Adjusting a 10 × 20 ft rectangular pool: pH down by 40

Muriatic acid to add

2,992.21fl oz

moves pH by 40 in 5,984 gallons

Dose	2,992.21fl oz
Pool volume20 × 10 ft rectangular, 3–5 ft deep (avg 4 ft)	5,984gallons
Change requestedpH	40
Standard ratemoves 0.2 pH	25 fl oz per 10,000 gal
Target range	7.2–7.8
Side effect	lowers total alkalinity too — they move together
— 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

Pool size: 10 × 20 ft rectangular
Shallow end: 3 ft
Deep end: 5 ft
Adjustment: pH down
Change by: 40

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 10 × 20 ft rectangular pool at 3–5 ft holds 5,984 gallons. Moving pH down by 40 takes 2,992.21 fl oz of Muriatic acid.

Target range: 7.2–7.8.

What it drags along with it

Muriatic acid lowers total alkalinity too — they move together.

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.
Muriatic acid lowers total alkalinity too — they move together. That is why "high pH with low alkalinity" cannot be fixed with one chemical: acid brings both down together. The fix is aeration to drive pH up without touching alkalinity, then bicarbonate, repeated.
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 40 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.

Nearby sizes

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.
Cyanuric Acid Too High: Why the Pool Goes Green With Plenty of ChlorineA pool that reads 5 ppm chlorine and is still green almost always has one problem, and adding more chlorine is not the fix.
How Much Chlorine to Add: Dose per 10,000 Gallons, and What Each Product Also AddsThe dose is the easy half. What the product also adds to the water is the half that decides whether the pool still works in September.
How to Calculate Pool Volume in Gallons — the Number Every Dose MultipliesGet the gallons wrong by a fifth and every dose for the life of the pool is wrong by a fifth, in the same direction, forever.