How many gallons is my pond?
Volume is the number every other pond decision hangs off — pump size, filter rating, fish load, treatment doses. The math is plain geometry with one conversion:
ft³ = length × width × average depth (rectangular)
ft³ = π × (diameter ÷ 2)² × depth (circular) · ft³ = π × (L÷2) × (W÷2) × depth (oval)
US gallons = ft³ × 7.48 · liters = gallons × 3.785
A 10×6 ft pond averaging 2 ft deep is 120 ft³ ≈ 898 gallons. The trap is using the maximum depth: most ponds have plant shelves and sloped sides, so the average depth is often 60–75% of the deepest point. For genuinely irregular shapes, the average-dimension method gets within ordering accuracy; for dosing-critical work (medication), measure by metered fill instead.
Pond liner size calculator
The liner has to travel down one wall, across the bottom, up the far wall, and still leave an apron to anchor under the edging — on both axes. Hence:
liner length = pond length + 2 × max depth + 2 × overlap
liner width = pond width + 2 × max depth + 2 × overlap
The same 10×6 ft pond, 2 ft deep with a 2 ft overlap, needs a liner 10+4+4 = 18 ft by 14 ft. Flexible liner is commonly sold in 5-foot width increments, so round each dimension up to the next increment your supplier stocks — the calculator shows the next 5-ft sizes as a note. Cut the underlayment the same size: it is the cushion between liner and soil, and a liner that outlives its warranty usually has underlayment to thank. Excess liner trims in minutes; a short liner restarts the project.
Pond pump size: GPH from turnover
Filtration is specified as turnover — how often the pump pushes the whole pond through the filter. The generic guidance: a planted garden pond turns over about every 2 hours; a koi pond, with its heavy fish load, every hour. So:
pump GPH = pond gallons ÷ turnover hours
An 898-gallon garden pond wants roughly a 450 GPH pump; the same pond stocked with koi wants ~900 GPH. One critical caveat the simple math hides: pumps are rated at zero lift, and every foot of head height (vertical rise to a waterfall or filter, plus pipe friction) cuts real flow. Read the pump’s flow curve at your head, not the number on the box — it is common to buy one size up once head is counted.
Waterfall flow: GPH per inch of weir
Waterfall look is set by flow per inch of spillway width. The generic rule of thumb: about 100 GPH per inch for a gentle sheet, up to ~150 GPH per inch for a strong, noisy fall. A 12-inch weir therefore wants 1,200–1,800 GPH at the top of the falls — often more than the filtration turnover alone, which is why waterfall ponds frequently run a dedicated circuit. Enter your weir width above and the calculator adds the waterfall requirement beside the turnover figure so you can size for whichever is larger.
Koi pond volume: gallons per koi
Koi grow large and load the water heavily, so stocking guidance is conservative: a commonly cited generic figure is around 250 gallons per adult koi, with serious keepers planning 500+ per fish and a practical minimum pond around 1,000 gallons before koi are a good idea at all. Goldfish are far less demanding (tens of gallons each). These are husbandry rules of thumb, not formulas — filtration quality, aeration and climate all move the real number, and a koi specialist’s advice beats any calculator for valuable fish.
Pond salt calculator
Salt dosing is plain arithmetic on the volume: water weighs 8.34 lb per gallon, so
salt (lb) = gallons × 8.34 × dose% ÷ 100
At 0.1%, an 898-gallon pond takes about 7.5 lb of salt. Two cautions that are biology, not math: dose in stages over days, and remember many pond plants and some fish are salt-sensitive — confirm the dose for your livestock before adding anything, and never re-dose without accounting for water you have not changed (salt does not evaporate).