Adjusting a 16 × 36 ft rectangular pool: alkalinity up by 2

Sodium bicarbonate to add

0.45lb

moves ppm alkalinity by 2 ppm in 15,081 gallons

Dose	0.45lb
Pool volume36 × 16 ft rectangular, 3–4 ft deep (avg 3.5 ft)	15,081gallons
Change requestedppm alkalinity	2 ppm
Standard ratemoves 10 ppm alkalinity	1.5 lb per 10,000 gal
Target range	80–120 ppm
Side effect	raises pH very slightly as well
— Order of operations —
1. Total alkalinitythe buffer — set this first or pH will not hold	80–120 ppm
2. pHadjust only after alkalinity is in range	7.2–7.8
3. Calcium hardnesslast, and it only goes up	200–400 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.
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 2 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.

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 3–4 ft holds 15,081 gallons. Moving alkalinity up by 2 takes 0.45 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.

Nearby sizes

Calcium Hardness — Water That Cannot Get Calcium Takes It From the PoolA vinyl pool tolerates almost any hardness. A plaster pool with soft water is quietly dissolving, and the damage is not reversible.
Total Alkalinity — The Number That Decides Whether pH Stays PutPeople chase pH and never fix the reason it moves. The reason is almost always alkalinity, and it is one adjustment away.
How Much Muriatic Acid to Lower pH, and Why Alkalinity Decides the AnswerThe same jug of acid moves pH by a full point in one pool and by a tenth in another. Total alkalinity is the reason, and dosing without it is guesswork.
Pool Water Balance: Alkalinity First, Then pH, Then HardnessAdjust pH while alkalinity is out of range and the alkalinity pulls pH straight back. That is a fortnight of chasing your own tail with a test kit.