In short: UV dose is intensity × time. A system delivering 30 mW/cm² for 1.4 seconds of contact time provides 42 mJ/cm² — enough to clear the common NSF/ANSI 55 Class A residential disinfection minimum of 40 mJ/cm².
This is a planning estimate from the intensity and time you enter. It does not replace third-party reactor validation or a certified system's rated performance.
How UV dose is calculated
UV dose measures the total UV energy an organism receives as water passes through the reactor. It is simply the UV intensity striking the water multiplied by how long the water is exposed to it — a milliwatt applied for one second is a millijoule, so the units work out directly to millijoules per square centimetre:
Dose (mJ/cm²) = Intensity (mW/cm²) × time (s)
Intensity should be the manufacturer's rated output at the point in the water furthest from the lamp, ideally already de-rated for end-of-lamp-life and your water's UV transmittance (UVT). Contact time is the reactor's water volume divided by the flow rate through it.
Worked example. A residential system rated at 30 mW/cm² with a reactor sized to give 1.4 seconds of contact time delivers 30 × 1.4 = 42 mJ/cm² — above the 40 mJ/cm² NSF/ANSI 55 Class A minimum. The same dose equals 420 J/m² or 42,000 µW·s/cm².
Common UV dose targets
Most residential systems are certified against one of two NSF/ANSI/CAN Standard 55 classes. Municipal systems instead validate a specific dose against the pathogens and log-inactivation levels required by their regulator, per the EPA's UV Disinfection Guidance Manual.
| Class | Minimum dose | Intended use |
|---|---|---|
| Class A | 40 mJ/cm² | Disinfection of microbiologically unsafe water |
| Class B | 16 mJ/cm² | Supplemental treatment of water already deemed safe |
Source: NSF/ANSI/CAN Standard 55. Certification also depends on validated reactor testing, not intensity and time figures alone.
Why intensity and time both matter
Intensity falls over the system's life. A new lamp puts out far more UV than one nearing its rated replacement interval, and manufacturers size systems to still meet the target dose at end-of-lamp-life. Sleeve fouling, mineral scale, and lower water temperature all reduce intensity further, and cloudy or tea-coloured water (low UV transmittance) absorbs UV before it reaches the far side of the reactor — see the UV transmittance calculator to convert %UVT into an absorbance figure you can use to de-rate intensity.
Time is set by flow, not by the clock. Contact time is the reactor's water volume divided by the flow rate through it, so a higher flow rate — from a bigger household, more fixtures running at once, or an oversized pump — shortens contact time and lowers dose even though the lamp hasn't changed. Check the flow your system will actually see with the pump flow rate calculator before sizing the reactor.
Worked scenarios
New lamp, sized reactor
42 mJ/cm²
30 mW/cm² × 1.4 s. Clears the 40 mJ/cm² Class A minimum with a small margin.
End-of-lamp-life derate
22.4 mJ/cm²
Same reactor, intensity down to 16 mW/cm² × 1.4 s. Fails Class A (40) but still clears Class B (16).
Flow above rated capacity
15 mJ/cm²
Full-intensity lamp at 30 mW/cm², but flow cuts contact time to 0.5 s. Falls just short of even the 16 mJ/cm² Class B minimum.
What else affects the result
Water clarity (UVT). The intensity you enter should already reflect your water's UV transmittance — a manufacturer's data sheet often lists dose at a stated %UVT (commonly 95% or 75%). Using a clean-water rated intensity for turbid or tannin-stained water will overstate the delivered dose.
Flow variability. Peak-hour flow, not average flow, sets your worst-case contact time. Size the reactor and sensor setpoint against the highest flow the system will see, not a typical one.
Sensor drift and fouling. Most certified systems monitor dose indirectly through a UV sensor and flow signal; a fouled sleeve or miscalibrated sensor can report a dose that no longer matches reality, which is why manufacturers specify periodic sleeve cleaning and sensor checks.
Frequently asked questions
What is UV dose and how is it calculated?
UV dose is the amount of UV energy delivered to the water, in millijoules per square centimetre (mJ/cm²). It is calculated as UV intensity (mW/cm²) multiplied by contact time (seconds), since a milliwatt applied for one second is a millijoule.
What UV dose is required for drinking water disinfection?
Residential point-of-use and point-of-entry systems are commonly certified to NSF/ANSI/CAN Standard 55: Class A systems must deliver at least 40 mJ/cm² for disinfection of microbiologically unsafe water, while Class B systems deliver at least 16 mJ/cm² as supplemental treatment for water already deemed safe. Municipal systems instead follow dose targets validated under the EPA's UV Disinfection Guidance Manual.
How do I find the contact time for my UV system?
Contact time is the reactor volume divided by the flow rate. Use the advanced panel to enter the reactor volume in liters and the flow rate in liters per minute, and the calculator converts that to seconds and fills in the time field for you.
Does water clarity affect the UV dose I actually deliver?
Yes. Low UV transmittance (UVT) water absorbs more UV light before it reaches organisms away from the lamp, which effectively lowers the delivered dose even though the lamp's rated output has not changed. Check your water's UVT with the UV transmittance calculator and use a manufacturer's UVT-derated intensity here rather than the lamp's sleeve-face output.
Why does the dose fall as the lamp ages?
UV lamps lose output gradually, and most manufacturers rate systems using an end-of-lamp-life intensity, often well below the intensity of a brand-new lamp. Sleeve fouling, mineral scale, and lower water temperature can reduce output further, so a system sized against fresh-lamp figures can fall short of its target dose well before the lamp is due for replacement.
Can this calculator confirm my system is certified or safe?
No. This is a planning calculator for the dose arithmetic only. Certification to NSF/ANSI 55 or compliance with a regulatory dose requirement depends on third-party reactor validation testing, not just the intensity and time you enter here. Confirm system performance with the manufacturer's validated data sheet or a water treatment professional.
Sources & references
- U.S. Environmental Protection Agency (EPA). Ultraviolet Disinfection Guidance Manual (UVDGM), 2006.
- NSF/ANSI/CAN Standard 55. Ultraviolet Microbiological Water Treatment Systems.
- International Ultraviolet Association (IUVA). UV dose guidelines for drinking water applications.
Sarah Bennett, MS
Water Quality & Sustainability Specialist · Portland, Oregon
Environmental scientist (MS, Water Resources) with 9+ years in residential water quality, treatment, and usage efficiency, and 150+ completed assessments. Sarah writes and reviews the treatment, usage, and cost calculators on watercalculator.us. Read full bio
MS Environmental Science, Water Resources (2018) · BS Environmental Engineering (2016)