01
The purpose
What does this calculator estimate?
Contact time to inactivate Giardia, Crypto, or E. coli to target — and when UV is required.
Commercial Pools · G5
Commercial Pool Disinfection & CT Calculator
Contact time to inactivate Giardia, Crypto, or E. coli to target — and when UV is required.
Measure the inputsCT is the product of chlorine concentration and contact time — higher FC means less time.
Read the full resultCrypto is extremely chlorine-resistant — a dual UV + chlorine system is required.
Check the boundaryCommercial pools usually run CYA-free so chlorine stays fully active.
Useful forCommercial PoolsPlanningComparisonVerification
Live calculation
Disinfection inputs
✓ Checked May 2026
Enter volume, chlorine levels, and the target pathogen.
Reference result
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Enter the required values to calculate.
Enter the known values. Let the engine handle the calculation.
✓ Formula checked↻ Live result○ No account needed
Planning estimate only. Confirm product labels, equipment specifications, qualified-professional advice and local requirements before acting.
The result, explained
Use the Commercial Pool Disinfection & CT Calculator result with context.
Contact time to inactivate Giardia, Crypto, or E. coli to target — and when UV is required.
02
The tool
What information does it use?
- Pool volume: enter a measured or verified value
- Current free chlorine: ppm
- Target free chlorine: ppm
- Inactivation target: Giardia, Crypto, or E. coli to a log reduction.
- Chlorine type: Liquid or on-site electrolytic.
- Cyanuric acid: Commercial pools are usually CYA-free.
03
The method
How to use this calculator
- Pool volumeEnter the value you can verify.
- Current free chlorineppm
- Target free chlorineppm
- Inactivation targetGiardia, Crypto, or E. coli to a log reduction.
- Chlorine typeLiquid or on-site electrolytic.
- Cyanuric acidCommercial pools are usually CYA-free.
04
The relationship
The formula in plain English
Contact time (min) = CT value ÷ free chlorine (ppm)
CT at pH 7.5, 77°F: E. coli 4-log = 0.4 · Giardia 3-log = 143 · Crypto 2-log = 15,300 (UV required).05
Worked through
A worked example: Giardia 3-log at 3 ppm FC
- Giardia 3-log CT = 143
- Contact time = 143 ÷ 3 = 47.7 min
- ≈ 48 minutes at 3 ppm free chlorine
| Pathogen / target | CT value | At 2 ppm | At 3 ppm |
|---|---|---|---|
| E. coli 4-log | 0.4 | 0.2 min | 0.1 min |
| Giardia 3-log | 143 | 72 min | 48 min |
| Crypto 2-log | 15,300 | 127 hrs (UV req.) | 85 hrs (UV req.) |
06
Why it matters
How the result supports the next decision
CT is the product of chlorine concentration and contact time — higher FC means less time.
Crypto is extremely chlorine-resistant — a dual UV + chlorine system is required.
07
Good inputs
Improve the estimate before relying on it
- CT is the product of chlorine concentration and contact time — higher FC means less time.
- Crypto is extremely chlorine-resistant — a dual UV + chlorine system is required.
- Commercial pools usually run CYA-free so chlorine stays fully active.
- After a fecal incident, follow the published remediation CT for the pathogen.
- Higher pH slows disinfection — keep pH near 7.5 for the listed CT values.
08
The boundary
How to interpret the result
The enforcing authority and the facility operating plan override this general reference.
The engine reports invalid inputs and safety warnings beside the result. Resolve those messages before using the estimate.
09
Before acting
What should override this estimate?
- Measured site conditions and current test results
- Product labels and manufacturer specifications
- Qualified-professional advice and applicable local codes
10
Common questions
Frequently asked questions
What is CT in pool disinfection?
CT is the disinfection metric equal to free chlorine concentration in ppm multiplied by contact time in minutes. Each pathogen and target log reduction has a required CT value. Dividing that value by your chlorine level gives the contact time needed. Higher chlorine shortens the time; lower chlorine lengthens it.
Why is Cryptosporidium so hard to kill?
Crypto has a tough outer shell that resists chlorine extraordinarily well — its CT for even a 2-log reduction runs into the thousands, meaning impractically long contact times at normal chlorine levels. That's why the MAHC requires a supplemental UV or ozone system to inactivate Crypto in high-risk commercial pools.
How long does chlorine take to kill Giardia?
Giardia's 3-log inactivation CT is about 143 at pH 7.5 and 77°F. At 2 ppm free chlorine that's roughly 72 minutes of contact time; at 3 ppm, about 48 minutes. The exact time scales inversely with your chlorine level, which the calculator computes from your inputs.
Do commercial pools use cyanuric acid?
Generally no, or only minimal amounts, because CYA dramatically slows chlorine's disinfection speed and lengthens CT contact times. Keeping commercial pools CYA-free or very low ensures chlorine stays fully active for rapid pathogen inactivation, which matters under heavy public bather loads and code requirements.
What CT do I need after a fecal incident?
It depends on the contamination type. A formed-stool incident is treated as a Giardia-level event with its CT; a diarrheal incident is treated as Crypto, requiring the much higher CT or a UV-assisted remediation per published health guidance. Follow the official remediation procedure for the specific incident type.
What is CT value for pool disinfection?
Chlorine ppm times contact minutes; each pathogen has a required CT (e.g., Giardia 3-log ≈ 143).
Can chlorine kill Cryptosporidium?
Only over impractically long times — UV or ozone is required for Crypto in commercial pools.
How long to kill Giardia in a pool?
About 72 minutes at 2 ppm FC (CT 143 ÷ 2), shorter at higher chlorine.
Primary sources
Use the result with current guidance.
These official references support safe planning context. Product labels, local requirements and qualified-professional advice take priority over a general calculator estimate.