poolchemistrycalc

Lower pH

Into 21,100 gal of water, add

2.64lb 1.20kg

HASA Dry Acid — sodium bisulfate, 93.5%. pH from 8.0 pH to 7.5 pH.

pH on one scale: the band 7.2–7.8, the ideal band 7.4–7.6, the reading now at 8.0 pH, and an arrow to 7.5 pH carrying 2.64 lb. reads now 8.0 pH target 7.5 pH ideal code 7.2 7.4 7.6 7.8 2.64 lb −0.5 pH with HASA Dry Acid

Your pool

feet, the long side
feet
inches of water, not wall height
inches of water
what your kit reads
where you want it

The answer, the drawings, every reading and every table on this page are worked for the values in these fields, and worked again as you type. The questions further down are answered for the example this page was published with.

At 21,100 gal

Every product that does this

Every product with a fetched strength that moves this reading by -0.5 pH. A row we can compute a quantity for comes first.
Product Add Also moves
HASA Dry Acid
sodium bisulfate 93.5% 		2.64 lb
1.20 kg		 Total alkalinity
−5.8 ppm
HASA Muriatic Acid
hydrochloric acid 31.4% 		no quantity — see the label Total alkalinity
moves it, by an amount no fetched source states

The manufacturer's own dose sentence reads: 0.5 lbs for each 0.1 pH unit reduction per 20,000 gals of water. The figures above are computed from the product's strength.

In steps, if you would rather

One dose split five ways. Add a step, run the pump, and test again before the next.
To reach Add Total added Also moves
7.9 pH 8.4 oz 8.4 oz −1.2 ppm
7.8 pH 8.4 oz 1.05 lb −2.3 ppm
7.7 pH 8.4 oz 1.58 lb −3.5 ppm
7.6 pH 8.4 oz 2.11 lb −4.7 ppm
7.5 pH 8.4 oz 2.64 lb −5.8 ppm

Where the band comes from

Source Range
COMAR 10.17.01.45 A(1)
conventional pool 		7.2–7.8 pH
Florida 64E-9.004 (1)(d)1
conventional pool 		7.0–7.8 pH
COMAR 10.17.01.45 A(1)
spa or hot tub 		7.2–7.8 pH

Pool and spa rules are adopted and amended state by state, and most of the ranges here are written for public pools. Confirm every figure against the code your jurisdiction has adopted and against your own product label.

Always dilute acid before it goes in: ten parts water to one part acid, into the water and never the other way round. Run the pump while you add it and for an hour afterwards, then test again before you add any more.

What people ask

How much acid lowers pH in a 21,100 gal pool?
2.64 lb of HASA Dry Acid for a 0.5 pH change, computed from the manufacturer's own step of 0.5 lbs for each 0.1 pH unit reduction per 20,000 gals of water. HASA's own limit on muriatic acid is 1.00 qt per 20,000 gal in 24 hours, so a large correction is several doses rather than one.
Can baking soda lower pH in a pool?
No. On the products this site has fetched, sodium bicarbonate appears only as something that raises alkalinity; it is never listed as lowering pH. The products listed above are the ones whose own labels state that they do.
Do I adjust pH or shock first?
Alkalinity first, then pH, then chlorine. Alkalinity buffers pH, so a pH you correct before the alkalinity moves again when the alkalinity does. Add one thing at a time and test again before the next; no product page we have read states how long to wait.

Other adjustments

Updated September 2026

Sources and dates

Ranges are quoted from the code named beside them, product strengths from the manufacturer's own product page, each retrieved on the date shown.