Free pond volume calculator: enter pond length, width, and average depth to get gallons, litres, liner size, and pump flow rate.
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Measure pond length and width at the waterline in feet, and average depth by measuring depth in five or more spots, adding them, and dividing by the number of measurements. The calculator multiplies those three numbers for cubic feet, multiplies by 7.48 for US gallons, converts to litres, sizes the liner by adding twice the depth plus 3 feet of overlap to each side, and gives the pump flow for one full turnover per hour for a fish pond.
A rectangular pond is a box of water: length times width times average depth gives cubic feet, and one cubic foot holds 7.48 US gallons, because a gallon is 231 cubic inches and a cubic foot is 1,728 cubic inches. A 12 foot by 8 foot pond at 4 feet average depth is 384 cubic feet, which is 2,872 gallons and 10,873 litres. A 10 foot by 5 foot pond at 2 feet average depth is 100 cubic feet, or 748 gallons. In metric units there is no conversion factor at all: cubic metres are length times width times depth in metres, and litres are cubic metres times 1,000, so a 3 by 2 by 1.2 metre pond holds 7.2 cubic metres, or 7,200 litres.
Measure length and width at the waterline, not at the top of the bank. Measure depth in at least five spots across the pond, add them, and divide by the count for average depth. Enter those three numbers and read cubic feet, gallons, litres, liner size, and pump flow together. For a pond that is not yet dug, use the planned waterline dimensions, and re-run the numbers once the hole is dug, because the finished depth often differs from the plan.
Most garden ponds have sloped or stepped sides, so the edges are much shallower than the middle. Using the deepest point instead of the average inflates the volume, which leads to an undersized pump and overdosed treatments, the two most common ways pond owners harm fish. Measure depth at the shallow edge, the middle, and the deep end, and average them. If the pond is irregular, build the box around the widest outline and multiply by 0.8 to allow for irregular banks, a trade shortcut that stays within about 10 percent for equipment sizing.
A round pond uses pi times radius squared times average depth, where radius is diameter divided by 2: a 10 foot diameter round pond at 3 feet average depth is 235.5 cubic feet, or about 1,761 gallons. An oval pond uses length times width times average depth times about 6.7 gallons per cubic foot equivalent in the pond trade, though the exact pi divided by 4 times 7.48 is 5.88, so treat the oval figure as an estimate. For a truly irregular pond, split it into rectangles, triangles, and circles, calculate each section, and add the totals, or use the 0.8 box method above.
For a fish pond, the pump should turn over the full volume about once per hour, so a 2,872 gallon pond needs roughly a 2,900 GPH pump at its actual head height and tubing size, not the zero-head rating on the box. For a planted pond with no fish, half that turnover is often adequate. Treatment dosage is set per 100 gallons, so an accurate volume keeps fish safe, while an inflated volume risks overdosing. For fish capacity and stocking, allow about 240 gallons per adult koi and no more than 1 inch of fish per 10 gallons as a general guide, so a 2,872 gallon pond supports roughly 10 to 12 koi with good filtration.
Size the liner from the same three measurements: liner length equals pond length plus twice the depth plus about 3 feet of overlap, and the same for width. A 10 by 8 foot pond at 2 feet depth needs a liner of about 17 by 15 feet. Give yourself the full overlap, because trimming a liner that is too small is a much bigger problem than trimming excess. Water weight matters too: one gallon weighs 8.34 pounds, so the 2,872 gallon example weighs nearly 24,000 pounds, just under 12 tons, which is why above-ground and raised ponds need engineered support.
Using maximum depth instead of average depth, dividing by 7.48 instead of multiplying, and rounding the conversion to 7.5 without saying so are the three math mistakes competitors warn about. Measuring at the bank instead of the waterline, forgetting rocks and gravel displacement, and sizing a pump on its zero-head rating are the practical ones. Rocks and gravel can displace a noticeable share of the volume, so some owners multiply the final figure by 0.8 for a heavily rocked pond. When in doubt, verify with a water meter on filling, which is the only exact measure.
Multiply length times width times average depth in feet, then multiply by 7.48 for gallons. A 12 by 8 by 4 foot pond holds 384 cubic feet, which is 2,872 gallons. Always use average depth, not the deepest point.
One cubic foot holds 7.48 US gallons. A US gallon is 231 cubic inches, and 1,728 divided by 231 gives 7.4805, conventionally rounded to 7.48.
Use average depth. Most ponds have sloped sides, so maximum depth overstates the volume. Measure in five or more spots, add them up, and divide by the number of measurements.
In metric units, multiply length times width times average depth in metres for cubic metres, then multiply by 1,000 for litres. A 3 by 2 by 1.2 metre pond holds 7,200 litres.
For a fish pond, turn over the full volume about once per hour: a 2,872 gallon pond needs roughly a 2,900 GPH pump at its actual head height. Check the pump chart at your head height, not the zero-head rating.
Allow about 240 gallons per adult koi and no more than 1 inch of fish per 10 gallons as a general guide. A 2,872 gallon pond supports roughly 10 to 12 koi with good filtration.
Add twice the depth plus overlap to each dimension: liner length equals pond length plus 2 times depth plus about 3 feet of overlap, and the same for width. A 10 by 8 foot pond at 2 feet depth needs about 17 by 15 feet.
One gallon of water weighs about 8.34 pounds. The 2,872 gallon example weighs roughly 23,952 pounds, just under 12 tons, so above-ground ponds need structural support designed for that load.