Fecal incident (hyperchlorination) calculator
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Quick answer
For a diarrheal incident CDC recommends a CT of 15,300: hold time in minutes = 15,300 ÷ free chlorine ppm, so 20 ppm for 12.75 hours at pH 7.5 or lower. With 1–15 ppm CYA, use 20 ppm for 28 hours, 30 ppm for 18 hours or 40 ppm for 8.5 hours. For formed stool or vomit, hold 2 ppm for 25 minutes.
- Diarrheal, no CYA: 15,300 ÷ ppm minutes (20 ppm → 12.75 h; 10 ppm → 25.5 h).
- Diarrheal, CYA 1–15 ppm: 20 ppm × 28 h, 30 ppm × 18 h or 40 ppm × 8.5 h.
- CYA above 15 ppm: partially drain to 15 ppm or less first.
- Formed stool or vomit: 1 ppm × 45 min, 2 ppm × 25 min or 3 ppm × 19 min.
CDC hyperchlorination options
| Incident | Water | Free chlorine | Hold time (pH ≤ 7.5, ~77°F) |
|---|---|---|---|
| Formed stool / vomit | Any | 1 ppm / 2 ppm / 3 ppm | 45 / 25 / 19 minutes |
| Diarrheal | No CYA | 20 ppm (or 10, 40) | 12.75 h (25.5 h, 6.4 h) — CT 15,300 |
| Diarrheal | CYA 1–15 ppm | 20 / 30 / 40 ppm | 28 / 18 / 8.5 hours |
| Diarrheal | CYA > 15 ppm | — | Drain and refill to CYA ≤ 15 ppm, then hyperchlorinate |
How much chlorine does hyperchlorination take?
Raising free chlorine from 2 to 20 ppm in a 50,000-gallon pool takes about 7.2 gallons of 12.5% liquid chlorine or 12 lb of 65% cal-hypo. A 100,000-gallon pool needs double. Many facilities do not keep that much product on hand — run the numbers for your pool now and note the quantity in your contamination response plan.
After the hold: reopening checklist
- Confirm the full CT was reached — log the free chlorine and pH at the start, during and at the end of the hold.
- Diarrheal incidents: backwash the filter, sending the water to waste.
- Bring free chlorine back down to the normal operating range (dilution, time or a dechlorinator, per your code).
- Check pH is back in range, then reopen and record the time and who approved it.
- Fill in the incident log. See CDC fecal incident response for the full procedure.
How to hyperchlorinate after a diarrheal incident
- Close and clear. Close the pool, clear bathers and remove as much material as possible.
- Check CYA and pH. If CYA is above 15 ppm, partially drain first. Lower pH to 7.5 or below.
- Raise free chlorine. Add unstabilized chlorine to reach 20 ppm (or the level you choose).
- Hold for the full time. Hold 12.75 hours at 20 ppm with no CYA, or 28 hours at 20 ppm with 1–15 ppm CYA.
- Backwash, restore and log. Backwash to waste, return chlorine to normal range, reopen and complete the incident log.
Frequently asked questions
How do you calculate CT value for a pool?
CT = free chlorine (ppm) × time (minutes). CDC’s target for a diarrheal incident is 15,300, so hold time in minutes = 15,300 ÷ free chlorine. At 20 ppm that is 765 minutes (12.75 hours).
How long to close a pool after diarrhea with stabilizer?
With 1–15 ppm CYA: 28 hours at 20 ppm, 18 hours at 30 ppm or 8.5 hours at 40 ppm free chlorine, pH 7.5 or lower. Above 15 ppm CYA, partially drain first.
How much chlorine do I need to reach 20 ppm?
About 10.2 fl oz of 12.5% liquid chlorine per 1 ppm per 10,000 gallons. Going from 2 to 20 ppm (+18 ppm) in 50,000 gallons takes about 7.2 gallons.
When does the hyperchlorination clock start?
When the whole pool reaches the target free chlorine and pH is 7.5 or lower. Test at several points before starting the clock.
Is vomit treated like a fecal incident?
Yes. CDC guidance treats vomit like a formed stool: 2 ppm free chlorine for 25 minutes at pH 7.5 or lower.
This page is an educational summary, not legal advice. Pool and spa codes differ by state and county and change over time — always confirm requirements with the health department that inspects your facility.