ChlorBenchPool volume and chemical doses worked out, not guessed.

Adjusting a 20 × 44 ft rectangular pool: hardness up by 10

Calcium chloride to add
8.23lb
moves ppm hardness by 10 ppm in 49,371 gallons
Order of operations1. Alkalinity80–120 ppm2. pH7.2–7.83. Hardness200–400 ppm8.23 lb of Calcium chloridenothing else — hardness only comes down by dilution
Dose8.23lb
Pool volume44 × 20 ft rectangular, 5–10 ft deep (avg 7.5 ft)49,371gallons
Change requestedppm hardness10 ppm
Standard ratemoves 6 ppm hardness1 lb per 10,000 gal
Target range200–400 ppm
Side effectnothing else — hardness only comes down by dilution
— Order of operations —
1. Total alkalinitythe buffer — set this first or pH will not hold80–120 ppm
2. pHadjust only after alkalinity is in range7.2–7.8
3. Calcium hardnesslast, and it only goes up200–400 ppm
Pool size
20 × 44 ft rectangular
Shallow end
5 ft
Deep end
10 ft
Adjustment
hardness up
Change by
10

Order matters more than dose

Alkalinity first. Then pH. Then calcium hardness.

Total alkalinity is the buffer that holds pH steady. Adjust pH while alkalinity is out of range and the alkalinity simply pulls pH back where it was — and you spend a fortnight chasing your own tail with a test kit.

The dose

A 20 × 44 ft rectangular pool at 5–10 ft holds 49,371 gallons. Moving hardness up by 10 takes 8.23 lb of Calcium chloride.

Target range: 200–400.

What it drags along with it

Calcium chloride nothing else — hardness only comes down by dilution.

That is not a footnote. Acid brings pH and alkalinity down together, which is why "high pH with low alkalinity" cannot be fixed with a single chemical — the fix is aeration to drive pH up without touching alkalinity, then bicarbonate, repeated.

Never mix, always wait

One chemical at a time, hours apart, pump running. Muriatic acid and chlorine together produce chlorine gas, and people are hospitalised by that every summer.

Retest after a full turnover — six to eight hours on most residential pumps. A reading taken ten minutes after dosing tells you about the water beside the return jet.

Move in halves

If you need more than this dose, add half, wait a turnover, test, and add the rest. Overshooting alkalinity or hardness is far harder to undo than undershooting: both come down only by draining and refilling.

Notes

  • **Order matters more than dose.** Alkalinity first, then pH, then hardness. Total alkalinity is the buffer that holds pH steady — adjust pH while alkalinity is out of range and the alkalinity will simply pull pH back where it was, and you will spend a fortnight chasing it.
  • Calcium chloride nothing else — hardness only comes down by dilution.
  • Add it slowly with the pump running, and never mix chemicals in the same bucket — muriatic acid and chlorine together produce chlorine gas, and people are hospitalised by that every summer. One chemical at a time, hours apart.
  • Retest after a full turnover, not after ten minutes. Most residential pumps take 6–8 hours to turn the water over once, and a reading before that tells you about the water near the return jet.
  • Move in steps. This dose changes 10 ppm; if you need more than that, add half, wait a turnover, test, and add the rest. Overshooting alkalinity or hardness is far harder to undo than undershooting — both only come down by draining and refilling.
  • Calcium hardness is a one-way street. It never leaves the water except by dilution, and too much of it scales the heater, the tile line and the filter. If you are already at the top of the range, do not add more "for safety".

Nearby sizes

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