Pool Chemical Dosage Calculator
Not All Chlorine Products Dose the Same
A gallon of 10% liquid chlorine and a scoop of 90% trichlor tablets both raise free chlorine — but by wildly different amounts per unit of product, because "10%" and "90%" refer to how much active chlorine is actually in the container. Dosing by weight or volume alone, without accounting for purity, is how pools end up over- or under-treated. This calculator uses the standard water-treatment dosing equation, which explicitly divides by the product's purity fraction, so the dose is correct for whichever chemical is actually on hand.
The Formula
The 8.34 constant is the weight in pounds of one gallon of water, which converts the million-gallons figure into a pounds-of-chemical figure. Dividing by purity fraction scales the result up for weaker products and down for concentrated ones.
Same Pool, Same PPM Target, Five Different Products
| Chemical | Purity | Amount needed |
|---|---|---|
| Liquid Chlorine (10%) | 0.10 | 80.06 oz |
| Liquid Chlorine / Sodium Hypochlorite (12.5%) | 0.125 | 64.05 oz |
| Sodium Dichlor (56%) | 0.56 | 14.30 oz |
| Calcium Hypochlorite (65%) | 0.65 | 12.32 oz |
| Trichlor (90%) | 0.90 | 8.90 oz |
Going from 10% to 90% purity cuts the required amount by roughly 9× — concentration, not brand, is what drives dosage size.
Where This Calculation Matters
- Weekly maintenance dosing — matching the dose to whatever chlorine product is currently stocked, rather than guessing, keeps free chlorine in a consistent range.
- Shock treatments — a large PPM jump (shocking to 10–15 ppm after an algae bloom or heavy bather load) needs an accurate volume-and-purity calculation to avoid overshooting.
- Switching chemical brands — recalculating dosage when switching from tablets to liquid (or between purity grades) prevents under- or over-treating during the transition.
How to Use This Calculator
- Enter your pool volume in gallons.
- Enter your current free chlorine reading in PPM.
- Enter your target PPM level.
- Select the chemical product you're using from the list.
- Select Calculate to see the exact amount needed, in ounces.
Related Calculations
If you specifically use 12.5% liquid chlorine, the Pool Chlorine Calculator offers a quick rule-of-thumb version, or start from the Pool Volume Calculator if you don't yet have an accurate gallon figure.