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Pond construction

How deep should a koi pond be?

Dig a koi pond at least 3 feet deep, and go to 4 feet or more if you can, with steep sides and few or no plant shelves. The only major voice that disagrees is Aquascape, whose ecosystem pond kits are built to 2 feet and whose fish guidance says "generally about 2 to 2½ feet." That depth suits a water garden that happens to have koi in it. If you want plant shelves, keep the first one 6 to 10 inches deep and no step between shelves taller than 12 inches, which is how contractors already dig ecosystem ponds.

What the authoritative sources say

The extension literature frames depth as a climate question. Clemson's Home and Garden Information Center says "Many pools are 18 to 24 inches deep, but in cooler areas of the state these pools may require heaters," and that "In warmer climates during the summer, plants and fish should have areas at least 3 to 4 feet deep to allow for a cool refuge." It also gives the only extension shelf dimension we could find: "Most earthen and/or flexible liner ponds have a 10-inch shelf approximately 18 inches wide around the perimeter of the pond before dropping off to a depth of 20 to 36 inches." The Southern Regional Aquaculture Center's Water Gardens fact sheet (SRAC 435, hosted as a PDF by Texas A&M and by Oklahoma State) says "Many year-round outdoor water gardens have a section at least 3 or 4 feet deep that does not freeze in the winter and gives fish a cool retreat during hot weather," names koi as the fish that suffer without it, and adds that "A depth of 18 to 24 inches is sufficient in the deep south, as long as only a few fish are stocked and plenty of floating aquatic plants are provided for shade." The Oklahoma State HTML page refused the fetch, so those quotes come from the Texas A&M PDF.

Aquascape's own pages also refuse automated fetches, so we checked its position three other ways. A search snippet of its Pond Fish Health page reads "The pond should be deep enough (generally about 2 to 2½ feet) to give the fish a chance to get out of the way of predators." A snippet of its pond kit catalog lists finished sizes of 4 by 6, 6 by 8 and 8 by 11 feet, each with a 2 foot maximum depth. And an article by Ed Beaulieu, Aquascape's vice president of field research, was republished by Pond Trade Magazine, where it does load; Aquascape's own How to Excavate a Pond page carries the same figures. Beaulieu writes that "The first pond ledge is typically 6 to 10 inches deep," that "a ledge that is 6 inches deep will become a 4-inch-deep ledge after the gravel is installed," that you should use "narrow ledges that are 6 to 10 inches wide in the foreground and wider ledges of 16 to 24 inches in the background," and, if the stone is hand placed, "keep your ledges a maximum of 12 inches tall." Treat all of that as Aquascape's rule for its own kits.

On herons, Pond Informer's heron deterrent guide says "both the great blue and great egret usually hunt within a depth range of 5-11 inches (13-28 cm), with the most frequent hunting depth being 7-9 inches (18-23 cm)" and that "depths of 13 inches+ (33 cm) are very rarely, if ever, hunted in by the larger heron species." Pond Informer is a hobby site, so we looked for a primary source. A 1980 Ohio State University report, Great Blue Herons as Environmental Indicators by Gerald Grau and Robert Parris, plots the "heron feeding zone" on a managed marsh as water between 0 and 40 centimeters deep, which is about 16 inches. Neither source gives a depth that stops a heron standing on a shelf and reaching down, which is the case that matters for a pond.

Depth and shelf dimensions by pond type, with the source for each row
Pond typeWorking depthShelvesSource
Ecosystem pond, Aquascape style2 feet maximum in the kits; "generally about 2 to 2½ feet"First ledge 6 to 10 inches deep, 6 to 10 inches wide in front, 16 to 24 inches wide behind; no step over 12 inches tallAquascape (search snippets); Ed Beaulieu in Pond Trade Magazine
Water garden with fish, extension guidance18 to 24 inches in the deep South with few fish; a section "at least 3 or 4 feet deep" for year round ponds; 20 to 36 inches below the shelfOne shelf 10 inches deep and about 18 inches wide around the perimeterSRAC 435 (Texas A&M PDF); Clemson HGIC
Koi pond, mild climate"A minimum depth of 3 feet is acceptable in areas with mild climates"; 4 to 6 feet idealSteep sides; "no plant shelves"Pond Informer; Grass Roots Pond and Garden
Koi pond, northern winters"at least 4 feet"; keepers in Zone 5 and 6 report 48 inches; Minnesota society says 3 to 5 feet or more so the pond cannot freeze solidSame as above; any shelf is a stepping stone for predators and a freeze zone for plantsPond Informer; Garden Pond Forum; Minnesota Water Garden Society

What contractors and owners do

The Aquascape contractors are open about building shallow. Premier Ponds of Glenwood, Maryland, which says it has built over 2,000 ponds as a Certified Aquascape Contractor, writes "Most guides tell you a koi pond has to be three to six feet deep. We build most of ours about two," because in Maryland "two feet clears our local frost depth line." Dreamscapes Watergardens in Pennsylvania puts the freeze zone at 18 inches and recommends digging "at least two feet (2' or 24") deep," with a "marginal shelf" at 0 to 8 inches and a "mid-zone lilies shelf" at 12 to 16 inches "before your pond drops to the 24" depth." Dreamscapes also notes a limit of the design: with an Aquascape skimmer, "anything deeper than 2.5-3 feet will need to be stirred up in order to be able to pull that water into the skimmer box."

Koi specialists go the other way. Grass Roots Pond and Garden of New Boston, Michigan, puts the ideal at "4 to 6 feet," says "In colder climates, freezing can be a real issue. You'll want at least 4 feet," and argues against shelves outright: "A well-designed koi pond with steep sides and no plant shelves gives fish a safe zone," because shelves "can create a stepping stone effect for predators." Meyer Aquascapes, an Aquascape distributor, nonetheless publishes "The general rule of thumb is to keep pond depths between 4 and 5 feet for koi ponds." Southern California Koi Rescue is blunter still: "A proper koi pond should have a minimum depth of 4 feet," and it titles its assessment of Aquascape design ponds "Unfit for Koi," noting that "Most Aquascape design ponds are less than 2 feet deep."

Owners in cold states land near the specialists. On Garden Pond Forum in June 2018, Dosry5 in Des Moines, Iowa, described a 3 foot hand dug pond and a landscaper who said "no chance any fish survive with it being at least 42 inches." mrsclem replied "I would say if subzero for weeks, go deeper. 4' or more," and qclabrat in New Jersey said "I went with 48", for you I suggest at least that deep," while j.w suggested raising the walls with berms or blocks instead of digging further. On Pond Informer's heron page, a commenter named Henry wrote that "Designing a pond 3′ to 4′ deep with steep walls is ideal to keep them out" and that a 4 foot pond "is ideal for the cold weather as well as a Heron deterrent."

Where the tiers differ and why it matters

The 2 foot and 4 foot camps are describing different ponds. Aquascape's 2 to 2½ feet comes with gravel, plants, a skimmer that stops pulling water at 2.5 to 3 feet, and a light fish load; the extension sources accept 18 to 24 inches only "in the deep south" with "only a few fish." Koi keepers stock heavily and keep large fish for years, so their 4 feet is about water stability and predators as much as ice. If a client says koi and means a dozen fish they intend to keep for twenty years, the kit depth is the wrong answer even in Maryland.

Two numbers circulate without a source. "Two feet below the frost line" appears on blogs with no extension or engineering citation behind it, and the frost depth the contractors quote (18 inches in Pennsylvania, per Dreamscapes; Premier says only that two feet clears Maryland's) is a soil figure that says nothing about how thick ice gets on standing water. Likewise the heron hunting depths are a hobbyist's figures; the 1980 Ohio State marsh study supports a feeding zone up to about 16 inches but says nothing about shelves. The contradiction on shelves is real: Clemson calls the shelf "excellent plant habitat," Grass Roots calls it a predator step, and both are right, which is why a koi pond either skips shelves or pushes them away from the edge.

What to do

  1. Pick the pond type first. An ecosystem pond with a few fish needs 2 to 2½ feet in the mid Atlantic and south, and more where winters are longer. A koi pond needs at least 3 feet in a mild climate, 4 feet or more north of that, and 4 to 6 feet if the koi are the point.
  2. Set the water level and the filter elevation before digging. As Beaulieu puts it, "The biological filter is always set first, and the flexible piping follows." Use a laser or line level around the whole perimeter.
  3. Cut the outline and dig the first ledge around the full perimeter, 6 to 10 inches deep for an ecosystem pond. Skip it for a koi pond.
  4. For an ecosystem pond, step down to a second shelf at 12 to 16 inches and then to the floor at 24 to 30 inches. For a koi pond, drop near vertical from the rim to full depth, with one deep point for a pump during cleanouts. Keep any vertical you intend to face with hand placed rock at 12 inches or less, and remember a 6 inch ledge becomes 4 inches after gravel.
  5. Check the gallons before the liner arrives. On a 10 by 14 foot rectangle, 2 feet of average depth gives about 2,100 gallons and 3½ feet gives about 3,700. The extra depth adds about three quarters to the volume and to the filtration it needs, and it makes the liner bigger too.

Do not treat a 2 foot kit as a koi pond, do not dig a skimmer only pond past 3 feet without adding a bottom drain or circulation, and do not run a plant shelf around a koi pond at the edge where a heron can stand on it. At 1,000 gallons and 4 feet deep the footprint is only about 33 square feet. At 20,000 gallons a center 5 or 6 feet deep is normal, and the shelves become a safety matter as much as a planting one.

The PondTools tools for this

Depth drives everything downstream. The PondTools volume calculator (also standalone) takes length, width, average depth and a shape setting and returns gallons, so you can run the same footprint at 2 feet and 4 feet in a minute. Feed that volume and the surface area into the PondTools stocking calculator (standalone), which takes pond volume in gallons, surface area in square feet, and a count and current length for each fish type, and tells you whether the fish will still fit at adult size; how many koi per gallon covers the rules it uses. In the PondTools app, trace the outline from a satellite or drone photo and set the depth of each shelf separately, so the ecosystem profile above (first ledge, lily shelf, floor) or a sheer koi profile comes out as real volume and real liner size; pond liner size with shelves explains why the liner is sized from the deepest point rather than the average depth.

Sources