Skimmer sizing2026-10-04

What size pond skimmer do I need?

Choose a skimmer whose surface-area rating covers your pond and whose flow range contains the flow your pump actually delivers at the waterfall. The weir opening then has to be wide enough to pass that flow. For a 3,000 gallon ecosystem pond moving about 3,000 gallons per hour, a 6 inch weir skimmer rated to 4,000 GPH or more will do. A 5,000 gallon pond moving 5,000 gallons per hour needs an 8 or 9 inch weir. Once the surface passes about 1,000 to 1,400 square feet, or the flow passes what the largest single box is rated for, the makers' tables run out and the answer is two skimmers.

What the manufacturers say

Our article on GPH for koi and ecosystem ponds picks the flow and matches skimmers to it by surface area. This page covers what that article leaves out: the weir, the water level, where the box goes, and when one box is not enough.

Weir width sets the flow ceiling

Every maker publishes a weir width and a maximum flow. Dividing one by the other gives the most flow the maker allows per inch of opening.

Weir width, rated surface and maximum flow, manufacturer and dealer figures (flow per inch is our division, rounded)
Skimmer	Weir	Rated surface	Maximum flow	Maximum flow per inch of weir
Aquascape Signature Series 200	6 inches	200 square feet	3,000 GPH	500 GPH
Aquascape Signature Series 400	6 inches	400 square feet	4,000 GPH	667 GPH
Aquascape Signature Series 1000	8 inches	1,000 square feet	10,000 GPH	1,250 GPH
Atlantic PS3900	6 inches	200 square feet	3,900 GPH	650 GPH
Atlantic PS4600	6 inches	400 square feet	4,000 GPH	667 GPH
Atlantic PS7000	9 inches	900 square feet	7,000 GPH	778 GPH
Atlantic PS15000	14 inches	1,400 square feet	15,000 GPH	1,071 GPH
EasyPro Pro-Series Small (PS1)	8 inches	600 square feet	6,000 GPH	750 GPH
EasyPro Axiom PSA5000	6 inches	not published	5,100 GPH	850 GPH
EasyPro Axiom PSA7000	8 inches	not published	7,080 GPH	885 GPH
EasyPro Axiom PSA9000	8 inches	not published	9,000 GPH	1,125 GPH
EasyPro Axiom PSA11000	10 inches	not published	11,100 GPH	1,110 GPH
Savio Skimmerfilter, 6 inch weir	6 inches	300 square feet	3,500 GPH	583 GPH
Savio Skimmerfilter, 8.5 inch weir	8.5 inches	500 square feet	5,000 GPH	588 GPH
Savio Skimmerfilter, 16 inch weir	16 inches	1,200 square feet	8,500 GPH	531 GPH

Atlantic's figures are from its water garden instruction manual (Atlantic has been part of Atlantic-Oase since Oase bought it in 2018). EasyPro's own pages list the PSA7000 at "118 gpm (7080 gph)" and the PSA9000 at "150 gpm (9000 gph)," and the other EasyPro rows come from the dealers Midwest Ponds and Pond Warehouse. Aquascape's site refuses automated fetches, so its rows are from Pond and Garden Depot. Savio's are from PondUSA, which also lists minimums of 500, 2,500 and 5,000 GPH.

No single GPH-per-inch figure holds across brands. The makers allow anywhere from 500 to 1,250 GPH per inch, and Aquascape gives its 200 and 400 the same 6 inch weir with different ceilings, so the box and basket limit flow too. Read the maker's own table for the unit you buy.

Why too much flow through a narrow weir fails

Water reaches the pump only by falling over the lip of the floating weir door. The Pond Guy's page on water dropping below the skimmer opening puts it plainly: "Skimmers are rated for maximum water flow. If your pump exceeds this rating, the water will leave the skimmer faster than it enters, leaving you with a low water level." Savio's Compact Skimmerfilter manual lists "Weir not large enough for pump flow" as a cause of a pump chamber that "runs dry during operation," with "Install larger weir assembly" among the remedies, and it lists "Pump flow rate too high" as a cause of a filter pad that needs frequent cleaning.

The Bureau of Reclamation's Water Measurement Manual shows why width matters. Its Francis equation for a contracted rectangular weir gives the flow in cubic feet per second as 3.33 times the weir length (less a fifth of the head) times the depth of water over the lip raised to the 1.5 power. The manual calls it "convenient for weirs operating under favorable prescribed conditions," and a skimmer door is not a measuring weir, so treat the inches below as the shape of the problem.

Water depth over the weir lip needed to pass each flow, from the Francis equation (our arithmetic, approximate for a floating door)
Flow	6 inch weir	8 inch weir	14 inch weir
3,000 GPH	2.1 inches	1.7 inches	1.1 inches
4,000 GPH	2.5 inches	2.0 inches	1.4 inches
7,000 GPH	3.8 inches	3.0 inches	2.0 inches
10,000 GPH	5.0 inches	3.9 inches	2.6 inches

The door has to sit that far below the pond surface to pass the flow. When the pump asks for more than the door's travel allows, the level inside the box falls and the pump draws air. A weir that is too wide fails the other way, because the water crosses the lip too slowly to pull leaves in.

Water level

Where each maker sets the pond's water level on the skimmer face
Source	Water level
Aquascape Signature Series 400 manual	"approximately ¾” below the top of the opening"
EasyPro Axiom instructions	“Ideal standing water level is 1" down from top of skimmer opening.”
The Pond Guy	"about an inch below the top of the skimmer opening"
Atlantic manual	"between 1¼” and 1 ½” below the top of the weir door opening"
Savio Skimmerfilter manual	at the mark on the faceplate, with levels allowed to "run 3/4” above or below this mark"

Get the box height right when you dig. Aquascape's manual for its older 6.0 and 8.0 skimmers says the adjustable faceplate "is for making final adjustments and tweaks to the water level," and that otherwise "You may find you’ll need to reset the entire skimmer box." EasyPro finds the level "with a transit or laser level." Atlantic wants soil in front of the box "at least 10” below water level" so the door can swing, and The Pond Guy says to "ensure the overflow's bottom is not lower than the ideal water level."

Placement and wind

Aquascape's Signature Series 400 manual says "the skimmer should be placed directly across from a waterfall. If you do not have a waterfall, use your area's prevailing winds to determine where you should place your skimmer along the longest axis." Atlantic and EasyPro both put the skimmer at the opposite end from the waterfall filter. Savio's Skimmerfilter manual ranks the two: the downwind side "is less important than placing the Skimmerfilter opposite the waterfall but it is a significant design consideration." Savio also warns that "Anything placed in front interferes with skimming action."

When you need two

Atlantic's manual says that if the skimmer and falls sit too close together "or the pond has an unusual shape, dead areas can occur. These dead areas can be eliminated with the use of multiple falls or multiple skimmers." The other trigger is size. Atlantic's largest box in that manual is rated to 1,400 square feet and 15,000 GPH, and a 20,000 gallon pond turned once an hour needs more than that.

What contractors and owners do

Kent Wallace, a pond builder writing in POND Trade Magazine (2014), puts a floor under the flow: "1,500 GPH: The minimum flow rate required to get most skimmers on the market to work properly." On the Garden Pond Forum (2015), Burgmetz's Aquascape MicroSkim did not skim on about 300 GPH, and at about 720 GPH "the wier door seems to function correctly and stuff is pulled in pretty good." In that thread Skydog runs his water "around 2" or so" below the opening, and Meyer Jordan replied that Aquascape skimmers are "engineered to provide maximum skimming action with the water level a 3/4" below the top of the faceplate opening."

The high end shows up as a box that will not stay full. In a 2010 thread, acleme1's 3,360 GPH pump was "pumping lots of air," and after asking a local koi shop he concluded "my skimmer is too small for my pump." bthis suggested a valve "so your skimmer won't drain so quickly." Meyer Jordan, in a 2017 thread, said "The size of the intake weir opening is the prime controlling factor in determining maximum flow rate for a skimmer," and that it should not be so wide that it would "greatly reduce the velocity of the water flow through the weir." Waterbug added: "IMO a bigger skimmer doesn't clean better than a correct size," and "Multiple skimmers are used for large ponds rather than larger skimmers."

In a 2015 thread DrCase runs "a 4800 gph pump in my savio with a 8" weir" and hoses the pad "every few days" at that time of year. Roger and Melody Sears, who have built ponds for more than 30 years, wrote in POND Trade (2019) that skimmers with 8 inch openings "usually begin to starve the pump after a water level drop of only a couple of inches." JFKVirginia (2026) sees cavitation start "when the water level over the bottom of the skimmer inlet drops below about 1/2 inch."

Where the sources differ and why it matters

Savio's ranges differ between sources. PondUSA lists 500 to 3,500 GPH for the 6 inch weir, while Meyer Jordan quoted Savio in 2015 as "6" 500 - 2500." The water level differs by brand, from 3/4 inch below the opening for Aquascape to 1 1/4 to 1 1/2 inches for Atlantic, so use the mark for the box you bought. The common idea that a bigger skimmer is the safe choice has no maker behind it. Savio's 16 inch weir has a 5,000 GPH minimum, and Wallace's 1,500 GPH floor holds even for a 1,000 gallon pond, so on a small pond the skimmer can set the pump size.

Every rating applies to the flow that actually passes through the box. A pump's box rating is measured at zero head, and the same pump moves less once it lifts water to a falls.

What to do

Get the surface area by tracing the pond in the PondTools app, which computes it from the outline. The PondTools surface-area calculator (also standalone) then takes that area, water temperature, the coldest night and the kind of fish, and sizes aeration, de-icer wattage and a stocking limit from them.
Find the delivered flow with the PondTools pump calculator (also standalone). It takes pond volume, static lift, pipe inside diameter, one-way pipe length, a flow to check and points from the pump's curve, and returns the head and the flow the pump will actually deliver.
Pick a skimmer whose surface rating covers the pond and whose range contains that delivered flow, with some room below the maximum. EasyPro's recommended ranges stop at about 90 percent of each Axiom's maximum (8,100 of 9,000 GPH on the PSA9000), a margin worth leaving on any brand for a basket half full of leaves.
Put it across from the waterfall, on the downwind end if you can, with nothing in front of the opening.
Set the box to its own maker's water level with a laser or transit before backfilling, and set the overflow at or above that level.
Add a second skimmer when the pond has coves or an odd shape that leaves dead areas, or needs more flow or surface than one box is rated for. Atlantic's skimmer manual sketches one such layout with two skimmers and two FilterFalls.
If the box runs low, check the basket first, then for a leak (see leak or evaporation), then throttle the pump with a valve on the discharge.