Breakpoint Chlorination Math for Commercial Pools

Add free chlorine equal to about 10 times the combined chlorine reading. That is the 2024 MAHC Annex rule (glossary entry "Breakpoint Chlorination"; check which MAHC edition, if any, your state adopted). To turn that into product, multiply the ppm rise by pool volume in millions of gallons and by 8.34 to get pounds of available chlorine, then divide by the available chlorine fraction on the label.

What does the MAHC require at 0.4 ppm combined chlorine?

MAHC 5.7.4.4.2 says the owner must ensure the facility takes action to reduce combined chlorine when it exceeds 0.4 ppm. The listed actions are superchlorination, water exchange, or bather hygiene, and the list is not exclusive.

The Annex makes a point that matters in an inspection: 0.4 ppm is an action concentration, not a limit that produces a violation by itself. The violation an inspector would cite is failing to test regularly, or exceeding 0.4 ppm with no plan to reduce it. Your record needs the reading, the action and the retest. Combined chlorine is total chlorine minus DPD free chlorine (Annex glossary: CC = TC - DPD-FC), and 5.7.5.1 requires testing before opening each day. Item 3 of the daily record list in 6.4.1.6 is the combined chlorine concentration.

Why does the dose need to be about 10 times the reading?

The Annex glossary describes the chain. Chlorine added to water containing ammonia first forms monochloramine. More chlorine converts monochloramine to dichloramine, which decomposes to nitrogen gas, hydrochloric acid and chlorine. The apparent residual falls where that happens, and the Annex calls that the breakpoint. Past it, the inorganic combined chlorine compounds disappear.

A short dose leaves you stuck in the middle of that chain. The Annex does not state outright that a short dose makes things worse, but its odor figures show why it can feel that way. Odors are unlikely below these concentrations: monochloramine 5.0 ppm, dichloramine 0.8 ppm, trichloramine 0.02 ppm (Annex 5.7.4.4.2). Converting monochloramine toward dichloramine without finishing the job swaps a species with a high odor threshold for one with a much lower threshold. The kit may show no improvement, and the deck may smell worse. Annex 5.6.2.8 also notes that a higher ratio of chlorine to nitrogen in the water favors trichloramine formation.

How do you size the dose in pounds or gallons?

The 8.34 is the weight of a US gallon of water in pounds. NIST SP 811 Appendix B gives 1 gallon as 3.785412 L and 1 pound as 0.4535924 kg, and 3.785412 / 0.4535924 = 8.345 lb at 1 kg/L. So 1 ppm in 1 million gallons is about 8.34 lb of active ingredient.

Formula:

• Rise (ppm) = 10 x combined chlorine (ppm)
• Pounds of available chlorine = rise (ppm) x (gallons / 1,000,000) x 8.34
• Pounds of product = pounds of available chlorine / available chlorine fraction on the label

Worked example (illustrative numbers): a 90,000 gallon pool tests at 3.8 ppm total chlorine and 3.0 ppm free chlorine, so combined chlorine is 3.8 - 3.0 = 0.8 ppm.

1. Rise by the Annex rule: 10 x 0.8 ppm = 8.0 ppm.
2. Pounds of available chlorine: 8.0 x (90,000 / 1,000,000) x 8.34 = 8.0 x 0.09 x 8.34 = 6.0 lb.
3. If the label states 65 percent available chlorine (an assumed label for this example), granular product is 6.0 / 0.65 = 9.2 lb.
4. If the label states 12.5 percent available chlorine by weight (also assumed), liquid product is 6.0 / 0.125 = 48 lb. Say the SDS lists 10 lb per gallon (assumed), then 48 / 10 = 4.8 gallons.

Use the strength and density printed on your own product's label and safety data sheet. If the label states strength by volume instead of weight, convert with the density on the SDS.

CDC's chloramines page words the target differently: raise the free chlorine level to 10 times the combined chlorine level. At 3.0 ppm free chlorine that target is 8.0 ppm, so the dose is 8.0 - 3.0 = 5.0 ppm, or 3.8 lb of available chlorine. The Annex reading adds 8.0 ppm on top of what is there and peaks near 11 ppm. Pick one reading, write it into your SOP, and retest. Both say "approximately".

What can go wrong with the dose?

• Stabilized product adds cyanuric acid. Using the molecular weights on PubChem (trichlor 232.41, cyanuric acid 129.07, dichlor 219.94, chlorine 70.90), trichlor leaves 129.07 / (3 x 70.90) = 0.61 ppm CYA per ppm of chlorine and dichlor leaves 129.07 / (2 x 70.90) = 0.91. An 8.0 ppm dose adds about 4.9 ppm (trichlor) or 7.3 ppm (dichlor) of CYA. The MAHC requires non-stabilized chlorine only for fecal and vomit remediation (6.5.2.3, item 5); it does not say this about chloramine shocks, but the CYA it adds is a good reason to shock with unstabilized product anyway.
• The ceiling. 5.7.3.1.1.5 says free chlorine shall not exceed the product label. Annex 5.7.3.1.1.5 says a shock dose when bathers are not present can be part of a water quality plan, and re-entry must be consistent with the label. Item 3 of 6.6.3.1 makes chlorine above 10 ppm with bathers present an imminent health hazard.
• Testing at the top end. The same Annex section warns that operators sometimes keep adding chlorine when they cannot measure free chlorine and the test is bleaching out. Know how your test method behaves at high readings before you trust a low one after a shock.
• pH. The Annex reports HOCl at 1 ppm free chlorine, 77 F, no CYA: 0.4286 ppm at pH 7.6 against 0.3212 ppm at pH 7.8 (Table 5.7.3.4.2). Check pH before you dose. This is about efficacy, not breakpoint speed, which the Annex does not quantify.
• Ventilation. CDC says to close the pool, ventilate the swimming area, raise free chlorine to 10 times combined chlorine, then bring it back to the operating range.

When is breakpoint the wrong tool?

Water exchange is plain dilution. For a drain and refill, with full mixing and replacement water carrying no combined chlorine, the fraction to replace is 1 - (target / current). From 0.8 to 0.4 ppm that is 1 - 0.4/0.8 = 50 percent, or 45,000 gallons in the example. The Annex notes that fill water from a chloraminated supply can carry monochloramine, which sets a floor.

Situation	First move	Basis
CC above 0.4 at the pre-opening test, shock can finish before opening	Close, ventilate, dose, retest, log, reopen when free chlorine is back in range	5.7.4.4.2, 5.7.5.1
CC above 0.4 while open, bathers in	Hold free chlorine in the normal range, enforce showers and bathroom breaks, schedule the shock after close	6.6.3.1 item 3, 5.7.4.4.2
Indoor pool, CC above 0.4 on repeat days	Shock, then work the air plan: fresh air, exhaust, ventilation setpoints	5.6.2.8
Dose done, CC still above 0.4	Recheck arithmetic and test, then partial water exchange	5.7.4.4.2
CC at or below 0.4	No action required; log the reading	6.4.1.6 item 3

How do you run this across 20 or 60 pools?

Treat one high reading and a chronic site differently. One rule you could set (pick your own thresholds): one reading above 0.4 in 14 daily logs is an event, so shock and log. Three or more in 14 is a site problem. Look at bather load, the CYA and free chlorine ratio, filtration, and for indoor sites the air handling.

Keep sign-off simple. Breakpoint chlorination is a required topic in operator training courses under 6.1.2.1.1.6, so name the trained people who may authorize a shock and who may sign the dose arithmetic. Keep the dose math on the log line: reading, rise, pounds, product, retest.

Say a company runs 20 pools and 5 need a shock on the same night. Stagger them: assign each pool a shock slot after close and a retest slot before open, and cap each person to the sites they can finish in the gap. Poolean keeps each pool's water readings with test photos and builds daily or monthly logs, and you can download a CSV, so a supervisor can review every site's combined chlorine from one place instead of pulling binders. If you want to see how it works across sites, book a 30-minute call.

For the basics on the reading itself, see combined chlorine in a commercial pool and how often to test commercial pool water. For ordering enough product, see pool chemical usage forecasting.

The short version

• Breakpoint dose is about 10 times combined chlorine in ppm (MAHC Annex); CDC's wording targets a free chlorine level of 10 times combined chlorine. Write down which one your SOP uses.
• Pounds = ppm x (gallons / 1,000,000) x 8.34, divided by the label's available chlorine fraction.
• Stabilized shock products add CYA: about 0.61 ppm per ppm of chlorine for trichlor and 0.91 for dichlor.
• Shock after close, stay under the label maximum, and never run over 10 ppm with bathers in.
• Repeat high readings are a source problem: bather load, ventilation, fill water. Dose math will not fix them.