Garden Pond Calculator - Volume, Liner Size and Fish Capacity

Every cubic meter of pond water weighs one metric ton - here is what that means for your garden

A modest 4 m round pond at 1.2 m depth holds 15,080 liters of water - that is 15 metric tons sitting in your garden. The EPDM liner to hold it measures 7.4 m x 7.4 m and costs around $383 at $7/m2. The calculator works out surface area, volume, liner dimensions, how many plants you need, and how many fish the pond can safely support.

Four numbers that define your pond

12.6 m2

Surface area (4 m round)

15,080 L

Water volume (1.2 m deep)

7.4 x 7.4 m

EPDM liner needed

75 fish

Max capacity (5/m3)

How to use the garden pond calculator - step by step

Pond shape - round is simplest to build and calculate. Kidney-shaped looks most natural. Rectangular suits formal gardens.
Length or diameter - the main dimension. A 3-4 m pond is a good starting point for most gardens.
Width - only for rectangular and kidney shapes. Leave empty and the calculator defaults to 70% (rectangular) or 60% (kidney) of length.
Depth - 0.5 m for a water feature, 0.8 m for plants, 1.0-1.5 m for fish. Fish need at least 1 m to survive winter.
Fish? - if yes, the calculator shows maximum stocking density (5 fish per m3) and warns if depth is too shallow.
Liner price - enter your local EPDM price per m2 to see total liner cost. US average: $5-10/m2.

Pond volume and liner size by dimensions

Pond (round)	Surface (m2)	Volume at 1 m	Volume at 1.2 m	Liner size (1.2 m)	Max fish
2 m diameter	3.1	3,140 L	3,770 L	5.4 x 5.4 m	18
3 m diameter	7.1	7,070 L	8,480 L	6.4 x 6.4 m	42
4 m diameter	12.6	12,570 L	15,080 L	7.4 x 7.4 m	75
5 m diameter	19.6	19,630 L	23,560 L	8.4 x 8.4 m	117
6 m diameter	28.3	28,270 L	33,930 L	9.4 x 9.4 m	169

Fish capacity at 5 fish per m3 (1.2 m depth). Liner = diameter + 2x depth + 1 m overlap.

Did you know?

A 4 m round pond weighs as much as 3 cars. At 15,080 liters (15 metric tons), it is heavier than most SUVs combined. The ground beneath must be compact and level.

EPDM rubber liner lasts 30-50 years. PVC lasts 10-15 years but costs half as much. Over a 30-year period, EPDM is cheaper because you never replace it.

Water lilies need exactly 80 cm of depth. Too shallow and the leaves burn. Too deep and they cannot reach the surface. The 0.8 m depth option is designed for them.

A goldfish produces 1 gram of ammonia per day. With 75 fish in a 15 m3 pond, the pump must cycle the entire volume twice per hour to keep water safe. That is 30,000 liters per hour.

Practical examples

Small decorative pond, round, 2 m, 0.5 m deep: Surface: 3.1 m2. Volume: 1,570 L. Liner: 4.0 x 4.0 m (16 m2). No fish - too shallow. Plants: 9. Liner cost: ~$112.

Mid-size fish pond, round, 3 m, 1.2 m deep: Surface: 7.1 m2. Volume: 8,480 L. Liner: 6.4 x 6.4 m (41 m2). Max fish: 42. Plants: 21. Liner cost: ~$287.

Kidney-shaped, 5 x 3 m, 1.0 m deep: Surface: 12.8 m2. Volume: 12,750 L. Liner: 8.0 x 6.0 m (48 m2). Max fish: 63. Plants: 38. Liner cost: ~$336.

Large rectangular, 6 x 4 m, 1.5 m deep: Surface: 24.0 m2. Volume: 36,000 L. Liner: 10.0 x 8.0 m (80 m2). Max fish: 180. Plants: 72. Liner cost: ~$560.

Tiny water feature, round, 1.5 m, 0.5 m deep: Surface: 1.8 m2. Volume: 880 L. Liner: 3.5 x 3.5 m (12.3 m2). No fish. Plants: 5. Liner cost: ~$86.

EPDM vs PVC liner comparison

Feature	EPDM (rubber)	PVC
Lifespan	30-50 years	10-15 years
Price per m2	$5-10	$2-5
UV resistance	Excellent	Degrades after 10 yrs
Flexibility	Very flexible, even in cold	Stiffens in cold weather
Fish safe	Yes (non-toxic)	Yes (if fish-grade)
30-year cost (50 m2)	$350 (1 liner)	$350-500 (2-3 liners)

FAQ

How deep should a fish pond be?

Minimum 1.0 m for fish to survive winter in temperate climates. Recommended: 1.2-1.5 m. The deeper zone stays above freezing even when the surface freezes, giving fish a safe refuge. Koi ponds should be at least 1.5 m.

How many fish can a pond hold?

The safe rule is 5 fish per cubic meter of water. A 3 m round pond at 1.2 m depth holds 8.5 m3 = about 42 fish. Overstocking causes ammonia spikes and kills fish. Start with fewer and add gradually.

How do I calculate EPDM liner size?

Liner length = pond length + (2 x depth) + 1 m overlap. Liner width = pond width + (2 x depth) + 1 m overlap. For a 4 m round pond at 1.2 m deep: 4 + 2.4 + 1 = 7.4 m in both directions. Always round up to the nearest available roll size.

How much does it cost to build a garden pond?

For a mid-size 3 m round pond (1.2 m deep): EPDM liner $287, pump $120-200, filter $80-150, plants (21 plants) $126-252, fish $40-200. Total: roughly $650-900 DIY. Professional installation adds $500-1,500.

Do I need a pump for a garden pond?

Yes, unless it is a very small decorative water feature with no fish. The pump circulates and filters the water, preventing stagnation, algae bloom and mosquito breeding. Minimum flow: 2x the pond volume per hour. A 15 m3 pond needs a pump rated at 30,000 L/hr.

Can I keep koi in a garden pond?

Koi need at least 1.5 m depth and 1,000 liters per fish (more than goldfish). A 4 m round pond at 1.5 m holds 18,850 L, enough for 15-18 koi. They also need higher filtration capacity and produce more waste than goldfish.

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