How to use the pool calculator
- Measure the pool. Pick the shape and enter the length, width (or diameter) and the depth at both ends. If you already know your gallons, pick “I know the gallons”.
- Test the water. Use a test kit or strips for free chlorine, pH, total alkalinity, calcium hardness and stabilizer (CYA). Salt pools also need a salt reading.
- Enter what you tested. Leave a box empty to skip it. The targets are filled in with the usual ideal, and you can change them.
- Pick your product. Chlorine comes in different strengths, so the amount changes with the jug or bucket you have.
- Add one chemical at a time. Run the pump, let it mix, and retest before the next one. Fix alkalinity before pH.
How the math works
Pool volume is length times width times average depth times 7.48, because one cubic foot holds 7.48 gallons. Round and oval pools use the same math times 0.785 (pi divided by 4). Average depth is the shallow end plus the deep end, divided by two.
Each product has a published amount per 10,000 gallons for a set change. The calculator scales that amount to your gallons and to the change you want. The per 10,000 gallon amounts come from PHTA (the Pool and Hot Tub Alliance) tables, EPA-registered product labels and makers’ guides, listed under each result.
| Product | Per 10,000 gallons | Change |
|---|---|---|
| Liquid chlorine 12.5% | 10 fl oz | +1 ppm free chlorine |
| Cal-hypo 65% | 2.1 oz | +1 ppm free chlorine |
| Baking soda | 1.4 lb | +10 ppm alkalinity |
| Muriatic acid 31.45% | 26 fl oz | -10 ppm alkalinity |
| Calcium chloride 77% | 1.2 lb | +10 ppm calcium hardness |
| Cyanuric acid | 13 oz | +10 ppm stabilizer |
| Pool salt | 83.4 lb | +1,000 ppm salt |
pH does not move in a straight line, because alkalinity buffers it. For pH the calculator uses the published HTH pH Up and pH Down charts, and when your reading falls between chart rows it tells you to follow your product label and retest.
Targets the calculator uses
- Free chlorine: 2 to 4 ppm. The CDC asks for at least 1 ppm, or at least 2 ppm when you use stabilizer.
- pH: 7.4 to 7.6 is ideal; the CDC range is 7.0 to 7.8.
- Total alkalinity: 80 to 120 ppm.
- Calcium hardness: 200 to 400 ppm.
- Stabilizer (CYA): 30 to 50 ppm.
- Salt: the number your salt cell maker gives, such as 3,200 ppm for Hayward AquaRite or 3,600 ppm for Pentair IntelliChlor.
Stop. Never mix pool chemicals with each other, and never mix chlorine with acid: it can release toxic gas. Add chemicals to water, never water to chemicals, and keep them dry and apart in storage.
Common questions
How do I calculate how many gallons are in my pool?
Multiply length by width by average depth in feet, then by 7.48. For a round pool, multiply the diameter by itself, then by the average depth, then by 5.9. A 32 by 16 foot pool that is 3.5 feet deep at one end and 6 feet at the other holds about 18,200 gallons.
How much liquid chlorine do I add to raise chlorine by 1 ppm?
About 10 fl oz of 12.5% liquid chlorine raises 10,000 gallons by 1 ppm, per the maker’s label. Weaker 10% chlorine needs about 11.6 fl oz. The calculator scales this to your pool.
Should I adjust pH or alkalinity first?
Alkalinity first. It holds pH in place, so pH keeps drifting until alkalinity is in range. Fix alkalinity, retest, then adjust pH.
Can I use household bleach in my pool?
Plain, unscented bleach is sodium hypochlorite, the same chemical as liquid pool chlorine, just weaker. You need more of it for the same change. Use only plain, unscented bleach, check the strength on the label and pick it in the calculator.
Why does my stabilizer or calcium only go up?
No chemical lowers stabilizer or calcium hardness. The only fix is to drain part of the water and refill with fresh water, and the calculator shows roughly how much to replace.