Pick your target dose, then enter your UV unit's rated flow to check it against your water use
1 · Target UV dose
Target
Log Reduction
Custom UV Dose
mJ/cm²
2 · Your UV unit (from its spec sheet)
Rated Flow
…at a Dose of
mJ/cm²
Spec sheets list the maximum flow at a stated dose — usually 40 mJ/cm² (NSF Class A) or 30 mJ/cm² — and at a stated water UV transmittance (UVT), often 95%.
3 · Your water use
Peak Flow (same unit)
L/min
Use the highest flow your system can reach — every tap and appliance that might run at once, or your pump's maximum output.
3 · Your tank
Tank Volume
UV only treats water as it flows past the lamp. This mode shows how long a recirculating pump would take to pass the whole tank through the UV unit once, at the flow the unit can handle for your dose.
⚠ Please enter valid values greater than zero for all required fields.
WaterTankCalculator.com — UV Disinfection Results
Your UV Results
Required UV Dose
—
mJ/cm²
Exposure Time
— min
UV Intensity
— mW/cm²
Effective Power
— W
Log Reduction
— LRV
% Inactivation
— %
System Status
—
UV Reactor Schematic
📖 How To Use
How to Use This UV Disinfection Calculator
Choose your target doseFor household drinking water, use the NSF/ANSI 55 Class A standard of 40 mJ/cm² — it's what certified point-of-entry UV systems are built to deliver. To target a specific organism, pick it and a log reduction and the calculator uses the US EPA dose table.
Enter your UV unit's ratingFind the maximum flow and the dose it's rated at on the spec sheet or label, for example "12 GPM at 40 mJ/cm²".
Flow Check: enter your peak flowThe calculator adjusts the unit's maximum flow for your dose and tells you whether it's big enough.
Tank Turnover: enter your tank volumeIf you're recirculating a tank through a UV unit, you'll see how long one full pass takes.
Important: UV only works on clear water. Cloudy water, iron, manganese and hardness scale on the quartz sleeve all block UV light and cut the dose. Put a sediment filter (typically 5 micron) before the UV unit, and follow the manufacturer's water quality limits.
📐 The Formula
How UV Dose Works
UV dose (also called fluence) is how much UV energy each part of the water receives:
UV dose (mJ/cm²) = UV intensity (mW/cm²) × time in the light (s)
The intensity inside a UV unit depends on the lamp, the chamber shape, the quartz sleeve and how clear the water is (its UV transmittance, or UVT). That's why you can't work out a real dose from lamp wattage and tank size alone — manufacturers measure it by testing each unit, and publish a maximum flow rate for a given dose.
What you can calculate is how that flow rating changes for a different dose. For the same unit and water, the time the water spends in the light is inversely proportional to flow, so:
Max flow at your dose = Rated flow × (Rated dose ÷ Your dose)
Example: a unit rated 45 L/min at 40 mJ/cm², needed for 22 mJ/cm²
= 45 × (40 ÷ 22) = 81.8 L/min (for the same water quality)
This is a first-order estimate. It assumes your water's UVT is at least what the unit was rated at; lower UVT means a lower dose than the rating.
US EPA UV dose table
The US EPA's Long Term 2 rule sets these UV doses for inactivating each type of organism:
Log reduction
Cryptosporidium
Giardia
Viruses
2-log (99%)
5.8 mJ/cm²
5.2 mJ/cm²
100 mJ/cm²
3-log (99.9%)
12 mJ/cm²
11 mJ/cm²
143 mJ/cm²
4-log (99.99%)
22 mJ/cm²
22 mJ/cm²
186 mJ/cm²
The virus values are set by adenovirus, which is unusually resistant to UV. That's why water systems that need to control viruses usually add chlorine rather than relying on UV alone.
NSF/ANSI 55 classes for household UV
Class
Dose
Meant for
Class A
40 mJ/cm²
Disinfecting water that may be contaminated with bacteria and viruses; must have a UV sensor and alarm
Class B
16 mJ/cm²
Supplemental treatment of water that's already considered safe; not for contaminated water
🎯 Use Cases
Using UV With a Water Tank
Put the UV unit on the tank outlet
UV has no lasting effect — it treats water as it passes the lamp and leaves nothing behind to protect it afterwards. For drinking water from a storage tank, the reliable setup is: tank → pump → sediment filter → UV unit → taps. Every litre is treated on its way to use, no matter what's happening in the tank.
Recirculating a tank
Pumping a tank through a UV unit and back reduces germs in the stored water, but because the water mixes, some of it goes through more than once and some doesn't go through at all. Use it to keep a tank in good condition, not as the only barrier for drinking water. A lamp dropped into a tank doesn't work — UV intensity falls off within centimetres in water.
Well and rainwater
UV is a good fit for private wells and harvested rainwater, provided the water is clear. Filter first, check for iron, manganese and hardness, and use our first flush diverter calculator to keep the dirtiest roof runoff out of the tank.
What UV doesn't do
UV doesn't remove chemicals, metals, nitrate or tastes, and it doesn't keep water safe after treatment. Keep the tank clean and covered — see the tank disinfection calculator — and disinfect the pipework downstream of the UV unit when it's first installed or after a lamp failure.
❓ FAQ
Frequently Asked Questions
What UV dose do I need for drinking water?
For household systems, 40 mJ/cm² — the NSF/ANSI 55 Class A standard for water that may be contaminated. For specific organisms, the US EPA table gives 22 mJ/cm² for 4-log Cryptosporidium or Giardia, and 186 mJ/cm² for 4-log virus inactivation. Because viruses need such a high dose, water systems often combine UV with chlorine.
Can I work out UV dose from lamp wattage and tank size?
No. The dose depends on the chamber design, how far the water is from the lamp, the quartz sleeve and the water's UV transmittance. Two units with the same lamp can deliver very different doses. Use the manufacturer's rated flow at a stated dose, which comes from testing the actual unit.
What size UV system do I need?
One rated for at least your peak flow at the dose you need. If your peak flow is 40 L/min and you want 40 mJ/cm², choose a unit rated for 40 L/min or more at 40 mJ/cm² — ideally with some headroom. Check that its rating assumes a UVT your water actually reaches.
Can I put a UV lamp inside my water tank?
It won't disinfect the tank reliably. UV is absorbed quickly by water, so only water very close to the lamp gets a useful dose. Install a proper UV unit on the outlet so every litre passes through it on the way to the taps.
Does UV kill all bacteria and viruses?
At the right dose, UV inactivates bacteria, Giardia and Cryptosporidium very effectively. Viruses — especially adenovirus — need much higher doses. UV damages the organisms' genetic material so they can't reproduce; it doesn't remove them, and it leaves no disinfectant behind to protect the water afterwards.
What water quality does UV need?
Clear water with good UV transmittance. Particles shade organisms from the light, and iron, manganese and hardness coat the quartz sleeve. Use a sediment filter before the unit and check the manufacturer's limits for turbidity, iron, manganese and hardness — pre-treat if your water is above them.
How often should the UV lamp be replaced?
Follow the manufacturer's schedule — for many household lamps that's about once a year, because output falls as the lamp ages even though it still glows. Clean the quartz sleeve when you change the lamp, and don't ignore a UV sensor alarm.
✅ Sources & Review
Sources and review
The doses and standards on this page come from these references:
NSF/ANSI 55, Ultraviolet Microbiological Water Treatment Systems — Class A (40 mJ/cm²) and Class B (16 mJ/cm²).
US EPA, Ultraviolet Disinfection Guidance Manual (2006) — how UV dose depends on reactor design and UV transmittance.
Reviewed by John Mac, PE, Water Systems Specialist
Texas-licensed Professional Engineer with 13+ years in residential water storage and 180+ tank projects. John writes and reviews every calculator on this site.
