Pool CSI and LSI, explained

The calcium saturation index (CSI) tells you whether your water wants to dissolve calcium or deposit it. Near zero is balanced. Below zero the water is corrosive: it slowly etches plaster and grout and attacks metal. Above zero it is scaling: white crust on tile, in heaters and on salt cells. Keep plaster, pebble and tile pools between −0.3 and +0.3; vinyl, fiberglass and acrylic can sit between −0.6 and +0.6.

CSI comes from six readings: pH, total alkalinity, calcium hardness, CYA, salt and water temperature.

What the number means

CSI	Plaster, pebble, tile	Vinyl, fiberglass, acrylic
−0.3 to +0.3	balanced	balanced
−0.6 to −0.3, or +0.3 to +0.6	watch: could become corrosive or scaling	balanced
−1.0 to −0.6, or +0.6 to +1.0	act: likely corrosive or scaling	watch
below −1.0 or above +1.0	act	act

Vinyl, fiberglass and acrylic have no calcium to lose, so they get the wider range. Heaters and salt cells still scale, whatever the pool is made of.

Worked examples

Readings	CSI	On plaster	On vinyl
pH 7.5, TA 70, CH 350, CYA 50, salt 1,000, 80°F	−0.18	balanced	balanced
pH 7.8, TA 80, CH 300, CYA 40, no salt, 84°F	+0.26	balanced, near the top	balanced
pH 7.2, TA 70, CH 200, CYA 40, no salt, 70°F	−0.66	act: corrosive	watch

The third row is why the surface matters: the same water that etches plaster is only worth watching in a vinyl pool.

Temperature moves CSI too. The water in the first row reads −0.36 at 60°F, −0.18 at 80°F and −0.05 at 95°F. Cold water leans corrosive and warm water leans scaling, so water that is balanced in spring can drift by midsummer.

How to bring CSI back into range

1. Test all six readings. pH, TA, calcium hardness, CYA, salt (for a salt pool) and water temperature.
2. Change pH first. It is the quickest lever: raising pH raises CSI and lowering it lowers CSI. Stay within 7.2 to 7.8.
3. Adjust alkalinity or calcium if pH alone isn't enough. Raising either one raises CSI.
4. Lower calcium by replacing water. No product removes calcium, so a CSI that is high because of calcium needs a partial drain.
5. Retest as the season warms. The same water scores higher in summer heat.

Whenever you add a product, add it separately, with the pump running, and wait 30 minutes before the next.

Calcium hardness by surface

Surface	Calcium hardness (ppm)	Aim for
Plaster, pebble, tile	250 to 650	350
Vinyl	up to 550	no need to add any
Fiberglass	up to 300	no need to add any
Acrylic hot tub	100 to 250	150
Any pool with a gas heater	at least 200	250

Vinyl and fiberglass pools don't need calcium, unless a gas heater is on the system: it needs at least 200 ppm to protect its heat exchanger. To raise calcium, dissolve calcium chloride in a bucket of pool water first, add it separately, with the pump running, in portions, and not while pH is above 7.8.

CSI and LSI

The Langelier saturation index (LSI) answers the same question with an older method built on lookup tables. CSI uses a formula that also allows for salt and for the part of alkalinity that is really stabilizer. You read them the same way: near zero is balanced.

Questions people ask

What CSI causes scale?

On plaster, pebble and tile, above +0.3 the water can start to scale, and above +0.6 it likely will. Keep it between −0.3 and +0.3.

Is a low CSI a problem for vinyl or fiberglass?

Less so: those surfaces have no calcium to lose, so their range runs to −0.6. Below −1.0, act anyway, because corrosive water still attacks metal fittings and heaters.

My plaster pool reads −0.5. Is that an emergency?

No. It's in the watch range: the water could become corrosive. Nudge pH toward 7.6 and retest this week.

Does salt change CSI?

A little. Salt pulls the index slightly down, so enter your real salt reading for an accurate number. The target range is the same.

For a plan that puts pH, alkalinity and calcium in the right order, enter your readings in the water check.