Use an approved salt product and follow your equipment and pool-surface instructions for adding, circulating and retesting. Confirm volume and reading before acting on a small correction. The calculated mass is not a recommendation to add it all at once. Salt concentration alone does not establish safe swimming water.

Evaporation removes water, not dissolved salt; do not automatically add salt when topping up evaporated water. Splash-out, backwashing and leaks are different.

Purity adjusts the product mass arithmetically; it does not approve additives. The Hayward Salt & Swim 3C manual specifies greater than 99% NaCl and excludes rock salt, iodized salt and anti-caking additives. Follow your own model’s requirements.

For dilute pool water, ppm is approximated as mg/L. Product kg = liters × (target − current ppm) ÷ (1,000,000 × purity fraction). Bags to buy = round up(max(0, required mass − owned mass) ÷ bag mass). Exact input fractions determine purchase boundaries; display rounding never changes the number of bags.

Preview ppm = current ppm + product kg × purity fraction × 1,000,000 ÷ liters. The diagram shows concentration, not water depth. Optional bounds combine supplied volume and reading extremes, treating target and purity as fixed; they are not statistical confidence levels.

Ideal replacement fraction = (current − target) ÷ (current − fill-water ppm). Compared final ppm = current × (1 − fraction) + fill-water ppm × fraction. This assumes one removal from an initially well-mixed pool, refill to its initial volume and complete mixing. It does not model continuous overflow, repeated exchanges or a safe work procedure.

1 US gallon = 3.785411784 L; 1 lb = 0.45359237 kg. Imperial gallons are not supported. Decimal dot or comma, up to 12 decimal places. Arithmetic limits: volume 0.001–1,000,000,000 L; current and fill 0–20,000 ppm; target 0.001–20,000 ppm; purity 99–100%; bag 0.000001–10,000,000 kg; stock and preview 0–1,000,000,000 kg; volume uncertainty 0–50%; reading uncertainty 0–20,000 ppm. These are software limits, not equipment recommendations. Mass displays round to 0.001 of the chosen unit; use a suitable scale.