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 quantity, the drawing, every product row and every step on this page are worked again as you type.

At 21,100 gal

Every product that does this

One product here has a quantity, and it makes the whole change of −0.5 pH on its own.
Product Add Also moves
HASA Dry Acid
sodium bisulfate 93.5% 		2.64 lb
1.20 kg		 −5.8 ppm
total alkalinity
HASA Muriatic Acid
hydrochloric acid 31.4% 		no quantity — see the label amount not stated
total alkalinity

The HASA Dry Acid label says: “0.5 lbs for each 0.1 pH unit reduction per 20,000 gals of water”.

In steps, if you would rather

Split into 5 equal steps of 8.4 oz each: add one, run the pump, and test again before the next.

To reach Total added Also moves
7.9 pH 8.4 oz −1.2 ppm
7.8 pH 1.05 lb −2.3 ppm
7.7 pH 1.58 lb −3.5 ppm
7.6 pH 2.11 lb −4.7 ppm
7.5 pH 2.64 lb −5.8 ppm

How much at once

The label's limit in 24 hours

1.05qt

Test again 30 minutes after each dose, before you add any more.

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 test again before you add any more.

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.

Other adjustments

Updated October 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.

This site publishes calculated water volumes and chemical quantities for home pool and spa owners. It is not a health, safety or professional service, and it does not replace the instructions printed on the product you are holding. Not affiliated with, endorsed by or sponsored by HASA, Pyrock Chemical, Pentair, Hayward, Sonoma Hot Tubs, the Centers for Disease Control and Prevention, or any state health department. Product names are the trademarks of their owners and appear here only to identify the strength of what is in the container.