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Measure. Estimate. Retest.

Pool Chlorine / Shock Calculator

Estimate the product amount needed for a chosen free chlorine (FC) increase. Select your exact chlorine product strength. This tool does not choose a target or a universal shock level.

Enter your pool and free chlorine readings

Verify your pool volume and measured free chlorine. Choose a target appropriate to your pool conditions and product label; this tool cannot validate the target.

Not sure of your volume? Use the Pool Volume Calculator.

Enter a measured free chlorine result, not total chlorine.

Use a target appropriate to your CYA, pool conditions and product label.

Check the label’s concentration basis. Liquid choices mean sodium hypochlorite by weight, not trade percent. For cal-hypo, choose the product's AVAILABLE CHLORINE percentage, not just the calcium hypochlorite ingredient percentage. If these do not match your label, use the label instructions instead.

Use digits and a decimal point if needed, without commas or unit text. For example, enter 20000 for twenty thousand.

Estimated chlorine to add

Enter your values and calculate to see an estimate.

Estimate only. Always follow the product label. Never mix chlorine products together or chlorine with acid. Add chemicals separately according to label instructions. Retest after adequate circulation before adding more.

How the chlorine calculator works

For water, 1 ppm means 1 mg/L. We convert US gallons to liters (1 US gallon = 3.785411784 L), then multiply liters by the positive difference between target and current FC. Required available chlorine in grams = liters × FC increase ÷ 1000. This assumes complete mixing and no chlorine demand; it estimates an addition, not a guaranteed final reading. Check your volume with the Pool Volume Calculator.

Why product strength matters

Liquid choices here mean 6%, 10% or 12.5% sodium hypochlorite (NaOCl) by weight. Available chlorine as chlorine-gas equivalent is approximately the NaOCl weight fraction ÷ 1.050. Assumed solution densities are respectively 1.079, 1.145 and 1.186 g/mL. Liquid dose in mL = required grams ÷ available chlorine fraction ÷ density. These reference densities vary with formulation and temperature; actual product label directions control.

Dry choices mean 65% or 73% available chlorine by weight. Dose in grams = required grams ÷ 0.65 or 0.73. A label showing 73% calcium hypochlorite ingredient may declare only 70% available chlorine; that product does not match the 73% available-chlorine choice. Likewise, trade-percent liquid strength is a different basis. Do not select a mismatched concentration. Liquid chlorine can weaken during storage.

Liquid chlorine vs cal-hypo

Liquid sodium hypochlorite is measured by volume; dry cal-hypo is measured by weight. Cal-hypo also adds calcium, so consider your water’s calcium hardness and product directions. Neither of these unstabilized products adds CYA. Trichlor, dichlor, blends and products with other concentration bases are not supported.

Why free chlorine targets depend on pool conditions

This calculator does not prescribe a target, determine swimming safety or label an FC target as a shock level. Pool conditions, pH, contamination, sunlight and stabilizer affect the appropriate treatment. Read Low Chlorine in a Pool when investigating a low reading and Pool Shock Explained before considering shock treatment. Appropriate shock/SLAM targets depend on chemistry, especially CYA; there is no universal shock target here.

Why CYA matters

Cyanuric acid stabilizes chlorine against sunlight but also affects disinfection. Review your measured CYA and relevant product or professional guidance when choosing FC targets. See Cyanuric Acid (CYA) in Pools and the Pool Water Testing Guide.

When to retest

Retest free chlorine after adequate circulation as directed by your product label before adding more. Actual demand can consume added chlorine. The pool pump runtime guide explains circulation considerations; it does not replace the chemical label’s mixing or retesting directions.

Chlorine safety basics

  • This is an estimate. Always follow the product label and use its required protective equipment.
  • Never mix different chlorine products together. Never mix chlorine with acid.
  • Do not combine chemicals in a bucket or feeder unless the manufacturer explicitly allows that exact use. Never treat this as permission to mix chlorine products or chlorine and acid.
  • Add chemicals separately according to label instructions. Do not predissolve or add to a skimmer unless the specific product label supports it.
  • Confirm actual pool volume and chlorine concentration before dosing. Retest after adequate circulation before adding more.
  • Product strength can degrade during storage, especially liquid chlorine.
  • Do not rely on this calculator for emergency contamination or fecal-response treatment. Follow local and public-health guidance.

Technical references

Concentration definitions and density assumptions: Olin Sodium Hypochlorite Product Stewardship Manual, concentration table and conversion formulas. Safety: CDC pool chemical injury prevention guidance. Testing and stabilizer context: CDC home pool water treatment and testing.

By PoolSpaKit Editorial