pH minus calculator: how much to lower your pool’s pH?
A high pH makes chlorine ineffective and encourages scale. pH minus (sodium bisulfate) brings it back down. The dose depends on volume, the target gap and your product — always work in small amounts.
A pH that will not come down usually means the alkalinity is too high. If the total alkalinity exceeds about 150 mg/L, the pH is "locked" and keeps rising. In that case, correct it in small successive steps, with filtration running, retesting each time — rather than with one large dose.
Result
pH minus to add (indicative)2.4kg
Dose based on your product’s stated value. Dissolve it in a bucket of water, pour with filtration running, and retest before any further addition.
Lowering the pH also slightly lowers the alkalinity: recheck both after correcting.
How this calculation works
This calculation is based on the dosing figure stated by pH minus (sodium bisulfate) manufacturers:
Grams of pH minus = your product’s dose × Volume (m³) × pH gap, where the "dose" is the amount, per 10 m³, that lowers the pH by 0.1.
That dose is about 80 g for a standard pH minus, but it varies by brand: that is why it is editable above. Refer to your product label. As with pH plus, the result is only a starting estimate: adjust gradually and retest. A pH that is hard to lower usually points to alkalinity that is too high, to correct first.
Reference for a standard pH minus (sodium bisulfate), by volume:
| Volume | −0.1 pH | −0.2 pH |
|---|---|---|
| 20 m³ | 160 g | 320 g |
| 30 m³ | 240 g | 480 g |
| 40 m³ | 320 g | 640 g |
| 50 m³ | 400 g | 800 g |
| 60 m³ | 480 g | 960 g |
Set your product’s dose above if it differs.
Frequently asked questions
How much pH minus to lower the pH?
As a rough figure, about 80 g of standard pH minus per 10 m³ lowers the pH by roughly 0.1. This dose varies by brand: check the label and enter the value in the calculator.
What is the right pH for a pool?
Between 7.0 and 7.4. In that range, chlorine is effective, the water stays comfortable and scale does not form.
Are pH minus and sodium bisulfate the same thing?
Powder pH minus is most often sodium bisulfate. Some liquid products use acid: the doses differ, which is why you enter your own product’s value.