ChlorBenchPool volume and chemical doses worked out, not guessed.

pH, Alkalinity & Hardness

Doses to move each one, and the order to move them in.

Balancing pool water is three adjustments in a fixed order, and the order matters more than the doses.

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 will spend a fortnight chasing your own tail with a test kit.

Target ranges

Total alkalinity 80–120 ppm. pH 7.2–7.8. Calcium hardness 200–400 ppm.

Everything drags something else with it

Sodium bicarbonate raises alkalinity and nudges pH up. Soda ash raises pH and takes alkalinity with it. Muriatic acid lowers both — it neutralizes bicarbonate, and bicarbonate is most of what total alkalinity measures.

Which is why "high pH with low alkalinity" cannot be fixed with a single chemical. The fix is aeration, which drives 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

Add half the calculated dose, wait a turnover, test, add the rest. Overshooting alkalinity or hardness is far harder to undo than undershooting, because both come down only by draining and refilling.

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 at the top of the range already, do not add more "for safety".

4 articles

Adjusting a 18 × 36 ft oval pool: pH up by 10

Soda ash (sodium carbonate) to add
571.07oz
moves pH by 10 in 19,036 gallons
Order of operations1. Alkalinity80–120 ppm2. pH7.2–7.83. Hardness200–400 ppm571.07 oz of Soda ash (sodium carbonate)raises total alkalinity at the same time
Dose571.07oz
Pool volume36 × 18 ft oval, 4–6 ft deep (avg 5 ft)19,036gallons
Change requestedpH10
Standard ratemoves 0.2 pH6 oz per 10,000 gal
Target range7.2–7.8
Side effectraises total alkalinity at the same time
— 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

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.
  • Soda ash (sodium carbonate) raises total alkalinity at the same time.
  • 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 pH units; 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.

pH, alkalinity and hardness

500 calculations · browse all