How Much Chlorine

How to Lower pH in a Pool (Approximate Muriatic Acid Amounts)

By Tam Vu · Updated

On this page
  1. What pH to aim for
  2. Approximate acid dose
  3. Steps
  4. Safety and the label
  5. Sources

Unlike chlorine or baking soda, pH has no simple formula that works for every pool. The same amount of acid moves pH by different amounts depending on where it starts and on your water. This page gives an approximate starting dose from a county health department chart, to be followed by a retest.

What pH to aim for

  • CDC: keep pool and hot tub pH between 7.0 and 7.8 (CDC).
  • Columbus Public Health (public pools): minimum 7.2, ideal 7.4 to 7.6, maximum 7.8 (Columbus Public Health).

The levels chart shows both with the other readings.

Approximate acid dose

Yakima County's chemical dosing sheet gives a rule of thumb: about 12 fluid ounces of muriatic acid (35%) lowers pH by roughly 0.2 in 10,000 gallons. The sheet notes the result depends on the starting pH, so it is an estimate only (Yakima County).

Estimate the acid

The pH values above are examples. This scales the Yakima County figure in proportion (12 fl oz of 35% acid per 0.2 pH per 10,000 gallons). Your water may respond differently, so add half, circulate, and test again.

Table: 35% muriatic acid per 0.2 drop in pH

Pool size Approximate fluid ounces
5,000 gallons 6
10,000 gallons 12
15,000 gallons 18
20,000 gallons 24
30,000 gallons 36

These are the Yakima figure scaled in proportion to volume. Example: a 15,000-gallon pool at pH 7.8 aiming for 7.4 needs a drop of 0.4, which is two 0.2 steps of 18 fl oz, about 36 fl oz in total. Add about 18 fl oz first.

Steps

  1. Test pH with a DPD kit if you can. The CDC says DPD kits are the most accurate and strips are less accurate.
  2. Work out a dose with the calculator above. Check the strength on your acid bottle: it may not be 35%, and a weaker acid needs proportionally more.
  3. Add about half the dose, pouring the acid into the water, with the pump running.
  4. Wait for circulation, then test again before adding more.
  5. Stop when pH is in range. It is easier to add a little more later than to undo too much.

Safety and the label

Muriatic acid is a strong acid. Virginia's Department of Health says to mix chemicals only as the label directs, add chemicals to water and never water to chemicals, never store liquid chemicals above dry ones, keep products in their original containers, and wear the protective gear the label lists (VDH). The label on your product is the final word.

Sources