How our calculators work
Every dose on this site comes from a formula we've written, tested, and checked against general water-chemistry references — never a guess. Here's the reasoning behind each one.
The core math
Most doses boil down to one idea: grams of active ingredient = change in ppm × liters of water ÷ 1000 (since 1 ppm = 1 mg per liter). From there, we divide by the product's purity or concentration to get a real-world amount — milliliters for liquids, grams or ounces for granulars, teaspoons for small hot tub doses.
Chlorine
Liquid chlorine is labeled by "trade %" (grams of available chlorine per 100 mL), so we convert directly. Granular products (cal-hypo, dichlor, trichlor) are listed as a percent purity, and each one has a documented side effect we account for: cal-hypo raises calcium hardness, dichlor and trichlor both raise cyanuric acid (CYA). We flag those side effects in every result.
pH and alkalinity
Lowering pH uses a simplified carbonate-equilibrium model: we estimate your water's total inorganic carbon from its current pH and alkalinity (corrected for CYA's buffering effect), then calculate how much that would shift at your target pH, and convert the difference into acid. This is an estimate — always retest and add gradually rather than dosing the full amount at once.
LSI (water balance)
The Langelier Saturation Index combines your pH, temperature, calcium hardness, and alkalinity (again corrected for CYA) into a single number: LSI = pH + temperature factor + calcium factor + alkalinity factor − TDS factor. A result between −0.3 and +0.3 is considered balanced; outside that range, water trends corrosive (low) or scale-forming (high).
Units and conversions
We work internally in metric (liters, grams) and convert to US units (gallons, ounces, fluid ounces, pounds) for display, since most of our audience measures their pool in gallons. Hot tub doses are small enough that we also show teaspoons and tablespoons, using each product's typical density.
Where the numbers come from
Our ideal ranges and dosing factors are based on general water-chemistry principles used across the pool and spa industry — not any single brand's recommendation. We don't copy text, tables, or graphics from other sites; we only use other calculators to sanity-check our own numbers during development.
The limits of a calculator
Every result here is an estimate based on the numbers you enter. Test strips and drop kits have their own margin of error, water chemistry interacts in ways a simple formula can't fully capture, and product labels vary by manufacturer. Always follow your product's label, and when in doubt, add less than the calculated amount and retest before adding more.