you use borates, a small amount of your reading comes from them

For the CSI, PoolFu subtracts the cyanurate and borate contributions from your TA reading. The cyanurate part depends on your CYA level, pH, and temperature (the Wojtowicz method):

At pH 7.5 and 82°F (28°C), about a third of your CYA level shows up in the TA test: 40 ppm of CYA adds 13 ppm. Borates add far less, under 2 ppm for 50 ppm of borates at normal pool pH.

This is why pools with higher CYA show a more negative CSI. That is the math working correctly.

Worked Examples

A chlorine pool at 82°F (28°C): pH 7.5, TA 80 ppm, calcium hardness 300 ppm, CYA 40 ppm, no salt.

The CYA correction is 13.0 ppm, so carbonate alkalinity is 67.0 ppm. CSI is -0.06. The water is balanced.

A salt pool at 84°F (29°C): pH 7.6, TA 70 ppm, calcium hardness 350 ppm, CYA 70 ppm, salt 3,200 ppm.

The CYA correction is 23.6 ppm, so carbonate alkalinity is 46.4 ppm. CSI is -0.25. The same water without salt would read -0.04, so the salt alone lowers CSI by 0.21. This pool is balanced, and its negative CSI is expected for a salt pool.

A hot tub at 102°F (39°C) on bromine: pH 7.6, TA 80 ppm, calcium hardness 150 ppm, no CYA, no salt.

There is no CYA correction, so carbonate alkalinity is the full 80 ppm. CSI is +0.04. That would be balanced in a pool. In a hot tub at this temperature the target is -0.3 and the highest balanced value is 0.0, so this water leans toward scale.

The Right Range for Your Water

For pools:

In pools warmer than 85°F (29°C) or colder than 60°F (16°C), the top of the balanced range drops to +0.3.

Calcium carbonate dissolves less readily as water gets warmer, so a CSI that is balanced in a pool can deposit scale in a hot tub. For hot tubs at 95°F (35°C) and above:

The bottom of the balanced range is -0.5 for every temperature.

Salt Pools Run Negative

SWGSWGShort for Salt Water Generator. Converts dissolved salt into chlorine through electrolysis, so you don’t have to add chlorine manually. pools usually run CYA at 60 to 90 ppm, which removes 20 to 30 ppm of carbonate alkalinity from the calculation. Salt also raises ionic strength. Both lower CSI. A CSI of -0.3 to -0.5 in a salt pool at proper CYA is normal and balanced for the system. Don’t push calcium beyond the safe range for your surface to make CSI positive.

In PoolFu, the Water Balance detail tells you when a negative CSI is expected for a salt pool with CYA between 60 and 90 ppm.

Your Surface Changes What a Number Means

Two pools with identical readings have the same CSI, whatever their SurfaceSurfaceYour pool’s interior finish. Plaster and pebble surfaces need higher calcium to prevent etching. Vinyl liners and fiberglass are non-mineral, so calcium targets are lower but still matter for equipment protection.. The consequences differ:

Between -0.3 and +0.3, every surface is in its best range. A plaster pool at -0.45 has more reason to adjust than a vinyl pool at the same reading.

When to Adjust

Inside your range, there is nothing to fix. A pool at -0.3 and a pool at +0.2 are both balanced, and neither needs a change.

• CSI too low: raise pH toward 7.6 to 7.7 first. Raise calcium only within the safe range for your surface.
• CSI too high: lower pH first. Calcium hardness only comes down by replacing some of the water with lower-calcium water.
• Cold water: below 50°F (10°C), balanced water may not be reachable within safe calcium levels. That is expected in winter.

CSI is about protecting surfaces and equipment over months and years. A slightly corrosive or scale-forming pool is still safe to swim in. Focus on sanitizer and pH for safety, and treat CSI as maintenance.

CSI in PoolFu

The Water Balance card shows your CSI on a scale from -0.5 to +0.5. Tap it to see the full calculation: your readings, the carbonate alkalinity after the CYA correction, and what the number means for your pool.