Koi Pond Calculator

Work out how many gallons your koi pond holds, then see what that volume means in practice — how many koi it supports, the pump flow it needs, and the liner size to buy. Rectangular, circular, oval and irregular ponds, in feet or metres.

Gallons & litres 4 pond shapes Feet or metres Always free

🪷 Koi Pond Volume Calculator

Results update as you type. Use the average depth of the pond, not the depth of the deepest point.

Measurement units
Irregular ponds use a 0.85 shape factor against the bounding rectangle, which is the usual allowance for curved edges.
Pond dimensions

Average water depth, measured from the water line. A pond with 2 ft shelves and a 5 ft centre averages closer to 3.5 ft than 5 ft, and using the deepest point is the single most common reason a pond calculation comes out far too high.

Common koi pond sizes

These are typical footprints for backyard koi ponds. Selecting one fills in the dimensions so you can see the volume, then adjust from there. Anything under about 1,000 US gallons and 3 feet deep is generally considered a goldfish pond rather than a koi pond.

How it works

How to Use the Koi Pond Calculator

Four steps from a tape measure to a gallon figure you can build a stocking and filtration plan on.

1

Pick the pond shape

Rectangular, circular, oval or irregular. The shape changes the surface area formula, and a kidney-shaped pond holds noticeably less than the rectangle it sits inside.

2

Measure at the water line

Measure the length and width of the water surface, not the excavation or the outside of the coping stones.

3

Work out average depth

Take depth readings across the pond and average them. Shelves and sloped sides pull the average well below the deepest point.

4

Read volume and capacity

You get gallons, litres and cubic feet, plus koi capacity, minimum pump flow and liner size derived from the same measurements.

How Much Water Does a Koi Pond Hold?

Pond volume is surface area multiplied by average depth. Everything else in koi keeping is downstream of that number: stocking density is quoted in gallons per fish, pump flow is quoted as turnovers of the pond volume per hour, medication and salt doses are quoted per hundred or per thousand gallons, and filter ratings are quoted against pond volume too. Get the volume wrong and every one of those figures is wrong by the same margin.

Two habits cause most of the error. The first is measuring the hole rather than the water — the excavation is bigger than the finished pond once the liner, underlay and any block work are in. The second is using the deepest point as the depth. A pond with 12 inch marginal shelves around a 4 foot centre can average barely 2.5 feet, so treating it as a 4 foot pond overstates the volume by more than half and leaves the filter and pump badly undersized.

If the pond is already full, there is a check that beats any calculation: fit a water meter to the hose, or count the fills of a container of known volume, and record the total as you fill. Use the calculator to plan and the meter reading to confirm.

Koi pond volume formulas

These are the formulas behind the calculator, if you want to work a figure out by hand or check one you have been given.

Rectangular pond
length (ft) × width (ft) × average depth (ft) × 7.48 = US gallons
Circular pond
3.1416 × radius (ft)² × average depth (ft) × 7.48 = US gallons
Oval pond
3.1416 × (length ÷ 2) × (width ÷ 2) × average depth × 7.48 = US gallons
Irregular or kidney shaped pond
length × width × average depth × 0.85 × 7.48 = US gallons

The 7.48 is the number of US gallons in a cubic foot. For imperial gallons use 6.23, and for litres use 28.32. Working in metres instead, a cubic metre is 1,000 litres or about 264 US gallons.

Koi Pond Volume by Pond Size

Common backyard pond footprints and the volume each one holds. Use it as a sanity check on the figure the calculator gives you.

Volumes are for a rectangular pond at the stated average depth. Adult koi capacity assumes 250 US gallons per mature fish.
Pond size (L × W × avg depth)Cubic feetUS gallonsLitresAdult koi
6 × 4 × 2 ft483591,3591 (goldfish pond)
8 × 6 × 3 ft1441,0774,0784
10 × 6 × 3 ft1801,3475,0975
10 × 8 × 3 ft2401,7956,7967
12 × 8 × 3.5 ft3362,5149,51410
15 × 10 × 4 ft6004,48816,99017
20 × 10 × 4 ft8005,98422,65323
20 × 15 × 4.5 ft1,35010,09938,22840

The jump from a 6 × 4 garden pond to a 12 × 8 koi pond is a factor of seven in water volume, and that is what makes koi keeping possible: a large volume dilutes waste, holds temperature steady overnight and through a cold snap, and gives adult fish room to grow into their full size rather than stalling.

What Size Pond Do Koi Need?

Koi are not garden pond goldfish. A well-kept koi reaches 18 to 24 inches, and jumbo bloodlines pass 30 inches, so the pond has to be planned for the adult fish rather than the 4 inch one you bring home. The widely used minimums among koi keepers are:

  • 1,000 US gallons minimum for a pond that will hold koi at all, and that is a starter figure rather than a comfortable one.
  • 3 feet minimum depth, with 4 feet or more in regions that freeze or that get intense summer sun.
  • 250 US gallons per mature koi as a long-term stocking figure, or around 150 gallons per fish on a pond with a bottom drain, a properly sized filter and dedicated aeration.
  • Length over depth for swimming: koi swim horizontally, so a long, wide pond beats a deep, narrow one of the same volume.

Depth does more than add gallons. It gives fish a thermally stable layer to sit in through both winter and a heatwave, it keeps them out of easy reach of herons and raccoons, and it slows the day-to-night temperature swing that stresses koi in shallow water. A 2 foot pond in an open garden can move several degrees between afternoon and dawn; a 4 foot pond barely moves.

How deep should a koi pond be?

Three feet is the practical floor. Four to five feet is the range most established koi keepers build to, and ponds deeper than about six feet mainly add cost and make netting a sick fish difficult without adding much benefit. In climates with hard winters the requirement is that a substantial body of water stays above freezing beneath the ice, which is a function of both depth and volume — a deep but small pond can still freeze through.

Surface area still matters

Oxygen enters a pond mainly at the surface, so two ponds with the same volume are not equivalent if one is deep and narrow. A wide surface, a waterfall or an air pump all raise dissolved oxygen, and that becomes the limiting factor on warm summer nights when water holds less oxygen and the fish need more of it. The calculator reports surface area alongside volume for this reason.

What Your Pond Volume Tells You Next

Once you have a gallon figure, four decisions follow directly from it.

Stocking

Divide by 250 for a conservative adult stocking level, or by 150 if the pond has strong filtration and aeration. The koi pond stocking calculator adjusts that for the size of your fish and the filtration you actually have.

Pump and filter

A koi pond should turn its full volume over roughly once per hour, so a 3,000 gallon pond wants a pump moving about 3,000 GPH at the head height it is actually working against — not 3,000 GPH on the box. The koi pond pump size calculator works out that head loss for you.

Liner

Liner size comes from the same measurements plus depth and overlap. The koi pond liner calculator gives the sheet size and the nearest standard liner.

Treatments

Salt and medication doses are quoted per volume, and a 20 percent error in your gallon figure is a 20 percent error in every dose. The koi pond salt calculator converts a target concentration into pounds or kilograms of salt.

Common Koi Pond Sizing Mistakes

  • Using the deepest point as the depth. Shelves, slopes and a planted margin all pull the average down. Measure in several places and average.
  • Measuring the excavation. Underlay, liner and block work eat into the hole. Measure the water surface once it is filled, or subtract the build thickness when planning.
  • Forgetting the filter and pipework. A gravity-fed filter chamber, a settlement chamber and the plumbing can hold hundreds of extra gallons. That helps stocking, but it also means salt and medication doses must be calculated on the total system volume, not just the pond.
  • Buying koi for the pond you have today. Six 5-inch koi look lost in a 1,500 gallon pond. The same six fish at 20 inches will be crowded in it.
  • Treating a bigger pump as a substitute for volume. Turnover moves water through the filter; it does not create swimming space or thermal stability.
  • Ignoring imperial versus US gallons. A UK gallon is about 20 percent larger. Product ratings and dosing instructions do not always say which one they mean, so check before dosing.
FAQ

Frequently Asked Questions

The pond sizing questions koi keepers ask most often.

Multiply the surface area of the water by the average depth, then multiply by 7.48 for US gallons. A rectangular 12 × 8 ft pond with an average depth of 3.5 ft is 336 cubic feet, which is about 2,514 US gallons, 2,093 imperial gallons or 9,514 litres. For circular ponds use 3.1416 × radius² for the area, and for irregular ponds multiply the bounding rectangle by about 0.85. If the pond is already full, a water meter on the hose during filling gives you an exact figure.
A commonly used minimum is 1,000 US gallons and 3 feet of depth, with 250 gallons per mature koi as the long-term stocking figure. That means a 1,000 gallon pond is realistically a four-koi pond once the fish are adult. Most established koi keepers build between 2,500 and 6,000 gallons with a depth of 4 to 5 feet, which gives room for the fish to reach full size and keeps water quality and temperature stable.
Three feet is the practical minimum and 4 to 5 feet is the usual target. Depth gives thermal stability in both summer and winter, puts fish out of easy reach of herons and raccoons, and reduces the day-to-night temperature swing that stresses koi in shallow water. In climates with hard freezes, depth combined with enough total volume is what keeps a body of unfrozen water beneath the ice. Below about 2 feet the pond behaves like a goldfish pond rather than a koi pond.
Working in metres, multiply length × width × average depth in metres to get cubic metres, then multiply by 1,000 for litres. A 4 × 2.5 m pond with an average depth of 1.1 m is 11 cubic metres, or 11,000 litres. Working in feet, multiply cubic feet by 28.32 to get litres. The calculator on this page reports litres alongside US and imperial gallons whichever units you enter.
Around 250 US gallons per mature koi is the widely used long-term figure, dropping to roughly 150 gallons per fish on a pond with a bottom drain, a properly sized biological filter and dedicated aeration. Juveniles need far less in the short term, but stocking to the juvenile figure is how ponds become overcrowded, because koi keep growing for years. Stock for the adult size of the fish you are buying.
The standard target is one full turnover of the pond volume per hour, so a 3,000 gallon pond needs a pump delivering about 3,000 GPH at its working head height. Pumps are advertised at zero head, and the flow drops as vertical lift, pipe length and elbows are added, so a pump rated 3,000 GPH may deliver only 2,000 GPH in a real installation. Size against the pump curve at your actual head, not against the headline number.
Liner length is pond length plus twice the maximum depth plus an overlap on each side, and liner width follows the same formula. A 12 × 8 ft pond at 4 ft deep with a 1 ft overlap on each side needs a liner of at least 22 × 18 ft. Round up to the next standard sheet size rather than trimming to the exact figure.
It is at the minimum end. A 1,000 US gallon pond at 3 feet deep will support around four mature koi at the conservative 250 gallon per fish figure, or six to seven with strong filtration and aeration. It works, but it leaves little margin: water quality moves faster in a small volume, temperature swings more, and there is no room to add fish later. If you have the space and budget, 2,500 gallons and up is a far more forgiving pond.
Free tools

Related Koi Pond Tools

Your pond volume is the input for every other decision — take the number above into these.

Koi Pond Stocking Calculator

Turn your gallon figure into how many koi the pond actually supports at adult size.

Koi Pond Pump Size Calculator

Size a pump by turnover rate and the real head height it has to push against.

Koi Pond Liner Calculator

Liner and underlay size from the same length, width and depth measurements.

Koi Feeding Calculator

Daily food weight by koi size and water temperature, including when to stop.

Also useful: the koi fish care guide for filtration, oxygen and seasonal care, the koi fish tank guide if you are growing koi on indoors first, and the water parameter checker for the readings a new pond needs watching.

Methodology and accuracy

Pond volume is calculated as surface area × average depth. Surface area uses length × width for rectangular ponds, π × radius² for circular ponds, π × (length ÷ 2) × (width ÷ 2) for ovals, and length × width × 0.85 for irregular and kidney shapes. The 0.85 factor is a common practical allowance for curved edges, not a measurement of your particular pond — a strongly lobed shape holds less and a nearly rectangular one holds more.

Unit conversions are exact and carried at full precision internally, rounded only for display: 1 ft = 0.3048 m, 1 ft³ = 7.480519 US gallons = 6.228835 imperial gallons = 28.316847 litres, 1 m³ = 1,000 litres = 264.172 US gallons.

The koi capacity, pump flow and liner figures shown alongside the volume are rules of thumb widely used in pond keeping — 250 US gallons per mature koi (150 with strong filtration and aeration), one full turnover per hour for pump flow, and pond dimension plus twice depth plus overlap for liner size. They are planning starting points, not measured results, and the dedicated calculators linked from this page refine each one with the variables that actually change it.

Pump flow shown here is the target flow only. It does not account for head loss, which can reduce a pump’s real output by a third or more; use the pump calculator for a figure you can buy against.