Pond filter and UV sizing calculator
UV clarifiers are sized by wattage per volume and by contact time, which means flow rate matters as much as the lamp.
% of maximum
hours per day
L/h
W
UV wattage recommended
24.0W
- Your lamp is big enough
- 0
- Maximum flow for that lamp
- 4500 L/h
- Flow is slow enough for the UV to work
- 0
- Turnover time
- 1.60 hours
- Biological media volume
- 40.0
- UV watts per 1000 litres
- 2.25
Show the working
- Pond volume = 8000
- Fish load = 50
- Direct sun = 6
- Pump flow = 5000
- UV lamp wattage = 18
- base_uv = 8000 / 1000 * 2 = 16
- load_factor = 1 + 50/200 + 6/24 = 1.5
- UV wattage recommended = 16 * 1.5 = 24
- Maximum flow for that lamp = 18 * 250 = 4500
- Flow is slow enough for the UV to work = 5000 <= 4500 ? 1 : 0 = 0
- Turnover time = 5000 > 0 ? 8000 / 5000 : 0 = 1.6
- Your lamp is big enough = 18 >= 24 ? 1 : 0 = 0
- Biological media volume = 8000 / 1000 * (50 > 50 ? 8 : 5) = 40
- UV watts per 1000 litres = 8000 > 0 ? 18 / (8000/1000) : 0 = 2.25
Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.
Scenarios & local history
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The formula
- a common baseline is 2 W of UV per 1000 litres, raised for fish load and sunlight
- contact time limits flow to roughly 250 litres per hour per watt
UV lamps lose most of their output long before they stop glowing. Replace them annually.
A UV clarifier kills free-floating algae. It does nothing about string algae on the rocks.