Pond UV Clarifier Size Calculator
Enter your pond volume and sun exposure to find the UV clarifier wattage you need to clear and prevent green water. Built on the 10 watts per 1,000 gallons rule, adjusted for how much sun your pond gets.
Not sure? Use our pond volume calculator first.
More sun fuels faster green-water blooms, so it needs more wattage.
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UV watts
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base watts (10/1,000 gal)
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target GPH through UV
Match the flow: aim to turn your pond over through the UV once every 1 to 2 hours, so roughly GPH passing through it. Check the UV unit's maximum flow rating and keep your flow at or below it so the light has enough contact time to kill the algae. Too fast and green water will not clear.
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How to Size a Pond UV Clarifier
A UV clarifier is the simplest reliable way to clear and prevent green water, and sizing one is mostly about your gallons. The widely used rule is 10 watts of UV for every 1,000 gallons of pond water. A 1,000 gallon pond starts at 10 watts, a 2,000 gallon pond at 20 watts, and a 4,000 gallon pond at 40 watts. Sun exposure then nudges that number up, because sunlight is the fuel that drives the algae bloom you are trying to control. The calculator above takes your volume, applies the 10 watts per 1,000 gallons baseline, multiplies by a sun factor of 1.0 for shade, 1.25 for partial sun, or 1.5 for full sun, then rounds up to the next common bulb size such as 9, 13, 18, 25, 40, or 55 watts. Rounding up is deliberate, since a slightly oversized UV clears faster and never hurts.
Green Water Is Single-Celled Algae
To understand why a UV works, it helps to know what green water actually is. That pea-soup color is billions of free-floating, single-celled algae suspended throughout the water. They are too small and too numerous for a filter to catch, which is why a pond can look murky green even with good filtration running. A UV clarifier is a sealed tube with an ultraviolet bulb inside. As your pump pushes water past the bulb, the UV light damages and kills the algae cells drifting through. The dead cells then clump together and your filter can finally trap them. Within a few days of running a correctly sized UV at the right flow, a green pond usually turns clear.
UV Does Nothing for String Algae
Here is the single most important thing to understand before you buy. A UV clarifier only treats green water. It does absolutely nothing for string algae, the long green hair-like strands that cling to your rocks, liner, and waterfall. String algae stays anchored in place and never travels through the UV tube, so the bulb never touches it. People buy a UV expecting it to wipe out all algae, then feel let down when the string algae on their waterfall is untouched. String algae is a different problem with different solutions: manual removal by hand or brush, cutting the nutrients that feed it with less fish food and more plants, adding beneficial bacteria, or using a barley straw product. If your issue is hairy strands on surfaces rather than cloudy green water, a UV is not the tool. See our guide on how to clear green pond water to confirm which problem you actually have.
Match the Flow Rate, Not Just the Wattage
Wattage is only half of sizing a UV. The other half is flow rate. UV light needs time to act on each algae cell, so the water has to move through the tube slowly enough for good contact. Push it through too fast and the cells survive the trip. A practical target is turning the whole pond over through the UV once every 1 to 2 hours, which is gentler than the once-per-hour circulation you size your main pump for. Every UV unit lists a maximum flow rating, and you should keep the flow at or below it. If your circulation pump is stronger than the UV can handle, run the UV on a slower side loop or tee, rather than blasting the full pump output through it. Start with your gallons using the pond volume calculator, size the UV here, then make sure your pump and UV flow ratings line up. A UV is a finishing polish that works alongside, not instead of, solid biological filtration.
Keep going: size the rest of your pond.
Frequently Asked Questions
What size UV clarifier do I need for my pond?
The standard guideline is about 10 watts of UV for every 1,000 gallons of pond water. So a 2,000 gallon pond wants roughly 20 watts and a 4,000 gallon pond wants about 40 watts. Ponds in full sun grow algae faster, so bump the wattage up. The calculator above starts at 10 watts per 1,000 gallons, multiplies by a sun factor, then rounds up to the next common bulb size such as 9, 13, 18, 25, 40, or 55 watts.
Does sun exposure really change the UV size I need?
Yes. Sunlight is the fuel for the single-celled algae that turns water green, so a pond in full sun blooms faster and harder than one in shade. A mostly shaded pond can often clear with the baseline 10 watts per 1,000 gallons, while a pond in full sun may need 50 percent more wattage to keep up. That is why the calculator applies a multiplier of 1.0 for shade, 1.25 for partial sun, and 1.5 for full sun before rounding to a real bulb size.
Why does flow rate through the UV matter so much?
A UV clarifier only works if the water moves through it slowly enough for the light to hit each algae cell. Push water through too fast and the contact time is too short to kill anything. A good target is turning the whole pond over through the UV once every 1 to 2 hours, not the rapid turnover you might want for circulation. Always check the maximum flow rating on the UV unit and match your pump to it, sometimes by teeing off a slower side loop.
Will a UV clarifier kill string algae too?
No, and this is the most common misunderstanding. A UV clarifier only treats green water, which is billions of free-floating single-celled algae suspended in the water. It works by killing those cells as they drift past the bulb. String algae, the long green hair that clings to rocks and waterfalls, never passes through the UV, so the light never touches it. String algae needs a different fix such as manual removal, better nutrient control, beneficial bacteria, or a barley product.
How long until a UV clarifier clears green water?
Once a correctly sized UV is running with the right slow flow, most green ponds clear noticeably within 3 to 7 days and fully within a couple of weeks. If yours is not clearing, check three things: the bulb is on and not past its life, the flow rate is slow enough for good contact time, and the wattage actually matches your gallons and sun exposure. UV bulbs weaken over time and should be replaced about once a year even if they still glow.
Should I leave the UV clarifier running all the time?
During the warm months when algae blooms, yes, run it continuously alongside your pump so the water is constantly being treated. Turning it on and off lets blooms rebound between cycles. Many keepers switch the UV off in winter when cold water slows algae and the fish are dormant, which also rests the bulb. Remember to replace the bulb roughly once a year, since the glass can still light up long after its algae-killing output has faded.
Do I still need good filtration if I run a UV?
Absolutely. A UV clarifier is a polish, not a substitute for biological filtration. The UV kills floating algae, but your biological filter is what processes the ammonia and nitrite from fish waste and keeps the water safe. In fact UV and filtration work together, because the filter removes the dead algae clumped after UV treatment. Size your filter and pump to your gallons first, then add UV to handle green water on top.