MathBench

Pool Chemical Calculator

Enter your water's current readings and targets, and get the actual doses — fluid ounces of chlorine, pounds of baking soda — plus a saturation-index verdict on the overall balance.

Don't know your volume? Estimate it

Doses

—saturation index (LSI)
—water balance verdict

How this calculator works

Enter your test-kit readings in the "now" fields and where you want each level in the "target" fields, and the calculator translates every gap into an actual dose of an actual product — fluid ounces of liquid chlorine, pounds of baking soda — scaled to your water volume. Where chemistry has no additive answer (lowering calcium or stabilizer), it says so plainly and tells you the real procedure instead. The saturation index at the bottom judges the water's overall balance from pH, alkalinity, calcium, temperature, and TDS.

Default targets follow public-health and industry practice: free chlorine 1–4 ppm for pools with 3+ for spas (CDC Model Aquatic Health Code minimums are 1 and 3), pH 7.2–7.8, alkalinity 60–120, calcium 200–400, stabilizer 30–50. The pH doses are inherently approximate — buffering varies pool to pool — so the tool scales them by your alkalinity and tells you to add two-thirds, circulate, and retest. That habit, more than any number, is what keeps water right.

The formula

Chlorine: available Cl needed = ppm × gallons × 3.785 mg/gal
  12.5% liquid: 10.24 fl oz / ppm / 10,000 gal   (125 g Cl per litre)
  cal-hypo 65%: 2.05 oz / ppm / 10,000 gal

Alkalinity: NaHCO₃ = Δppm × (84.01 ÷ 50.04) × water mass
  → 1.40 lb per +10 ppm per 10,000 gal
Calcium:    CaCl₂·2H₂O → 1.23 lb per +10 ppm per 10,000 gal
Stabilizer: CYA → 13.4 oz per +10 ppm per 10,000 gal

LSI = pH − pHs,  pHs = 9.3 + A + B − (C + D)
  A = (log₁₀TDS − 1)/10   B = −13.12·log₁₀(T°K) + 34.55
  C = log₁₀(CH) − 0.4     D = log₁₀(TA)

The chlorine, alkalinity, calcium, and stabilizer factors are pure stoichiometry — the alkalinity line shows the derivation: bicarbonate's equivalent weight over calcium carbonate's — and each lands exactly on the rule of thumb printed in pool-industry references. Target ranges follow the CDC's Model Aquatic Health Code (cdc.gov). The saturation index is Langelier's 1936 formulation as used in standard water-treatment references; pH doses are manufacturer label rates, scaled for buffering and labeled approximate.

Worked example

A 15,000-gallon pool tests at FC 1, pH 7.8, TA 70, CH 250, CYA 30, at 84 °F (the calculator's defaults):

  1. Chlorine to FC 3: (3 − 1) × 1.5 × 10.24 = 30.7 fl oz of 12.5% liquid — about a quart
  2. Alkalinity to 100: (30 ÷ 10) × 1.5 × 1.4 = 6.3 lb of baking soda
  3. pH 7.8 → 7.5: ≈ 20 fl oz of muriatic acid — added in stages with retests, and note the alkalinity addition will nudge pH up, so sequence TA first, then acid
  4. LSI at the current readings: +0.13 — balanced; after the adjustments (pH 7.5, TA 100) it lands at −0.01, still balanced and closer to ideal

The order matters more than beginners expect: alkalinity first (it stabilizes pH), then pH, then chlorine — and stabilizer any time, since it acts slowly.

Assumptions & tips

  • Never mix chemicals — especially chlorine types. Trichlor pucks contacting cal-hypo granules can ignite or explode; acid mixed with chlorine releases toxic gas. One chemical at a time, into the water, pump running, tools rinsed between.
  • Always acid into water, never water into acid. Pre-dilute muriatic acid in a bucket of pool water and pour slowly near a return jet. Gloves and eye protection are not optional.
  • Dose, circulate, retest — then dose again. Every estimate here assumes a retest before the next addition. Overshooting with chemicals is far more work to undo than two rounds of patience.
  • Mind what each product drags along. Trichlor adds CYA and lowers pH; cal-hypo adds calcium; dichlor adds CYA fast. The "free" side effects are how pools drift out of balance over a season — it's why liquid chlorine is the cleanest daily sanitizer.
  • Spas amplify everything. A 400-gallon spa is 1/40 of a pool: doses are tiny, drift is fast, and an extra ounce is a big miss. Measure with real measuring cups, not glugs.

Frequently asked questions

How often should I test pool or spa water?

Chlorine and pH two to three times a week in season (daily for a heavily used spa — small water heats fast and drifts fast), total alkalinity weekly, and calcium hardness and CYA monthly. Test before adding anything, wait a full circulation cycle after dosing, and retest before dosing again. Every dose this calculator suggests assumes you will confirm with a test, not stack chemicals on faith.

Why does my chlorine disappear so fast?

Usually sunlight or demand. Without cyanuric acid, UV destroys most free chlorine in a couple of hours — that is CYA's whole job. If CYA is fine, the chlorine is being consumed: heavy bather load, algae getting started, or combined chloramines (that "chlorine smell" is used-up chlorine, not excess). Persistent overnight loss with the pump running is the classic sign it's time to shock.

What should cyanuric acid actually be?

For a traditional chlorine pool, 30 to 50 ppm covers UV protection without blunting the chlorine's bite; saltwater-generator pools often run 60 to 80 per manufacturer guidance. The catch: CYA only leaves by dilution, and every puck of trichlor adds more. Above roughly 100 ppm, chlorine gets sluggish and the practical fix is a partial drain-and-refill.

What does "shocking" the pool mean, and how much do I add?

Raising free chlorine high enough to burn out combined chloramines and algae — conventionally about ten times the combined chlorine reading, or to roughly 40 percent of your CYA level for algae treatment. Use this calculator's chlorine dose with the target set to your shock level, add it at dusk with the pump running, and keep swimmers out until FC returns to normal range.

Is a saltwater pool different for balancing?

Only in where the chlorine comes from — the salt cell generates it, so you rarely dose chlorine by hand. Everything else on this page applies unchanged: pH (which tends to drift up in salt pools), alkalinity, calcium, CYA, and the saturation index all balance identically. Salt itself is dosed by the generator's manual, typically to about 3,200 ppm.

Sources

  1. 2023 Model Aquatic Health Code: Code Language, 4th Edition — Centers for Disease Control and Prevention, February 2023. cdc.govThe public-health floor the default targets here are set against: a minimum free chlorine of 1.0 ppm in pools, 2.0 ppm where cyanuric acid is used and 3.0 ppm in spas (§5.7.3.1.1.2), with pH maintained at 7.0–7.8 (§5.7.3.4.1).
  2. Home Pool and Hot Tub Water Treatment and Testing — CDC Healthy Swimming. cdc.govResidential-scale chlorine and pH recommendations behind the pool and spa FC defaults, why chlorine loses killing power as pH rises, and the test-twice-a-day habit every dose here assumes.
  3. Pool Chemical Safety — CDC Healthy Swimming. cdc.govBacks the handling rules in the tips: protect individual chemicals from mixing together or with other substances, and keep personal protective equipment at hand wherever pool chemicals are stored or used.
  4. The Analytical Control of Anti-Corrosion Water Treatment — W. F. Langelier, Journal of the American Water Works Association, vol. 28, no. 10, pp. 1500–1521, October 1936. DOI 10.1002/j.1551-8833.1936.tb13785.x The saturation index reported as this page's balance verdict, in the pHs = 9.3 + A + B − (C + D) form. Cited bibliographically: the publisher's copy could not be reached to verify a working link.
  5. Calcium carbonate (precipitated) — NIST Chemistry WebBook, Standard Reference Database 69 — National Institute of Standards and Technology. webbook.nist.govMolar mass 100.087 g/mol — the calcium-carbonate basis (equivalent weight 50.04) that both the alkalinity and the calcium-hardness dose factors are expressed against.
  6. PubChem Compound Summary for CID 516892, Sodium Bicarbonate — National Center for Biotechnology Information, U.S. National Library of Medicine, accessed 2026. pubchem.ncbi.nlm.nih.govMolar mass 84.007 g/mol, the figure in the baking-soda derivation shown above: 84.007 ÷ 50.043 gives 1.40 lb per +10 ppm of alkalinity per 10,000 gallons.
  7. PubChem Compound Summary for CID 6093260, Calcium chloride, dihydrate — National Center for Biotechnology Information, U.S. National Library of Medicine, accessed 2026. pubchem.ncbi.nlm.nih.govMolar mass 147.01 g/mol, which sets the calcium-hardness factor of 1.23 lb of calcium chloride dihydrate per +10 ppm per 10,000 gallons.

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