Pool Chlorine Calculator

Frequently asked questions

How much chlorine do I need to raise my pool by 1 ppm?

For a 10,000-gallon pool, 1 ppm of free chlorine needs: 9.11 fl oz of liquid chlorine 12.5%, 20.74 fl oz of household bleach 6%, 2.05 oz of cal-hypo 65%, 2.38 oz of dichlor 56%, or 1.48 oz of trichlor 90%. Scale linearly: a 20,000-gallon pool needs twice as much; 2 ppm needs twice as much. The Dose tab above computes this for your exact volume.

What is the pool chlorine dose formula?

For dry products: ounces = Δppm × gallons / (74.94 × % available chlorine). The constant 74.94 = 1,000,000 / (16 × 100 × 8.34), because one gallon of water weighs 8.34 lb and there are 16 oz in a lb. For liquids the calculator works by mass instead: it converts the target ppm to grams of chlorine, divides by the product's available-chlorine fraction (NaOCl → Cl₂ factor 70.906/74.44), then divides by density (1.18 g/mL for 12.5% liquid chlorine, 1.08 g/mL for 6% bleach).

How do I calculate a shock dose (breakpoint chlorination)?

Two standard approaches: (1) standard shock = raise FC to 10 ppm; (2) SLAM (TroubleFreePool protocol) = raise FC to 40% of your CYA level — e.g. CYA 50 ppm → shock to 20 ppm, which in a 20,000-gallon pool needs 82.12 oz (5.13 lb) of cal-hypo 65%. The old breakpoint rule of thumb is to dose about 10× your combined chlorine reading. Shock at dusk with the pump running, and do not swim until FC drops below 4 ppm.

Do chlorine tablets raise CYA (stabilizer)?

Yes — every stabilized chlorine product adds cyanuric acid. Trichlor adds ~0.61 ppm CYA per 1 ppm FC (so 3 ppm FC → +1.82 ppm CYA); dichlor adds ~0.91 ppm CYA per 1 ppm FC (2 ppm FC → +1.82 ppm CYA). Above ~50 ppm CYA, chlorine becomes noticeably less effective and you need higher FC targets; above ~100 ppm most guides recommend a partial drain. Dose daily with unstabilized chlorine (liquid chlorine or cal-hypo) once CYA is in range. Note: salt (NaCl) dosing for saltwater pools is not covered by this tool.

How many trichlor tablets does my pool need per week?

A common starting point: a 10,000-gallon pool losing 2 ppm FC per day needs 3 three-inch (~8 oz) trichlor tablets per week — each tablet adds about 5.40 ppm FC as it erodes over 5–7 days. Those tablets also add 9.82 ppm CYA per week, so starting from 30 ppm CYA you would hit an 80 ppm warning limit in about 5.1 weeks. The Tablet tab computes this for your pool.

How do I estimate my pool's volume?

Measure length, width and average depth (shallow + deep, divided by 2), then: rectangle = L×W×D, oval = (π/4)×L×W×D, round = π×(diameter/2)²×D, kidney ≈ 0.85×L×W×D — all in cubic feet, times 7.48 gallons per cubic foot. Example: a 40×20 ft pool averaging 5 ft deep holds about 29,922 gallons. The Volume tab does this math, or use the dedicated pool volume calculator for more shapes and fill-time estimates.

Why is this higher than the “1 gallon of 12.5% = 12.5 ppm” rule of thumb?

That rule of thumb is a simplification: it ignores the product's density and the difference between % NaOCl and % available chlorine. Exactly, one gallon of 12.5% liquid chlorine (density 1.18 g/mL, available chlorine 11.91%) adds 14.06 ppm to 10,000 gallons — not 12.5. Similarly, one gallon of 6% bleach adds 6.18 ppm (measured 6.17 by pool chemists), not 6.0. This calculator uses the exact mass-based math, which is why its liquid doses run a bit higher than the rough rule.

When is it safe to swim after shocking?

Wait until free chlorine falls back below 4 ppm — that is the reentry limit printed on HTH chlorine labels. After a standard 10 ppm shock this typically takes several hours to a full day depending on sun and bather load; test with a kit before swimming. Shocking at dusk avoids UV burning off the chlorine before it works.