Designing a koi pond: the complete specifications before digging

A koi pond rarely goes wrong when the digging begins. It goes wrong beforehand, on paper, when its size, depth, shape and filtration are decided. Once the liner has been laid and the excavated soil backfilled, adding a bottom drain, deepening a pit or moving a filtration chamber becomes a complete construction project. This specification brings together, in the order in which they need to be decided, the design choices for a koi pond, with approximate figures and a checklist to validate before the first turn of the spade. It does not describe the construction work itself, but everything that must be decided beforehand.

Set the target: number and size of the koi

All the design choices stem from one simple question: how many koi, and what size, will live in this pond in five or ten years? Not the size of the fish purchased initially, but the size they will reach. A 15 cm koi in spring can exceed 50 cm within a few years under good conditions, and adult fish from excellent bloodlines commonly exceed 60 to 70 cm.

So write down clearly:

• The target number of adult koi, knowing that every hobbyist eventually wants more.
• Expected adult size, which depends on bloodlines and rearing conditions.
• Pond use: contemplation, hand-feeding, quality breeding, presence or absence of plants.

These three answers determine the biological load, and therefore the size of the filter, flow rate, aeration and, indirectly, the operating budget.

Minimum volume and depth

The commonly accepted guidelines range from around 1 m³ per adult koi as an absolute minimum to 2 m³ or more for a comfortable, stable pond. Below about ten cubic metres, it becomes difficult to keep adult koi in good conditions. The article What size or volume pond for koi carp? explains these guidelines in detail.

Depth matters just as much as volume:

Deep water warms up and cools down slowly, protecting koi from sudden changes, both in summer and winter. It also keeps the fish out of reach of herons, which hunt by wading in shallow water.

Shape: why to avoid nooks

A classic garden pond is designed like a landscape; a koi pond is designed like a hydraulic system. The shape must allow water to circulate everywhere and waste to reach the bottom drains.

• Simple shapes: rectangle with rounded corners, oval, circle, or a very gentle freeform shape. Every nook is an area where sludge accumulates and water stagnates.
• Steep walls, almost vertical in the deep area: they deter herons, maximize usable volume, and avoid shelves where debris settles.
• A slightly sloped bottom toward each bottom drain, forming a basin, to guide sediment.
• No planting shelves in the main pond: koi rummage through and uproot plants. If you want vegetation, plan a separate area or a planted filter.

L-shaped, strongly kidney-shaped ponds or ponds with a peninsula are the most difficult to keep clean: circulation never truly reaches the extremities. If the garden layout requires such a shape, plan for a water return or an additional bottom drain in each area.

Bottom drains and returns

The bottom drain is the heart of a koi pond. Positioned at the lowest point, it continuously removes waste that settles before it decomposes. The guide Why a bottom drain is important and how to install it explains the principle.

Design guidelines, as a general guide:

• One bottom drain for approximately 15 to 25 m² of floor area, each connected to the filtration chamber by its own 110 mm pipe.
• A flow rate of around 8 to 12 m³/h per bottom drain in a gravity-fed system; above that, add another bottom drain rather than forcing the flow.
• One knife valve per pipe in the chamber, for balancing and isolating each line during maintenance.
• An aerated bottom drain, equipped with a diffuser membrane, combines suction and aeration at the deepest point.

The returns, through which filtered water flows back into the pond, are just as important as the intakes. Positioned in the wall, aimed tangentially and slightly downward, they create a slow rotation of the entire body of water, pushing sediment toward the bottom drains. One or two returns are sufficient for a medium-sized pond; the rule is that no area should remain stagnant.

The skimmer completes the system by drawing from the surface, generally accounting for a minor share of the total flow.

Gravity-fed filtration and sizing

In a koi pond, gravity-fed filtration is the standard: water flows freely from the bottom drains and skimmer into a chamber positioned at pond level, passes through the filtration stages, then a pump sends it back through the returns. The particles never pass through the pump and are not pulverized.

The typical layout, in order:

1. Mechanical prefiltration: drum filter, sieve, or settling tank, which removes particles before they decompose. The guide How to choose a drum filter? compares the options.
2. Biological filtration: high-surface-area media, moving bed, or matrices, where nitrifying bacteria work.
3. Pump at the filter outlet, suitable for gravity-fed operation.
4. UV-C on the return line, for green water.
5. Returns to the pond.

Sizing: the aim is to renew the entire volume every one to two hours. For a 20 m³ pond, this represents roughly 10 to 20 m³/h, typically using two bottom drains and a skimmer, each on its own 110 mm pipe, and a filter rated by its manufacturer for this volume with koi.

Waterproofing: liner, concrete, polyester

Three main solutions are used for koi ponds, each with its own advantages. For a general comparison, also see Liner or polyester for my garden pond?.

The choice depends on the budget, shape, and desired level of finish. For a rectangular shape with vertical walls, coated masonry is often the most consistent choice. For a gently curved shape, a properly installed EPDM liner remains an excellent solution. The guide How to build a pond or garden pond? explains the installation process.

Aeration and circulation

Koi consume a great deal of oxygen, as does the biological filter. In summer, at night, demand peaks while warm water dissolves less oxygen. Dedicated aeration is not optional in a koi pond.

• An air pump supplying diffusers at the bottom, in aerated bottom drains, or in the biological stage of the filter.
• General circulation provided by the returns, which mixes the water column and prevents summer stratification.
• An agitated surface, created by a waterfall or surface return, which promotes gas exchange.

Place the air pump in a dry location in the equipment room, with a clean air intake, and install a check valve on each line. For choosing the model, see How to choose an oxygenator for koi carp?.

Budget and phasing

The budget for a koi pond is divided into very different categories. The proportions vary according to the choices made, but the list itself is always the same:

• Excavation and removal of soil.
• Buried plumbing: bottom drains, pipes, returns, penetrations.
• Waterproofing.
• Chamber and equipment room.
• Filtration, pump, UV, aeration.
• Electricity.
• Surroundings, coping stones, cladding.
• Annual operating costs: electricity, consumables, food, tests.

The phasing principle is simple: everything that will be buried or poured must be done correctly from the outset. Bottom drains, 110 mm pipes, electrical conduits, filtration chamber, penetrations: these cannot be redone. However, some equipment can evolve: you can start with a simpler prefiltration system and switch to a drum filter later, provided you have planned for the space and connections. Similarly, you can plan for a spare pipe and bottom drain, capped off, for a future extension.

Specification checklist

Print this table, fill in the right-hand column with your own values, and have it reviewed before ordering anything. It is the most cost-effective document in the entire project.

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To determine the initial volume: What size or volume pond for koi carp?.

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Frequently asked questions

— How deep should a koi pond be?

As a general guide, at least 1.2 to 1.5 m over a large part of the pond, and ideally 1.5 to 2 m in the main zone, for thermal stability and protection against herons.

— How many bottom drains should be planned?

Approximately one bottom drain per 15 to 25 m² of floor area, each on its own 110 mm pipe, with a flow rate of around 8 to 12 m³/h per drain in a gravity-fed system.

— Can a koi pond have plants?

Rarely in the main pond, as koi will tear them out. A separate wetland zone or planted filter connected to the circuit is preferred.

— EPDM liner or concrete for a koi pond?

Both are suitable. EPDM is more economical and suited to gentle shapes; coated masonry allows smooth vertical walls without folds, at a higher cost.

In summary

• The number and adult size of the koi determine the entire design.
• Generous volume, a deep zone of at least 1.5 m, and a simple shape without recesses.
• Bottom drains, skimmer, and returns form a coherent hydraulic circuit.
• Gravity-fed filtration sized for a turnover time of one to two hours.
• Durable waterproofing, dedicated aeration, and everything underground completed from the outset.

Last updated: October 3, 2026.