Calculate pond volume in cubic feet and gallons for fish stocking, filter sizing, and chemical dosing.
Equipment sizing
Liner & water
Treatment dose
Usage Tip
Stock conservatively, around one inch of fish per ten gallons, and size the filter to the full volume with room to spare.
gallons = cubic feet × 7.48
For an irregular pond, use average dimensions; this assumes a roughly rectangular shape.
How many gallons is my pond?
Pond volume is the surface area times the average depth, converted to gallons. The key number is 7.48 — the gallons in a cubic foot. A rectangular pond is length times width times average depth times 7.48; round and oval ponds use the area of an ellipse. A 10 by 6 ft pond averaging 2 ft deep holds about 900 gallons. This calculator gives gallons, cubic feet and liters, then sizes the pump, filter, liner and even the water treatment dose from that volume.
Pond volume formulas
| Shape | Gallons |
|---|---|
| Rectangle | length x width x avg depth x 7.48 |
| Circle | 3.14 x radius squared x avg depth x 7.48 |
| Oval | 3.14 x (length/2) x (width/2) x avg depth x 7.48 |
| Irregular | length x width x avg depth x 7.48 x 0.85 |
If the bottom slopes, average depth is roughly the shallow plus the deep end divided by two.
Pond pump size calculator
A pump should circulate the whole pond regularly. For a koi pond, turn the entire volume over once per hour; for a garden pond, once every two hours is fine. So a 900 gallon garden pond wants a pump of about 450 gallons per hour at the pond, plus extra for the head height and any waterfall.
| Pond type | Turnover | Pump for 1,000 gal |
|---|---|---|
| Koi pond | Once per hour | about 1,000 GPH |
| Garden pond | Once per 2 hours | about 500 GPH |
A waterfall needs roughly 1,000 to 1,500 GPH for every foot of spillway width, so size the pump to the larger of the turnover and the waterfall.
Pond liner size guide
The liner has to reach across the pond, down both sides and overlap the edge. The rule is simple:
| Liner dimension | Formula |
|---|---|
| Liner length | pond length + (2 x max depth) + 2 ft overlap |
| Liner width | pond width + (2 x max depth) + 2 ft overlap |
Always size the liner from the deepest point, not the average, and add at least a foot of overlap on each side to anchor it.
Koi pond depth guide
Koi need depth to thrive and to overwinter safely. Aim for at least 3 ft, and 4 ft or more in cold climates so part of the pond stays below the freeze line. Garden ponds with goldfish can be shallower, around 1.5 to 2 ft. Deeper water also stays cooler and clearer in summer.
Common pond size examples
| Pond | Approx volume |
|---|---|
| Small garden pond (6 x 4 x 1.5 ft) | about 270 gallons |
| Medium pond (10 x 6 x 2 ft) | about 900 gallons |
| Koi pond (12 x 8 x 3 ft) | about 2,150 gallons |
| Large koi pond (16 x 12 x 4 ft) | about 5,750 gallons |
Frequently asked questions
How do I calculate pond gallons?
Multiply length by width by average depth in feet, then by 7.48. For round ponds use the area of the circle.
What size pump do I need?
Enough to turn the volume over once an hour for koi, or once every two hours for a garden pond, plus extra for waterfalls and head height.
How deep should a koi pond be?
At least 3 ft, and 4 ft or more in cold climates so the fish can overwinter below the freeze line.
What size liner do I need?
Pond length plus twice the max depth plus 2 ft of overlap, and the same for the width. Always use the deepest point.
How many fish can my pond hold?
Roughly one koi per 250 gallons or one goldfish per 20 gallons. Less is healthier; overstocking strains the filter.
How much water evaporates from a pond?
Often an inch or more per week in hot weather. The calculator estimates weekly loss and refill time from your hose flow.
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Estimates are for planning. Pump, filter and treatment needs vary with fish load, sunlight, plants and climate; size equipment with a margin and follow each product label for exact dosing. Confirm liner size against your final dig before ordering.