Pool Chemical Calculator

Pool chlorine dosing is real chemistry, and it can be derived rather than looked up in a chart. Parts per million is milligrams per litre, so to raise a volume of water by one ppm you need the weight of that water multiplied by one millionth.

Ten thousand US gallons weighs 83,400 pounds, so one ppm needs 0.0834 pounds of pure available chlorine. Divide that by the strength of your product and you have the dose: 65% cal-hypo needs 0.128 lb, which is 2.05 ounces. That is exactly the figure printed on the bag, and now you know where it comes from.

pH is messier and genuinely is a chart, because how far the pH moves depends on total alkalinity — the water's buffering capacity. The doses here are scaled from the standard reference of 10,000 gallons at 100 ppm alkalinity, which is why the alkalinity input changes the answer.

1. Step 1 Pool volume gallons = length × width × avg_depth × 7.48 round: π × radius² × avg_depth × 7.48
2. Step 2 Pure chlorine needed lb = gallons × 8.34 × ppm_increase ÷ 1,000,000 Water weighs 8.34 lb per US gallon.
3. Step 3 Convert to product product_lb = pure_lb ÷ product_strength Cal-hypo 65% or 73%, dichlor 56%, trichlor 90%, liquid 10% or 12.5%.
4. Step 4 Liquid products by volume fl_oz = product_lb ÷ density × 128 12.5% liquid chlorine weighs about 9.74 lb per gallon.
5. Step 5 pH adjustment dose = (gallons ÷ 10,000) × (|pH_change| ÷ 0.2) × base_dose × (alkalinity ÷ 100) Base dose is 12 fl oz of 31.45% muriatic acid, or 6 oz of soda ash, per 0.2 pH per 10,000 gal at 100 ppm alkalinity.

Example: 15,000 gallons going from 1 to 3 ppm free chlorine with 65% cal-hypo. That is 2 ppm × 15,000 ÷ 10,000 × 0.0834 = 0.25 lb of pure chlorine, divided by 0.65 = 0.385 lb of product, which is 6.2 ounces.

• Chlorine doses are derived from first principles using the weight of water (8.34 lb per US gallon) and the stated available-chlorine strength of each product. They match published dosing charts closely.
• Liquid chlorine loses strength on the shelf, roughly 1% of its available chlorine per week at room temperature and faster in heat. A jug that has been in the garage all summer is materially weaker than its label.
• pH doses are scaled from a reference of 10,000 gallons and 100 ppm total alkalinity. Alkalinity is what buffers pH, so the dose scales with it — this is an approximation of a non-linear relationship and should be treated as a starting point.
• Adjust pH in stages. Add about half the calculated amount, circulate for four hours, and retest. Overshooting downwards is much harder to correct than creeping up.
• Cyanuric acid, the stabiliser, is not modelled. It has a large effect on how much free chlorine you actually need: at 30 ppm CYA the target is around 2–4 ppm, but at 80 ppm you need 6–8 ppm to achieve the same sanitising power. Dichlor and trichlor both add CYA with every dose.
• Calcium hardness is not modelled. Repeated cal-hypo use raises it, and above about 400 ppm you get scaling on surfaces and equipment.
• Salt-water pools generate chlorine in the cell rather than from added product. Use these figures only for a manual top-up or shock, not for routine chlorination.
• Water temperature, sunlight, bather load and rainfall all consume chlorine. Test frequently in summer — a pool can drop from 3 ppm to zero in a single hot day with heavy use.