ChlorBenchPool volume and chemical doses worked out, not guessed.

Adjusting a 16 × 36 ft rectangular pool: alkalinity up by 10

Sodium bicarbonate to add
2.59lb
moves ppm alkalinity by 10 ppm in 17,235 gallons
Order of operations1. Alkalinity80–120 ppm2. pH7.2–7.83. Hardness200–400 ppm2.59 lb of Sodium bicarbonateraises pH very slightly as well
Dose2.59lb
Pool volume36 × 16 ft rectangular, 3–5 ft deep (avg 4 ft)17,235gallons
Change requestedppm alkalinity10 ppm
Standard ratemoves 10 ppm alkalinity1.5 lb per 10,000 gal
Target range80–120 ppm
Side effectraises pH very slightly as well
— 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
16 × 36 ft rectangular
Shallow end
3 ft
Deep end
5 ft
Adjustment
alkalinity 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 16 × 36 ft rectangular pool at 3–5 ft holds 17,235 gallons. Moving alkalinity up by 10 takes 2.59 lb of Sodium bicarbonate.

Target range: 80–120.

What it drags along with it

Sodium bicarbonate raises pH very slightly as well.

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.
  • Sodium bicarbonate raises pH very slightly as well.
  • 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.
  • Test with a decent kit rather than strips for anything you are dosing off. Strips are fine for a daily glance and poor for arithmetic.

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