How the calculation works
Raising alkalinity is straightforward: 1.5 lbs of sodium bicarbonate — ordinary baking soda — per 10,000 gallons raises total alkalinity by about 10 ppm. Scale that to your pool and the gap you need to close.
Lowering it is not a dose at all. Acid removes alkalinity, but it removes pH along with it, so the method is a cycle: add acid to bring pH down to about 7.0, aerate the water to push pH back up without restoring the alkalinity, then re-test and repeat.
Worked example
A 20,000 gallon pool at 70 ppm, aiming for 100 ppm. The gap is 30 ppm, so 1.5 × 3 × 2 = about 9 lbs of baking soda. Add it in two halves a few hours apart rather than all at once — it clouds the water briefly either way.
Assumptions
- Doses are calculated from the product strength you selected, not a generic chart.
- Pool volume is accurate — a volume that is out by a quarter puts every dose out by a quarter.
- Water is circulating while the chemical is added and for several hours afterwards.
Limitations
- Test kits vary. Strips are convenient but a drop-based kit is markedly more reliable for anything beyond a rough check.
- Products degrade. Liquid chlorine loses strength within weeks of manufacture, especially in heat, so an older jug delivers less than its label claims.
- These figures are estimates for a typical residential pool, not a prescription for your specific water.
- Bringing alkalinity down takes several rounds over days, not one dose. Any single figure for it would be misleading, which is why this calculator describes the method instead.
Questions people ask
What does alkalinity actually do?
It is pH's shock absorber. With alkalinity in range, pH drifts slowly and predictably. With it too low, pH swings wildly on rain, bather load or a single dose of anything — the classic "pH bounce".
Baking soda or alkalinity increaser?
They are the same chemical — sodium bicarbonate. Grocery-store baking soda works identically to the branded pool product and usually costs far less per pound.
How do I lower alkalinity?
Add muriatic acid to bring pH down to about 7.0, then aerate — run a fountain, aim the returns upward, or switch on the spa jets. Aeration raises pH back toward 7.5 without restoring alkalinity. Re-test and repeat until it is in range.
Should I fix alkalinity or pH first?
Alkalinity, almost always. It is what holds pH steady, so correcting it frequently brings pH into range without any separate pH adjustment.