Pool & spa
Pool Alkalinity Calculator
Adjusts total alkalinity, which is what stops pH swinging around.
Result
Dose against a test result, never against a schedule. Add chemicals in stages with the pump running, wait for a full turnover, then retest before adding more. Never mix chemicals, and never add water to acid. Where a product label differs from the figure here, the label wins. Raise alkalinity in stages of no more than about 20 ppm at a time, allowing a full turnover between doses. Lowering it is slower and is done with acid added in one spot with the pump off, which is a different technique entirely.
Get a Free Business AuditHow the calculation works
The dose is the gap between current and target total alkalinity times the volume, converted to the product used, normally sodium bicarbonate to raise it. Lowering it is done with acid, which lowers pH at the same time, so reducing alkalinity is a sequence of acid additions with aeration between them to bring pH back up while alkalinity stays down. Total alkalinity is the water's buffering capacity, meaning its resistance to pH change.
What moves the number
- Current reading against target, scaled by volume.
- Whether alkalinity is being raised or lowered, since lowering is a process.
- Fill water alkalinity, which sets where the pool naturally sits.
- Whether calcium hardness is also out, since the two interact in scaling.
Where this one goes wrong
Lowering alkalinity without dropping pH takes a sequence rather than a single dose, because acid reduces both together. The usual approach is adding acid to bring both down and then aerating to bring pH back up while alkalinity stays lowered, repeated as needed. Dumping a large acid dose in one go overshoots pH badly and can damage surfaces. On the other side, alkalinity that is too low leaves pH unstable and the water aggressive toward plaster and metal, so it is not a parameter to leave low in the hope of easier pH control.
Before you quote
- Lower alkalinity in steps with aeration between, not in one dose.
- Test the fill water, which determines where the pool tends to sit.
- Do not leave alkalinity low to make pH easier; it makes it unstable.
Where the rules come from
Questions
About this calculator
What does total alkalinity actually do?
It buffers pH, meaning it resists pH change. Too low and pH swings from small additions and the water becomes aggressive toward plaster and metal. Too high and pH becomes stubborn and tends to drift upward. It is the parameter that makes pH controllable.
How do I lower alkalinity without lowering pH?
In steps. Acid lowers both together, so the method is adding acid to bring both down, then aerating to drive pH back up while alkalinity stays lowered, repeating as needed. A single large acid dose overshoots pH and can damage surfaces.
Why does my alkalinity keep climbing?
Often the fill water, which in many areas is naturally high in alkalinity, so every top-up raises it. Some chlorine products also contribute. Testing the fill water tells you what the pool tends toward, which explains a lot of recurring chemistry battles.
The formula
about 1.5 lb of sodium bicarbonate per 10,000 gallons raises alkalinity 10 ppm
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How to use this calculator
- Pool volume (gallons). Enter your own figure; the page starts at 20,000.
- Current total alkalinity (ppm). Enter your own figure; the page starts at 60.
- Target total alkalinity (ppm). Enter your own figure; the page starts at 100.
- Price per pound or gallon ($). Enter your own figure; the page starts at $1.2.
The result updates as you type, so you can try a figure, see what it does to sodium bicarbonate to add, and change it back. Nothing you enter is sent anywhere or saved.
A worked example
These are the figures the calculator opens with. Every number in the results column below is produced by the same calculation the page runs, so you can follow it through before putting your own figures in.
What goes in
| Input | Value |
|---|---|
| Pool volume (gallons) | 20,000 |
| Current total alkalinity (ppm) | 60 |
| Target total alkalinity (ppm) | 100 |
| Price per pound or gallon | $1.2 |
What comes out
| Result | Value |
|---|---|
| Sodium bicarbonate to add main result | 12.0 |
| Change needed | 40.0 |
| Estimated cost | $14.40 |
| Dose per 10,000 gallons | 6.0 |
Add about 12.0 to move 20,000 gallons from 60 to 100.
How sensitive the answer is
Holding everything else steady and changing only price per pound or gallon, this is what happens to sodium bicarbonate to add. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Price per pound or gallon ($) | Sodium bicarbonate to add |
|---|---|
| $0.6 | 12.0 |
| $0.9 | 12.0 |
| $1.2 example above | 12.0 |
| $1.8 | 12.0 |
| $2.4 | 12.0 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Pool volume (gallons)
- A number, between 0 and 2000000, starting at 20,000.
- Current total alkalinity (ppm)
- A number, between 0 and 400, starting at 60.
- Target total alkalinity (ppm)
- A number, between 0 and 400, starting at 100.
- Price per pound or gallon ($)
- A money amount, starting at $1.2.
What the results tell you
| Result | What it is |
|---|---|
| Sodium bicarbonate to add | The headline result. A decimal figure. |
| Change needed | A decimal figure. |
| Estimated cost | A money figure. |
| Dose per 10,000 gallons | A decimal figure. |
Keep going
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