Pool, lawn and yard calculators

Pool Shock Calculator

This page computes breakpoint chlorination only: free chlorine at about 10 times the combined chlorine. It does not fill in a routine home shock target.

shock inputs
gallons

Do not know it? Use the Pool Volume Calculator.

ppm
ppm

Trade % is grams of available chlorine per 100 mL. Weight % is grams per 100 g; we convert it with the Olin table. Labels use both, so check yours.

Result: breakpoint dose

Enter combined chlorine

Enter the combined chlorine reading.

Follow the product label.

Chemical safety: read before you add anything

  1. Never mix pool chemicals together, in or out of the water. Add each one separately.
  2. Never mix chlorine with acid. Never mix cal-hypo (calcium hypochlorite) with trichlor.
  3. Add acid to water. Never pour water into acid.
  4. Split large doses into smaller portions. A numeric per-dose limit is not verified.
  5. This result is an estimate. Follow the product label.

How Much Shock Do You Need?

Breakpoint chlorination, in the 2023 Model Aquatic Health Code, is free chlorine at about 10 times the combined chlorine. Enter the combined chlorine from your test, the free chlorine you already have, the gallons, and the product.

Breakpoint free chlorine = combined chlorine × 10

A routine home shock target in ppm is not verified. The result box will not invent one. If combined chlorine is 0, the breakpoint dose is 0.

The annex lists 0.4 ppm as a combined-chlorine ceiling. Source: CDC, 2023 Model Aquatic Health Code (MAHC), §Annex 5.7.3. No CDC endorsement implied; available free at cdc.gov.

Source: CDC, 2023 Model Aquatic Health Code (MAHC), §Definitions: Breakpoint Chlorination. No CDC endorsement implied; available free at cdc.gov.

Fecal-Incident Schedules When Cyanuric Acid Is 15 ppm or Less

These are the MAHC hyperchlorination schedules for that incident case. They are not the calculator's default, and they are not a routine weekly shock.

Use the schedule that matches the incident instructions you are following. Source: CDC, MAHC §6.5.3.2.1.
Free chlorineHold time
20 ppm28 hours
30 ppm18 hours
40 ppm8.5 hours

Source: CDC, 2023 Model Aquatic Health Code (MAHC), §6.5.3.2.1. No CDC endorsement implied; available free at cdc.gov.

Common Mistakes With the Pool Shock Calculator

  • Typing a memorized 10 ppm shock when the test did not show combined chlorine. Ten times zero is zero.
  • Using the fecal-incident table for an ordinary afternoon. That table is a separate case, and only when cyanuric acid is 15 ppm or less.
  • Choosing a 65% cal-hypo figure from memory. We don't have a verified figure for 65% cal-hypo, so enter the percent on your label, or use 70 percent for a product labeled 73 percent.

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Pool Shock Calculator FAQ

What is breakpoint chlorination?

Free chlorine at about 10 times the combined chlorine. Source: CDC, 2023 Model Aquatic Health Code (MAHC), §Definitions: Breakpoint Chlorination. No CDC endorsement implied; available free at cdc.gov. Follow the product label.

Does this page pick a routine shock target such as 10 ppm?

No. A routine home shock target is not verified. Follow the product label.

What schedules does the MAHC list after a fecal incident when cyanuric acid is 15 ppm or less?

20 ppm for 28 hours, 30 ppm for 18 hours, or 40 ppm for 8.5 hours. Source: CDC, 2023 Model Aquatic Health Code (MAHC), §6.5.3.2.1. No CDC endorsement implied; available free at cdc.gov. Follow the product label.