Water chemistry

How Much Muriatic Acid to Lower Pool pH (Dose Chart)

Dose chart for 31.45% muriatic acid by pool size to lower pH and total alkalinity, with two worked examples and safe pouring steps.

Muriatic acid does two jobs on a route. It lowers pH and it lowers total alkalinity (TA). The chart below assumes 31.45% acid. Doses are approximate, so test, dose in steps and retest.

Dose chart for 31.45% muriatic acid

Notes for the chart:

• 128 fl oz is 1 gallon.
• The pH dose assumes a start near 7.8 at typical alkalinity. Use the low end of the range when TA is low and the high end when TA is high.
• If your jug is a different strength, multiply the chart dose by 31.45 divided by the label percentage. Weaker acid takes more.
• Not sure of the volume? Use the pool volume tool first.

Read TA before you pour

pH and TA move together, and acid lowers both. If TA is high, say 140 ppm, pH keeps creeping back up after you knock it down. A pH-only dose treats the symptom. If TA is already low, acid for pH can push TA lower still, and pH starts to bounce.

Use this to pick the dose:

• pH high, TA in range (60 to 120 ppm): dose for pH from the middle column.
• pH high, TA high: dose for TA first, then retest pH.
• pH in range, TA high: dose for TA, and expect pH to drop with it. Aerate afterward if pH falls too far.

Worked example 1: lower pH on a 14,000 gallon pool

Readings: pH 7.8, TA 90. You want pH 7.6, a drop of 0.2. The multiplier is 14,000 / 10,000 = 1.4.

12 to 16 fl oz x 1.4 = about 17 to 22 fl oz.

TA at 90 is in the middle of the range, so start with 17 fl oz. Let the pump run, then retest. If pH is still above 7.6, add a smaller second dose.

Worked example 2: lower TA on a 12,000 gallon pool

Readings: pH 7.7, TA 140. You want TA near 100, a drop of 40 ppm. The multiplier is 1.2.

26 fl oz x 4 x 1.2 = about 125 fl oz, just under a gallon.

Do not pour 125 fl oz in one go. Here is a practical way to run it:

1. Add about half (roughly 62 fl oz) in the deep end, diluted into water if the label allows, with the pump running.
2. Let the water circulate, then retest TA and pH.
3. Add the second half if TA is still high.
4. pH will drop well below range after a large TA dose. Aim return jets upward, run a waterfall or spillover if the pool has one, and let aeration bring pH back up. This leaves TA where you set it.
5. Retest the next visit.

Do not fix the low pH with soda ash. Soda ash raises alkalinity again and undoes the work.

How to pour

1. Wear gloves and eye protection. Work upwind of the container.
2. Fill a clean plastic bucket partway with pool water. Add the acid slowly to the water, never the other way around.
3. Pour the diluted acid slowly into the deep end near a return, with the pump running.
4. Keep the acid away from the skimmer, especially if a trichlor feeder or tabs are in there.
5. Cap the jug and store it upright, away from every chlorine product.

Check the label for how long swimmers stay out and how long to wait before retesting. Many techs retest after the water has circulated for a while, but the label controls.

Mistakes

• Dosing from pH alone when TA is 140 or higher.
• Pouring the full calculated dose in one pass. Dose in halves when you are far from target.
• Using the 31.45% numbers on a jug with a different percentage.
• Adding acid and liquid chlorine in the same spot at the same time.
• Treating a pH that keeps rising as a dosing problem. Check TA, aeration from water features, and salt cell output.

Safety

Follow the product label and SDS. Muriatic acid and chlorine products must never be mixed, and fumes from a spill of one onto the other can be dangerous. Add acid to water, never water to acid.

The pH and alkalinity dose tool does the scaling for you. When alkalinity is the low side, see how to raise alkalinity with baking soda. For all the ranges on one screen, use the pool chemistry cheat sheet.