Your pool has one number that rules all the others
Ask any pool question — how much chlorine, how much baking soda, how big a pump — and the answer begins with the same figure: how many gallons of water you are treating. Published dosing guidance is stated per 10,000 gallons, pump and filter sizing works from turning the whole volume over in roughly 8–12 hours, and even the cost of the first fill is just gallons times your water rate. Guess the volume 30% wrong and every dose you ever add inherits that error.
Happily, the pool industry publishes simple volume formulas keyed to shape. A rectangle is length × width × average depth × 7.5, where the average depth is just the shallow and deep ends averaged — (3 + 8) / 2 = 5.5 ft for a typical slope — and 7.5 is the industry’s rounding of the 7.48 gallons in a cubic foot. A round pool is diameter squared × average depth × 5.9; a kidney uses 0.45 × (sum of the two lobe widths) × length × average depth × 7.5. Measure at the water line, not the coping, because you are estimating water, not shell. Compute it once (our pool volume calculator handles all four shapes), write it down, and it serves every season you own the pool.
ppm is pounds per million pounds — literally
Test strips and kits report water chemistry in ppm, and the unit is exactly what it says: one part per million by weight, one pound of the measured chemical per million pounds of water. Since a US gallon of water weighs 8.34 lb, the arithmetic connecting a test reading to a scoop of product is short. To raise a parameter by some number of ppm, the pounds required are gallons × ppm increase × 8.34 ÷ 1,000,000, divided by the fraction of the product that is actually the measured chemical.
A concrete run-through: a 20,000-gallon pool testing 1 ppm free chlorine, target 4 ppm. The 3 ppm rise needs 20,000 × 3 × 8.34 ÷ 1,000,000 = 0.50 lb of chlorine. Using cal-hypo granules at 65% available chlorine, the product dose is 0.50 ÷ 0.65 = 0.77 lb — about 12.3 ounces. That is the entire theory behind every raising dose; only the strength factor changes from product to product.
The four levers you raise by the pound
Routine pool balance comes down to four parameters that you raise by adding a measured chemical. Free chlorine (FC) is the working sanitizer — the level your daily test watches. Total alkalinity (TA) is the water’s buffering reserve, part of the carbonate buffer system that resists pH swings; you raise it with sodium bicarbonate, which is chemically identical to grocery-store baking soda. Calcium hardness (CH) is the dissolved calcium level, raised with calcium chloride. Cyanuric acid (CYA) — the stabilizer — rounds out the four, and as the next sections explain, it is the lever most likely to move when you were not looking.
For chlorine, the products differ mainly in their available-chlorine strength and what else they carry: cal-hypo at 65% or 73%, dichlor at 56%, trichlor at 90%, and liquid chlorine at 12.5%, which doses by volume rather than weight. Per 10,000 gallons, raising free chlorine by 1 ppm takes about 2 oz of 65% cal-hypo, 2.4 oz of dichlor, 1.5 oz of trichlor, or 10 fl oz of liquid — four routes to the same ounce and a third of actual chlorine.
Raising is linear. Lowering pH is not.
Every dose above scales like a recipe: twice the gallons or twice the ppm rise means twice the chemical, because raising these parameters is stoichiometric — the added product becomes the measured species, pound for pound. This linearity is what makes a calculator trustworthy for raising doses, and it is why the per-10,000-gallon figures in published dosing tables can simply be multiplied out to any pool size.
Lowering pH obeys no such recipe. The acid a pool demands depends on where its pH currently sits on the carbonate buffer curve — which shifts with alkalinity, CYA, borates, and temperature — not just on the distance to the target. A linear formula would routinely overshoot, which is why a responsible calculator refuses to offer one and why your test kit includes an acid-demand test instead. The working method is deliberately unglamorous: add acid in small increments, circulate, retest, repeat. Treat any app or chart that hands you a single pH-lowering dose with suspicion.
Stabilizer creep: the tax on convenient chlorine
Dichlor granules and trichlor pucks are popular because they are stable and convenient — and both are chlorinated cyanurates, meaning every dose leaves cyanuric acid permanently behind. The exchange rate is unfavorable: dichlor adds about 0.9 ppm of CYA for every 1 ppm of free chlorine it delivers, trichlor about 0.6. Chlorinate all season on trichlor pucks and the pool can quietly gain 60+ ppm of CYA.
That matters because high CYA suppresses chlorine’s effectiveness — the classic symptom is a pool that tests "fine" but behaves under-sanitized — and nothing practical removes CYA except draining and refilling part of the pool. The defense is bookkeeping, not chemistry: know which of your chlorine sources are stabilized, track CYA over the season, and lean on unstabilized sources like cal-hypo or liquid chlorine once the stabilizer is where you want it. A dosing tool that prints the CYA side effect alongside every stabilized-chlorine dose makes this creep visible instead of surprising.
The safety rules are older than the chemistry
Pool chemicals are concentrated oxidizers, and the handling rules exist because the failure modes are violent, not messy. Always add chemical to water, never water to chemical — a splash of water into concentrated chlorine can react violently. Pre-dissolve granular products in a bucket of pool water where the label directs, and broadcast slowly over the deep end with the pump running.
Never mix products with each other, even two things both called "chlorine": trichlor plus cal-hypo can ignite. The same caution extends to sequencing — dose one chemical at a time, let the pump circulate the water fully (a few hours), and retest before adding anything else. Sequential small corrections cost you nothing but patience; simultaneous ones cost you the ability to know which chemical did what, and occasionally much more than that.
A season of balance, in one paragraph of numbers
Here is the whole toolkit at working scale, per 10,000 gallons. Raising free chlorine 1 ppm: about 2 oz of 65% cal-hypo, 2.4 oz of dichlor, 1.5 oz of trichlor, or 10 fl oz of 12.5% liquid. Raising total alkalinity 10 ppm: about 1.4 lb of sodium bicarbonate. Raising calcium hardness 10 ppm: about 1.2 lb of 77% calcium chloride dihydrate. Raising CYA 10 ppm: about 13.3 oz of stabilizer. Every one of these scales linearly with your gallons and your ppm gap — which brings us back to where this guide started: the one number that rules the others is your pool’s volume. Measure it once, and the rest of the season is small arithmetic between tests, with the pool chemical calculator doing the multiplication.