About Pool Chemical Calculator (Chlorine, Alkalinity, Hardness & CYA)
The pool chemical calculator turns a test-kit reading and a target level into a product dose for the four parameters you raise by adding a measured chemical: free chlorine, total alkalinity, calcium hardness, and cyanuric acid (stabilizer). Everything rests on one piece of pure chemistry — 1 ppm means 1 pound of the measured species per million pounds of water, and a US gallon of water weighs 8.34 lb — so the dose is gallons × ppm increase × 8.34 ÷ 1,000,000, divided by the product's strength factor. Solid chlorine doses come back in pounds and ounces; 12.5% liquid chlorine comes back in fluid ounces and gallons.
This tool computes RAISING doses only. Lowering pH (or alkalinity) with acid is deliberately excluded because acid demand is nonlinear — how much acid a pool needs depends on where its pH sits on the carbonate buffer curve, not just on the distance to the target. For lowering, use your test kit's acid-demand table and add acid in small increments with retests between. Shock/SLAM protocols and salt-water generator settings are also out of scope; the doses here are the routine adjustments between tests, verified against published CPO dosing tables.
How It Works
- Enter the pool volume in gallons — if you don't know it, the pool volume calculator computes it from the pool's shape and depths.
- Pick the parameter to raise (free chlorine, total alkalinity, calcium hardness, or stabilizer) and enter the tested current level and the target level. The target must be above the current reading — this tool only computes additions.
- For chlorine, choose the product: cal-hypo 65% or 73%, dichlor 56%, trichlor 90% (the percentages are trade available-chlorine strengths), or 12.5% liquid chlorine. For calcium hardness, choose 94% anhydrous or 77% dihydrate calcium chloride.
- Read the dose: pounds and ounces for solids, fluid ounces and gallons for liquid chlorine. When dichlor or trichlor is selected, the result also shows the cyanuric acid the dose adds (about 0.9 and 0.6 ppm CYA per 1 ppm of FC respectively) so stabilizer doesn't creep up unnoticed.
- Dose, let the pump circulate the water fully, then retest before adding more. Always add chemical to water (never water to chemical), pre-dissolve granular products where the label says to, and never mix products with each other.
Worked Example
A 20,000-gallon pool tests at 1 ppm free chlorine and the target is 4 ppm — a 3 ppm rise using 65% cal-hypo. The chlorine (as Cl₂) needed is 20,000 × 3 × 8.34 ÷ 1,000,000 = 0.5004 lb. Dividing by the 65% available-chlorine strength gives 0.5004 ÷ 0.65 = 0.770 lb of product, about 12.3 oz. Cross-check: the published CPO table calls for 2 oz of cal-hypo per 1 ppm per 10,000 gallons, which scales to 2 × 3 × 2 = 12 oz — the same dose within the table's rounding. Cal-hypo adds no stabilizer, but the same rise from dichlor (0.894 lb) would also add about 2.7 ppm CYA.
Formulas
- Core dosing relation
dose lb = gallons × Δppm × 8.34 / 10⁶ / strengthFactor- Solid chlorine strength (available chlorine)
factor = n(active Cl) × 70.91 / MM(product) → cal-hypo 0.65/0.73 (trade), dichlor 0.56, trichlor 0.90- Liquid chlorine 12.5% (volumetric)
dose gal = needed lb / 1.0432; trade % = g available Cl₂ per 100 mL- Total alkalinity (sodium bicarbonate)
factor = (100.09 / 2) / 84.01 = 0.5957 (CaCO₃ equivalence)- Calcium hardness (calcium chloride)
factor = (100.09 / 110.98) × purity = 0.9019 × {0.94 anhydrous, 0.77 dihydrate}- Cyanuric acid and the stabilized-chlorine side effect
CYA factor = 1; dichlor adds ≈0.9, trichlor ≈0.6 ppm CYA per ppm FC
Standards & References
- Pure chemistry: 1 ppm = 1 mg/L = 1 lb per million lb of water; molar masses CaCO₃ 100.09, NaHCO₃ 84.01, CaCl₂ 110.98, Cl₂ 70.91, CYA 129.07 g/mol
- "Dosages Required To Chemically Treat 10,000 Gallons Of Water" — Yakima County, WA public-health CPO dosing table (cross-check for every factor)
- Trouble Free Pool / PoolMath liquid-chlorine convention: trade % is w/v, so 1 gal of 12.5% raises 10,000 gal by ~12.5 ppm FC
- Orenda Technologies, "Understanding Labeled Chlorine %" — trade strengths cal-hypo 65/73%, dichlor 56%, trichlor 90%
Frequently Asked Questions
How much chlorine do I add to raise free chlorine by 1 ppm?
Per 10,000 gallons: about 2 oz of 65% cal-hypo, 2.4 oz of 56% dichlor, 1.5 oz of 90% trichlor, or 10 fl oz of 12.5% liquid chlorine. All four come from the same relation — 10,000 gal × 1 ppm × 8.34 ÷ 10⁶ = 0.083 lb of chlorine as Cl₂, divided by the product's available-chlorine strength — and match the published CPO dosing tables within their rounding. Scale linearly with volume and with the ppm rise.
Why doesn't this calculator lower pH?
Because acid demand is nonlinear. Raising alkalinity or hardness is stoichiometric — twice the ppm takes twice the chemical — but the acid needed to move pH depends on where the pool sits on the carbonate buffer curve, which shifts with alkalinity, CYA, borates, and temperature. A linear formula would routinely overshoot. Use your test kit's acid-demand test, add the indicated acid in small increments, and retest between additions.
What do the percentages on chlorine products mean?
They are available chlorine — the product's oxidizing power expressed as an equivalent mass of Cl₂ gas. Pure cal-hypo is 99.2% by this measure but trade granules are cut to 65% or 73%; dichlor dihydrate works out to 55.4% (sold as 56%) and trichlor to 91.5% (sold as 90%) straight from their molar masses. Liquid chlorine's 12.5% is a trade percent measured per volume: 12.5 g of available Cl₂ per 100 mL, which is why its dose is volumetric.
Does dichlor or trichlor raise stabilizer (CYA)?
Yes, permanently. Both are chlorinated cyanurates, so every dose leaves cyanuric acid behind: dichlor adds about 0.9 ppm CYA and trichlor about 0.6 ppm CYA for every 1 ppm of free chlorine delivered. Chlorinating a pool all season on trichlor pucks can add 60+ ppm CYA, which suppresses chlorine effectiveness — and nothing practical removes CYA except draining and refilling. The calculator prints the CYA added whenever you select a stabilized product.
How much baking soda does it take to raise alkalinity?
About 1.4 lb of sodium bicarbonate per 10 ppm of total alkalinity per 10,000 gallons. The factor is pure stoichiometry: TA is reported as ppm CaCO₃ (equivalent weight 50.045), and each 84.01 g of NaHCO₃ delivers one alkalinity equivalent, so the product strength is 50.045 ÷ 84.01 = 0.5957. Plain baking soda from the grocery store is the identical chemical to pool-store "alkalinity increaser".
What are the safety rules for adding pool chemicals?
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 "just chlorine types": trichlor plus cal-hypo can ignite. Dose one chemical at a time, let the water circulate fully (a few hours), and retest before adding anything else.