How acidic the water is. Right, chlorine works and eyes do not sting; wrong, it damages the pool.
pH you want
Good range: 7.2–7.8.
Alkalinity (if you know it)
ppm
It decides how far a dose moves pH. Empty = a typical 80.
Stabilizer (if you know it)
ppm
Stabilizer buffers pH too. Empty = a typical 30.
Using pH up.
Type the size and your pH reading.
How this is worked out
How far a dose moves pH depends on how much buffer your water holds — mostly alkalinity, plus stabilizer. We work it out from the water chemistry, so a pool with alkalinity 120 gets more acid than one at 60.
Leave alkalinity and stabilizer empty and we use typical values (80 and 30); type yours for a closer answer.
The product label always wins — if it says something different, follow the label.
Questions
Muriatic acid or dry acid?
Both lower pH. Muriatic acid is cheaper for pools; dry acid (sodium bisulfate) is easier to handle and the usual choice for hot tubs.
Why does my pH keep going up?
High alkalinity keeps pushing it up, and so do fountains, waterfalls and new plaster. Bring alkalinity into range and pH steadies.