| Level | Yours | Ideal | Add this |
|---|---|---|---|
| Fill in the fields above. | |||
How the calculation works
Every dose starts from one fact: raising 10,000 US gallons by 1 ppm takes 37.85 grams — about 1.34 ounces — of the active ingredient. From there the arithmetic is (ppm needed ÷ 10) × rate × (your gallons ÷ 10,000), with the rate set by the strength of the product you picked.
That is why the product selector matters. Cal-hypo at 73% and dichlor at 56% are not interchangeable: the same jug-full of each delivers a different amount of chlorine. Choosing your actual product means the number you get is for what is standing in your shed, not for an average one.
pH is the exception. It is not a simple dose, because alkalinity resists the change — the higher your TA, the more acid or soda ash it takes to move pH the same distance. The calculator scales the pH dose by your alkalinity reading for that reason.
Worked example
A 20,000 gallon pool tests at 1 ppm free chlorine, pH 7.9, and alkalinity 70 ppm, using 12.5% liquid chlorine.
Chlorine needs to come up 2 ppm, which takes roughly 43 fluid ounces — a third of a gallon. Alkalinity needs to come up 30 ppm, which takes about 9 lbs of baking soda. pH is high, but alkalinity is low, and raising alkalinity will lift pH with it — so the right order is to add the baking soda first, run the pump, re-test, and only then decide whether any acid is needed at all.
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.
- Readings were taken from water at least 12 inches below the surface, away from the return jets.
- pH dosing assumes muriatic acid at 31.45% and soda ash as the pH increaser.
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.
- Alkalinity and pH pull on each other. Fixing alkalinity first often moves pH into range on its own, which is why this calculator shows both rather than assuming an order.
- Calcium hardness and cyanuric acid cannot be lowered by adding anything — only by replacing water.
Questions people ask
Which order should I fix things in?
Alkalinity first, then pH, then calcium hardness, then cyanuric acid, and chlorine last. Alkalinity is what holds pH steady, so adjusting it first saves you chasing the same number twice.
Can I add two chemicals at once?
No. Add one, circulate for at least four hours, re-test, then decide on the next. Chlorine and acid added together react directly and release chlorine gas — this is the single most common way people are injured by pool chemicals.
My chlorine will not hold. Why?
Usually cyanuric acid. Below about 30 ppm, sunlight destroys chlorine within hours. Above roughly 60 ppm the opposite problem appears — there is plenty of chlorine but it is bound up and slow to act. Check CYA before reaching for more chlorine.
How often should I test?
Chlorine and pH twice a week in swimming season and weekly the rest of the year. Alkalinity and calcium hardness monthly. Always test after heavy rain, a big pool party, or topping up a large amount of fresh water.
Do I need every reading?
No. Leave any box empty and it is skipped. Enter just chlorine and pH for a quick weekly check, or fill in everything for a full monthly balance.