Pond Calculator gives the gallons in a rectangle, round, oval or kidney pond with straight or sloped walls, plus the liner sheet, pump flow, fill time and the dose from your label.
Measure length and width at the waterline and depth at the deepest point.
Pump and Waterfall
Liner and Filling
Dose From a Product Label
Volume by Water Depth
| Water Depth | Volume | Full |
|---|---|---|
| 0.5 ft | 299 gal | 17% |
| 1 ft | 598 gal | 33% |
| 1.5 ft | 898 gal | 50% |
| 2 ft | 1,197 gal | 67% |
| 2.5 ft | 1,496 gal | 83% |
| 3 ft | 1,795 gal | 100% |
Pump Flow by Waterfall Width
| Width | Residential | Heavier | Commercial |
|---|---|---|---|
| 1 ft | 1,500 | 2,000 | 3,000 |
| 1.5 ft | 2,250 | 3,000 | 4,500 |
| 2 ft | 3,000 | 4,000 | 6,000 |
| 3 ft | 4,500 | 6,000 | 9,000 |
| 4 ft | 6,000 | 8,000 | 12,000 |
| 5 ft | 7,500 | 10,000 | 15,000 |
Sizing a Garden or Koi Pond With the Pond Calculator
The Pond Calculator works out how many gallons a pond holds from its length, width and depth. It then sizes the liner, the pump flow and head, the fill time and the amount of any treatment from its label.
Homeowners use it before digging a backyard water garden. Pond installers use it to quote liner and pumps, and koi keepers use it to dose medication and salt without guessing at the volume.
Shape, Depth and Side Walls
Choose US units for feet and gallons or metric for meters and liters. Pick a rectangle, round, oval or kidney shape, then enter length and width at the waterline and depth at the deepest point. A round pond only needs its diameter.
Side Walls sets the profile from vertical to a 3:1 slope, which means 3 ft across for every 1 ft down. Three closed panels hold the pump and waterfall settings, the liner overlap and hose flow, and the dose from a product label.
How the Pond Calculator Turns Dimensions Into Gallons
A pond with vertical walls is its surface area times its depth. A cubic foot holds 7.48 gallons, so the volume in gallons follows directly from the cubic feet.
A 10 ft by 8 ft pond, 3 ft deep, holds 240 cu ft, or 1,795 gallons. Round and oval ponds use π/4 × length × width for the area, and kidney or free-form ponds use 0.8 × length × width, a convention published by pond supplier Aquacadabra for irregular outlines.
Sloped walls shrink the pond toward the bottom, so the area changes with depth. The calculator uses the prismoidal formula, which weights the top, middle and bottom areas and is exact for a pond whose sides slope at a steady rate.
The same 10 × 8 × 3 ft pond with 1:1 walls narrows to 4 ft by 2 ft at the floor. It holds about 853 gallons, 53% less than the vertical-wall version, which is why a dug pond with sloped sides needs far less water than its surface suggests.
A common slip is using maximum depth for a pond with plant shelves. Shelves cut the volume, so the volume tools from The Pond Guy and Pondliner.com both ask for average depth, while liner sizing still needs the maximum.
Length and width can run up to 1,000 ft and depth up to 50 ft, or 300 m and 15 m in metric. If the slope meets in the middle before it reaches the depth you entered, the tool stops and asks for steeper walls, a shallower depth or a bigger pond.
The Volume by Water Depth table shows gallons at each level below full. It helps when a pond is only partly filled, or when you need to know how much water a planned drawdown will remove.
Liner and Underlayment Size
The liner has to run across the floor, down one wall and up the other, then past the edge for anchoring. That is why the depth counts twice.
Here O is the overlap on each side, and width uses the same formula. Firestone liner dealer Garden Pond USA calls for at least 2 ft of overlap in total, which matches the 1 ft per side default.
For the 10 × 8 × 3 ft example, that gives a 16 ft by 18 ft sheet. Sloped walls use the slanted wall length in place of the depth, so the 1:1 version needs only about 12.5 ft by 14.5 ft.
The card also rounds up to a typical stock sheet, since liner is sold in set widths and lengths. Buy underlayment in the same size so it covers everything the liner touches.
Pump Flow, Waterfall Width and Head
Pump flow starts with turnover, the time it takes to move the whole pond through the pump. The Pond Guy sets a minimum of once every two hours for a pond with no fish and doubles that for fish, which gives once an hour, the same rate Aquascape uses.
A waterfall sets its own minimum. Aquascape lists 1,500 GPH per foot of spillway width for a residential waterfall and 3,000 GPH per foot for commercial work, and the tool picks whichever of turnover or waterfall flow is larger. The Pond Guy's 2,000 GPH per foot rate for a heavier flow sits between them as the middle setting.
Head is the height the pump lifts water plus friction in the tubing. The calculator uses the pond-trade convention of 1 ft of head for every 10 ft of tubing, the same rule The Pond Guy and Pond Informer publish.
A 5 ft lift with 25 ft of tubing gives 7.5 ft of head. Check the pump curve at that head, because a pump's rated flow is measured with no lift and drops as head rises.
For the default 1,795-gallon pond with fish, once-an-hour turnover needs about 1,795 GPH, but a 2 ft waterfall needs 3,000 GPH, so the waterfall sets the pump. When that happens, the card's Turnover Alone row reads 1× Volume per Hour rather than repeating the gallons.
For long or large-diameter runs, the Hazen Williams Equation Calculator gives a true friction loss instead of the rule of thumb. The Pump Pressure Calculator converts that head to psi if a pump lists pressure.
Water Weight, Fill Time and Digging
Fresh water weighs 8.34 lb per gallon, so the 1,795-gallon example carries about 14,973 lb. That matters for a raised pond or one built on a deck.
Fill time is the volume divided by hose flow. At 6 gpm, the example takes just under five hours, and the Gallons Per Minute Calculator finds your real hose flow from a bucket and a stopwatch.
Soil to dig equals the water volume in cubic yards, 8.9 cu yd for the example. Loose soil takes up more room once it is out of the ground, so plan for a bigger pile than the number suggests.
Treatment Dose and Pond Salt
The dose card scales any product label to your pond. Enter the label amount and the volume it treats, and the tool multiplies it out, such as 179.5 ml for 1,795 gallons at 10 ml per 100 gallons.
Salt is figured by the weight of the water, so 0.1% salt is 0.001 times the water weight. Kodama Koi Farm uses the same basis, with 1 kg per 1,000 liters for about 0.1%.
The popular rule of 1 lb per 100 gallons lands closer to 0.12%. The Midwest Pond and Koi Society puts that rate at 1.25 parts per thousand, so the card's 15.0 lb for 0.1% in the example is lower than a quick rule would give.
Measuring Mistakes That Skew Pond Volume
Measuring a kidney or free-form pond as a full rectangle overstates its volume. Use the kidney shape, or the longest and widest points with the 0.8 factor, for any pond that is not a true rectangle or circle.
Sizing a liner from average depth or the excavation depth leaves it short. Liner guides call for the maximum water depth, because the sheet must reach the floor at its deepest point.
Dosing salt from zero every time can build the level up. Salt does not evaporate, so a pond that already holds 0.25% needs much less than one starting fresh.
Common Questions From Pond Builders
What size pond liner do I need for a 10 x 8 ft pond?
For a pond 3 ft deep with 1 ft of overlap per side, the liner is 16 ft by 18 ft. Some koi guides call for 2 ft of overlap per side, which raises it to 18 ft by 20 ft.
Pick the overlap that matches how you will anchor the edge. A rock edge or trench needs more than a liner tucked under sod, and the overlap field changes the size right away.
Should a pond pump run all the time?
Pond supplier Half Off Ponds advises running pond pumps 24 hours a day to keep the pond's ecosystem working. Its pump cost tool exists for that reason, since a pump that never stops shows up on the power bill.
That is why the turnover figure matters even with no waterfall. It is the flow the pond sees around the clock, so size it for the fish you keep rather than for looks alone.
Will salt kill pond plants?
At low levels, most plants cope, but higher levels harm them. Krazy Koi Meds notes that most submerged plants will not survive 0.3%, the level often used against parasites.
Splash Supply Company advises against routine salt in koi ponds and suggests raising it to 0.3% only for sick fish, over about three days. Move plants out before a treatment, or treat the fish in a separate tank.
How can I find my pond's volume if I can't measure it?
Salt can measure a pond for you. Read the salinity, add a known amount of salt, let it mix and read it again, and the change tells you the volume.
The Midwest Pond and Koi Society formula works out to gallons = 120 × pounds of salt added ÷ the rise in parts per thousand. Add 10 lb and see a 0.5 ppt rise, and the pond holds about 2,400 gallons.